Additions to Table 1

This list in Table 1 should probably also include the famous tidal influenced Matawan Creek of New Jersey, a tributary of Raritan Bay where in July 1916 a couple of shark attacks on bathers with fatalities occurred at 25 km distance from the ocean, even though the identity of the involved species was never satisfactorily resolved (Klimley 2013). At that time, Fowler (1920) (and later on many further authors) presumed that Carcharodon carcharias Linnaeus, 1758 (great white shark) was the involved species. The attacks took place in proximity to the town of Matawan (40.45°N). Considering that this part of the inland waters of New Jersey is a low salinity environment, and because C. leucas undertakes expansive seasonal movements along the east coast of the United States to Massachusetts during the summer (see chapter 4.1), it is very likely that the culprit of these attacks was C. leucas (Klimley 2013). Carcharodon carcharias has never been reported from low salinity habitats or inland waters. However, not focusing on the attacks, this locality could represent the most northern fresh/brackish water occurrence of C. leucas in the world. Carcharhinus leucas may also occur in the more northern Hudson River (New York, USA) (Smith 1985; Berra 2007; Reefquest Centre for Shark Research 2018), but this needs verification. Mearns (1898) reported that sharks were frequently captured in the lower course of the Hudson River, and also in the East River. Moreover, Mearns (1898) reported that several specimens of Carcharhinus obscurus Lesueur, 1818 (dusky shark) were taken in the lower part of the Hudson River during the summer of 1881, one as far up the river as Peekskill, which is 65 km north of New York City. This record is probably based on a misidentification with another species of shark, and even Smith & Lake (1990) stated that the identification of C. obscurus by Mearns is in doubt. However, the tidal influence in the Hudson River reaches as far as 225 km upriver, which gives the lower reaches of this river the character of an estuary, so the occurrence of C. leucas, beside other carcharhinids, in the Hudson River seems possible.

Schwartz (1984, 1989) and Musick et al. (1999) reported the occurrence of C. leucas in estuaries and lagoons of Virginia, North Carolina, and South Carolina, but without naming any certain localities. Kushlan & Lodge (1974: 116) commented for C. leucas in the inland waters of Florida: “Large rivers such as the Caloosahatchee, St. Lucie and the numerous smaller rivers of the southwest coast such as the Shark, Broad and North Rivers of Everglades National Park provide suitable habitat.” Brame et al. (2019) provided evidence for C. leucas from the Everglades National Park. Hocutt & Wiley (1986) pointed out that in the southeast of North America, mostly on the Florida peninsula, C. leucas can be encountered in freshwater with some regularity. Loftus & Kushlan (1987) provided a small distribution map of freshwater occurrences of C. leucas in southern Florida. In Florida, C. leucas utilizes even the artificial freshwaters of the Miami Canals, where there have been numerous recent sightings of C. leucas, the sharks entering these waters via Biscayne Bay (Austin 2015), which brings sharks close to human beings and leads to increasing human-shark interactions. Swift et al. (1977) predicted C. leucas for the Ochlockonee River (FL, USA) due to its occurrence in the nearby Aucilla River (Table 1, No. 45) and its ability to enter freshwater. Rogillio (1975) mentioned C. leucas as an estuarine sportfish in southeastern Louisiana, and Wharton et al. (1981) mentioned this species as an occasional visitor of the inland open waters and the wetlands of bottomland hardwood forests of the Mississippi/Atchafalaya-Basin in the southeastern United States. In the Mississippi/Atchafalaya system, Gunter (1938) reported sightings by local fishers of sharks caught in the Black River at Jonesville (Louisiana), which is a tributary of the Red River (Table 1, No. 77). Parsons (2006) reported that C. leucas is common in and around the marshes of Louisiana. Hubbs (1958) listed C. leucas in the checklist of Texas freshwater fishes with the information that this euryhaline species enters coastal streams. For the rivers of Texas, Hubbs et al. (2008) noted that C. leucas may travel short distances upstream. Christensen et al. (1997) listed C. leucas as a species of the Gulf of Mexico estuaries that are located along the southern U.S. coast. Daugherty et al. (2018) stated that C. leucas is the most abundant shark species in Texas bays, especially specimens less than 2 m TL. Already Evermann & Kendall (1894) reported that Carcharhinus platyodon (= C. leucas) is said to be the most common large shark on the coast of Texas in summer.

García de León et al. (2005) listed C. leucas as a euryhaline marine species for the continental inland waters of Tamaulipas in northeastern Mexico, but without providing a certain locality. Additionally, Thorson (1976a) reported the occurrence of sharks (species not identified, but probably C. leucas) in the San Pedro River (a tributary of the Usumacinta River; Table 1, No. 112) as reported by fishers, local residents, and ichthyologists. Furthermore, for the inland waters of Mexico, Castro-Aguirre (1978) reported C. leucas from the district of Emiliano Zapata, which is drained by the Grijalva River (Tab. 1, No. 110) and the Usumacinta River (Table 1, No. 112). Jones (1985) reported the occurrence of unidentified sharks, in all probability C. leucas, observed by local fishers in the Champoton River in Campeche (Mexico). Macbeath (2014) included C. leucas in a list of fish species found in Mexican freshwaters, but without providing a certain locality.

No. 1: Schwartz (1957) published a “wanted call” for some shark species, including C. leucas, that he suspected to occur at Chesapeake Bay and the Atlantic Ocean off Maryland, to fill gaps in the knowledge about these species. Lawler (1976) reported two examined adult males of C. leucas from the Chesapeake Bay, one specimen (2.23 m TL) captured in July 1976 at Fishermen's Island and one specimen (2.39 m TL) captured in 1973 at the mouth of the Coan River, which is tributary of Chesapeake Bay. Lee et al. (1976), presumably referring to the reports of Schwartz from Chesapeake Bay, added C. leucas to a list of freshwater fishes of Maryland and Delaware with the information that C. leucas locally occurs in freshwater, but without naming a precise locality. However, Lee et al. (1976) and subsequently Flynn & Mason (1978) reported C. leucas from freshwaters of Maryland's coastal plain. Lippson & Lippson (1984) reported that C. leucas has been captured by fishers well up the Chesapeake Bay in Maryland waters, near Annapolis and the mouth of the Chester River. Additionally, Musick et al. (1993) reported catches of C. leucas from waters adjacent to Chesapeake Bay (Chesapeake Bight).

No. 5: Smith & Bean (1899: 180) reported Carcharhinus obscurus in the Potomac River from locations in Maryland and Washington, D.C.: “Occasionally observed in the Potomac between Fort Washington and Alexandria during dry weather when the water becomes brackish. An example 5 feet long, taken at Glymout in August, 1894, was examined by us in Center Market, where a cast of the specimen is now exhibited. Other sharks have also been taken in sturgeon nets at Glymont during dry weather, and many years ago one was captured at Port Washington.” This record by Smith & Bean seems doubtful, as C. obscurus is not known to enter low salinity habitats and normally does not penetrate brackish waters. In this context, Compagno (1984: 490) wrote about the habitat preferences of C. obscurus: “It does not prefer areas with reduced salinities and tends to avoid estuaries.” Moreover, the stretch of the Potomac River between Fort Washington and Alexandria is characterized by nearly pure freshwater conditions during the summer months (0–0.5‰ salinity) (Chesapeake Bay Program 2019). At the very least, the report by Smith & Bean has to be assessed as critical and questionable. Although Smith (1893) reported earlier about sharks in the freshwaters of Lake Nicaragua (see Table 3), these authors were not familiar with similar-looking carcharhinids, and presumably this record of C. obscurus is based on a misidentification with another member of the genus Carcharhinus, probably C. leucas. Thus, this historical account could maybe represent an early record of C. leucas in the Potomac River.

Nos. 13–16: Castro (1993) reported that sightings of juvenile C. leucas in South Carolina estuaries only occur occasionally, as well as reported a catch of a very large female with embryos in Bulls Bay. Ulrich et al. (2007) captured juvenile specimens of C. leucas in South Carolina estuaries, but without giving a precise localization of the catches.

Nos. 17–18: Additionally, and for completeness, Belcher (2008) and Belcher & Jennings (2009a, 2009b, 2010) reported only a few subadult individuals (1–2) of C. leucas in catches from some of the examined estuaries in Georgia, but without naming the precise locality of the catches. This may indicate that C. leucas only occasional utilizes the estuaries of Georgia as nursery areas, although these results are in contrast with those of Streich & Peterson (2011), who provided evidence of a C. leucas nursery in Georgia's Altamaha River Estuary (Table 1, No. 18).

No. 68: The occurrence of C. leucas in the Upper Mississippi River seems to be such a curiosity that Rasmussen (1979: 36) stated: “Only one straggler species is so unusual that it is worthy of note. This is the bull shark (Carcharhinus leucas).” As a result of the rare records from the Mississippi River, C. leucas was mentioned in the summary on the inland fishes of Mississippi by Ross (2001). However, recent records and reports of C. leucas for the Mississippi River are lacking.

Shell & Gardner (2021) reported that only two specimens of C. leucas were captured in the upper portion of the Mississippi River during the entire 20th century. These authors reported that two C. leucas swam up the Mississippi River and made it at least as far as St. Louis (Missouri) on two separate occasions. One specimen was reported from Alton (Illinois) and was captured in 1937 (record at first reported by Thomerson et al. 1977; see Table 1, No. 68). The second record was made just south of Festus in the vicinity of St. Louis (Missouri) near Rush Island Power Station in 1995 (Burr et al. 2004; Shell & Gardner 2021). This last record is not well documented except for a newspaper report. Thus, although Shell & Gardner (2021) reported a repeating large-scale migration of C. leucas in the Upper Mississippi River, this species seems to be rarely encountered in the upper reaches of this river. The limited number of C. leucas records from the Mississippi River Basin during such a wide span of time may lead to the conclusion that C. leucas is a cryptic species in the upper portions of this river system, or records are only poorly documented or events of river penetrations by C. leucas farther inland than the river's estuary are simply rare.

No. 69: Springer (1950: 6) reported for the mouth of the Mississippi River (Mississippi Sound): “The adults appear in great concentration near the mouth of the Mississippi from May through July and produce their young there.” Springer (1960: 33) later commented: “Bull sharks are extremely common around the mouths of the Mississippi and Orinoco Rivers.” Even Branstetter (1981) reported C. leucas as a common species near the mouth of the Mississippi River. Nakamura et al. (1980: 40) gave the information of occurrences of C. leucas in estuarine waters: “West of Mississippi River” and further “East of Mississippi River”, but the information provided was quite imprecise.

The large delta of the Mississippi River represents not only an important nursery ground for numerous shark species like Rhizoprionodon terraenovae Richardson, 1836 (Atlantic sharpnose shark) and C. leucas (Parsons & Hoffmayer 2005, 2007) but also an important feeding habitat for further shark species like Carcharhinus limbatus Müller & Henle, 1839 (blacktip shark) and Carcharhinus isodon Müller & Henle, 1839 (finetooth shark) (Hoffmayer & Parsons 2003). Moreover, the estuary of the Mississippi River delta system is highly productive and performs a function as an important nursery ground for juvenile marine and estuarine fishes (Madden et al. 1988). The availability of young and small sharks in this estuary may also attract adult C. leucas to move in, as this species is known as an intense elasmobranch consumer.

Nos. 70–73: Lac des Allemands, Bayou des Allemands, Lake Salvador, Little Lake, and Barataria Bay are part of the Barataria Basin, a vast Louisiana estuary characterized by a broad amplitude of salinities ranging from fresh to nearly seawater, bordered on the north and east by the Mississippi River and on the south by the Gulf of Mexico. Thompson & Forman (1987) reported C. leucas from the Barataria Basin and Alford (2012) from the Barataria estuary system.

Nos. 77–78: The Red River is the extension of the Atchafalaya River in the Mississippi River System of Louisiana State and is connected with the Mississippi River by some river branches. Hoese & Moore (1998) speculated that C. leucas may have reached the Red River and Saline Lake also via the Mississippi River. a historical record of an adult C. leucas (270 cm TL) from Red River County (Texas) in 1903 was published by Matich et al. (2020b), who investigated historical reports from newspapers. This record expands the distance of freshwater penetration of C. leucas in this river up to 950 km from the estuary and the Gulf of Mexico.

Nos. 81, 82, 86, 87, 89, 91: The C. leucas records from these rivers were originally taken from newspaper references and summarized by Matich et al. (2020b) (see therein for details on the primary references).

No. 105: Darnell (1962) expected C. leucas to occur in the lagoons in the Tampico area, including Laguna de Chairel.

Table 2.

Occurrences of Carcharhinus leucas in North American rivers, lakes, estuaries, and lagoons: Pacific Ocean coast. Abbreviations: WC = water conditions, F = freshwater, B = brackish water up to hypersaline conditions, F/B = salinity gradient from fresh to brackish, LHS = life history stage, Ad = adult, Sub = subadult, Juv = juvenile, Y-O-Y = young-of-the-year, N = neonate, U = unknown.

Additions to Table 2

Additionally, Anislado-Tolentino et al. (2016) reported a C. leucas attack at the mouth of the Pantia River (Guerrero, Mexico), so probably even this river and/or its estuary are utilized by C. leucas. The archaeoichthyological analysis of sediments from an archaeological site on the western Mexican coastal plain at Huatabampo, which is located along the Mayo River, by Guzmán (2008) revealed also remains of C. leucas. Possibly, this river was utilized by C. leucas in ancient times, but it could still be in use. Velázquez-Veláquez et al. (2016) and González-Acosta et al. (2018) reported C. leucas from continental waters and estuaries of the state of Chiapas (Mexico), but without providing any certain localities.

Additions to Table 3

Additionally, for the Atlantic Ocean side of Central America, Boesemann (1964: 10) reported on the occurrence of C. leucas: “Rivers of South America between the La Plata River and the Rio Magdalena.” Subsequently, even Thorson (1970b: 83) reported: “In South America sharks are found in Lake Maracaibo in Venezuela and in a plentitude of east coast rivers from the Magdalena in Colombia to the Rio de la Plata in southern Uruguay.” Furthermore, Thorson (1976a) later reported on the occurrence of sharks (species not identified, but probably C. leucas) from numerous additional rivers in Central America not listed in Table 3, based on reports of fishers, local residents, and ichthyologists. These include: A) Belize: Belize River; B) Guatemala: Motagua River; C) Honduras/Nicaragua: Coco River; D) Nicaragua: Grande de Matagalpa River, Huahuasan River, Escondido River, Indio River; E) Costa Rica: Pacuare River, Matina River. Also Burke (1979), who was referring to Smith (1893), reported the occurrence of sharks in the Escondido River and one of its tributaries, the Rama River. Gunter (1942) listed Carcharias platyodon (= C. leucas) in a list of euryhaline fishes that occur both in fresh and seawater from the east coast of Mexico to the southern limit at Panama. Jones (1985) reported the occurrence of unidentified sharks, in all probability C. leucas, based on reports by local fishers in the Sabun River (Belize). Possibly, occurrences of C. leucas even exist in Guatemala's Polochic and Cahabón rivers, which are tributaries of Lake Izabal (Table 3, No. 3).

Neal et al. (2009) reported C. leucas in a list of primarily marine and estuarine fish species collected in freshwater rivers of Puerto Rico, but without naming a precise locality or a particular river. Van Den Berghe (2015) supposed the presence of sharks in the Punta Gorda River (Nicaragua) in the Caribbean Lowlands of Nicaragua, but was not able to make a verified record. Bussing (1966) listed C. leucas as a component of the freshwater fishes of Costa Rica based on information by reliable observers, but without providing a certain locality. Alpirez (1984) and Angulo (2013) listed C. leucas as a component of the freshwater fish fauna of Costa Rica, also without naming a certain locality. Angulo & Farah-Pérez (2018) named members of the family Carcharhinidae (in all likelihood C. leucas) as migratory fishes in freshwater ecosystems in Costa Rica. Cala (1990), presumably referring to C. leucas, reported the taxon Carcharhinidae for Colombian freshwaters of the Magdalena and Amazon River basins. For the inland waters of the Orinoco River (Venezuela) there are only a few reports in the literature for C. leucas, but, interestingly, there are some cartographic records of C. leucas for this river in some distribution maps like the one provided by Van Der Sleen & Albert (2018). Thus, documented occurrences in the inland waters of this river system are rare. However, Anonymous (2013) reported sporadic captures of C. leucas by fishers from the Orinoco River, Lake Maracaibo, and from purely fresh waters at the mouth of the Catatumbo River, which is tributary to Lake Maracaibo, but these are unconfirmed records. Springer (1950: 6) commented, for C. leucas: “At the mouth of the Orinoco River adults are found in considerable numbers”. Le Bail et al. (2012), Mol (2012), and Mol et al. (2012) did not provide any riverine or estuarine records of C. leucas for Suriname and French Guiana. Mol (2012) did not report occurrences of C. leucas in inland waters of Suriname, but with the common knowledge of intrusions of C. leucas in tropical rivers he emphasized that there were no recorded incidents between sharks and swimmers in the Suriname River.

Table 3.

Occurrences of Carcharhinus leucas in South and Central American rivers, lakes, estuaries, and lagoons: Atlantic Ocean coast, including the Caribbean Sea. Abbreviations: WC = water conditions, F = freshwater, B = brackish water up to hypersaline conditions, F/B = salinity gradient from fresh to brackish, LHS = life history stage, Ad = adult, Sub = subadult, Juv = juvenile, Y-O-Y = young-of-the-year, N = neonate, U = unknown.

No. 5: At the present state of knowledge, the only river in Honduras with a verified freshwater record of C. leucas (Matamoros et al. 2009). The only reliable scientific record for this river was based on a photograph displayed by Strong (1934: 46) of a “fresh-water shark”. Moreover, Strong (1934: 47) reported: “While fishing here, some 180 miles from salt water, we caught a 4-foot fresh-water shark, the first to be recorded from these rivers.” Strong and his expedition team caught this specimen at the confluence of the Patuca River with the Yapowas Creek. Despite the circumstance that only this single record exists for this Central American river, the presence of sharks in the Patuca River seems to be well-known to local fishers, as a study based on interviews by Esselman & Opperman (2010) revealed; fishers also reported rarely captures of C. leucas from the Patuca River.

No. 9: The first observation of freshwater sharks in Lake Nicaragua by Europeans was made in 1535 by Gonzalo Fernández de Oviedo, a Spanish historiographer of the American Indies (Burke 1974, 1979). Interestingly, earlier, in 1526, Oviedo reported sharks from rivers of Central America, unfortunately without naming precise localities, but it appears from the context that he was referring to the mainland around Panama (Jones 1985). The sharks and the sawfishes of Lake Nicaragua were mentioned after the report of Oviedo by several early travelers and writers, but the first scientific treatment of both in a scientific journal appears to be that of Gill & Bransford (1877). Belt (1874: 4, 38) observed large sharks swimming at the outflow of Lake Nicaragua, i.e., the entrance of the San Juan River, and stated: “Beside the alligators, large freshwater sharks appear to be common in the lake.” Herre & Boeseman (1956) critically discussed the ability of sharks to pass the rapids in the upper San Juan River and reasoned that these impediments do not prevent sharks from entering Lake Nicaragua.

Geological studies of the area of Nicaragua by Riedel (1976) amplified the results of Thorson's tagging program, suggesting that the freshwater sharks of Lake Nicaragua must have an Atlantic origin. In the inland waters of Nicaragua, a natural physical barrier prevents the migration of sharks from Lake Nicaragua into Lake Managua due to a 3.7 m high waterfall on the Tipitapa River, a non-stable, periodical outlet only under flooding conditions, which prevents elasmobranchs to move into the lake (Thorson 1976a; Villa 1976). In former times, the entire stretch of the lake was occupied by C. leucas. Specimens of C. leucas, which were tagged by Thorson (1973) in the San Juan/Colorado river system, were recovered at the far end of Lake Nicaragua at the mouth of the Tipitapa River. Further historical records were made from the northwest end of Lake Nicaragua at Los Cocos and Zapatera Island (Thorson 1973; Watson & Thorson 1976), at the greatest known distance to the ocean (∼220 miles = 345 km). Camacho & Gadea (2005) reported catches of C. leucas from San Carlos and the Solentiname Archipelago at the southern end of Lake Nicaragua.

No. 32: There were earlier indications of the presence of C. leucas in the Magdalena River (Colombia) before Dahl (1971) verified C. leucas in this river, namely by Miles (1945, 1947). Miles (1945: 453) stated: “Carcharinus [sic] sp. (?). – Information obtained from fishermen at Calamar would seem to indicate that a species of shark ascends the Magdalena River at least as far as the junction with the Dique Canal, 112 kilometers from the ocean.” Later on, Miles (1947) also mentioned a shark in the fish fauna of the Magdalena River (“Carcharhinus spec.”), but did not provide any distributional or biological data. De Carvalho & Mceachran (2003: 14) also mentioned a shark occurrence for the Magdalena River, but they referred to the old information provided by Miles (1945, 1947: “Carcharhinus spec.”) and delivered no further information. Ramírez & Davenport (2013) reported that sharks, particularly of the genus Carcharhinus and mainly C. leucas, venture into Colombian rivers and that they have been reported to venture into some northern rivers of Colombia (presumably the Atrato, Sinú, and Magdalena Rivers).

No. 34: Remarkably, for Lake Maracaibo, Schultz (1949: 9) commented: “In Lago de Maracaibo, sharks, sawfishes, and large stingrays were reported, but I did not have an opportunity to fish for these. Sharks are caught by fishermen as far south as off the mouth of the Río Santa Ana. The occurrence of sharks in fresh-water lakes with access to the sea is not confined to Lago de Maracaibo. In Lake Nicaragua, Eulamia nicaraguensis occurs in abundance and reaches a large size.” a regional fisheries survey revealed that C. leucas was the only registered shark species in Lake Maracaibo (Tavares & Sánchez 2012).

No. 41: For the Peruvian, Brazilian, and Colombian Amazon, Thorson (1972a) delivered a detailed overview of locations and collectors of C. leucas along the river, and Soto & Nisa-Castro-Neto (1998) gave a detailed review of C. leucas records in Brazil, mainly from the Amazon river system. Thorson (1972a) summarized all records of C. leucas in the Amazon river system until 1972. a detailed map with records of C. leucas in the Amazon Basin and an extensive bibliography of this species in Brazil was delivered by Soto (2001). Reports estimate that eight to 10 sharks per year are caught near Leticia (Colombia) and sold in local markets (Thorson 1972a). Specimens of C. leucas do not appear to occur in large numbers at any point in the Amazon Basin, but they can be looked for occasionally in the Amazon River proper as well as its major tributaries at any place in the lowlands where the water temperature is suitable and the elevation gradient is moderate (Ramírez & Davenport 2013). Even Rosa & Lima (2005) reported that C. leucas occasionally enter freshwaters in the Amazon Basin. Verified occurrences of C. leucas exist as far as Iquitos (Peru) according to Myers (1952), who identified a single specimen by a photograph, and farther upstream from Pucallpa (Peru), at the confluence of the Ucayali and Maraňon rivers in the foothills of the Peruvian Andes, according to Thorson (1972a). Further records of C. leucas downstream the Amazon River were made at Leticia (Colombia), Manaus, Juruti, Santarém, and Belém (Brazil) (Soto & Nisa-Castro-Neto 1998; Soto 2001; Carneiro 2016; Fig. 2A). The specimen of the Colombian record from Leticia (catalog no.: CMNFI 1974-0095.1) was collected by C. G. GRUCHY in 1973 and later determined by the collector in 1974 as C. leucas. There exists photo material of a voucher specimen (adult female, 2.3 m TL, 118 kg) collected from the Solimões River (the upper stretch of the Amazon River above the confluence of this river with the Rio Negro) at Irinduba, nearly 30 km from Manaus, in the collection of the Itajaí Valley Museum of Oceanography, Itajaí, Brazil (catalog no.: MOVI 10179(1), collected by W. Damasceno, det. Soto; Soto & Mincarone 2004). Although specimens of C. leucas are sometimes found at the fish markets of Santarém, they are not utilized as food, but possibly kept there as tourist attractions (Ferreira et al. 1996). Moreover, from a deeper scientific viewpoint, Roberts (1972: 143) summarized: “Specimens of sharks and sawfishes from the Amazon River have yet to be examined by persons competent to identify them.” However, although the presence of C. leucas in the Amazon river systems is well-known today (Thorson 1972a; Campbell et al. 2006), there is a lack of data about the utilization and function of the tropical Brazilian river systems as nursery grounds for C. leucas, and records from the Amazon Basin are scarce (Feitosa & Nunes 2020).

Fig. 2.

Carcharhinus leucas Valenciennes. – A. Subadult male specimen (∼1.5 m TL) captured on 29 November 2016 by local fishers along the Amazon River (Table 3, No. 41) at Pinduri (Santarém, Pará, Brazil) (photo © Jeso Carneiro). The record of the farthest freshwater penetration by C. leucas (5,080 km) was made in the Amazon River. Although C. leucas records in this major river system are a rarity, these occurrences document its repeated use by this species. B. Juvenile specimen (∼70 cm TL) swimming in shallow water on the banks of the Sirena River Estuary, Costa Rica. This estuary functions as a nursery ground for this species (Table 4, No. 7). © Pedro Francisco Navarro Jimenez

For the Amazon Basin, Cavalcanti et al. (2019) listed C. leucas as a member of a group of marine-derived aquatic biota. Perrin et al. (2002) listed C. leucas as a potential predator on the Amazon dolphin (Sotalia fluviatilis Gervais & Deville, 1853). Probably, C. leucas occurs in further large tributaries of the Amazon River in the lowlands of the Amazon Basin that are not interrupted by human impediments and have a connection with the ocean. Rivers with suitable water parameters and a connection to the ocean, situated within the Amazon Basin, with a possible utilization by C. leucas, are the following: Araguaia, Iriri, Curuá, Juruena, Purús, Juruá, Javary, Marañon, Cuminá, Maicuri, Paru, Jari, Caquetá, Putumajo, Japurá, Trombetas, Napo, and Araguari. In future ichthyological investigation on these rivers, C. leucas should be expected or at least considered.

Ferreira (1993) reported that C. leucas is present in the Amazon and Madeira rivers, but without a confirmed presence in the Trombetas River, the study river of that author. Santos & Val (1998) reported C. leucas from Amazonia, but without naming a certain river. For the Rio Negro, one of the largest confluences of the Amazon, Thorson (1972a, 1976a) considered that probably neither sharks nor sawfishes occur there as a result of the dominating water parameters like acidity and hardness, but also as a result of the low productivity of the ionic-rich “black water” of this river, which may exclude the presence of C. leucas there. Maybe the distribution of C. leucas is restricted to the “white water” rivers of the Amazon river system. However, until today, no records of C. leucas for the Rio Negro are known (Beltrão et al. 2019). In contrast to the speculations of Thorson for the absence of C. leucas from black water rivers in Amazonia, Goulding et al. (1988) reported the rich fish life in the Rio Negro and pointed out numerous piscivorous groups from the Amazon Basin that are present within it, so probably there is enough food available to attract sharks and sawfishes.

Table 4.

Occurrences of Carcharhinus leucas in South and Central American rivers, lakes, estuaries, and lagoons: Pacific Ocean coast. Abbreviations: WC = water conditions, F = freshwater, B = brackish water up to hypersaline conditions, F/B = salinity gradient from fresh to brackish, LHS = life history stage, Ad = adult, Sub = subadult, Juv = juvenile, Y-O-Y = young-of-the-year, N = neonate, U = unknown.

Nowadays, intensive hydropower dam-building activities in the Andean Amazon Basin, including the Ucayali River, are endangering its unique freshwater fish fauna and are limiting the migrations of fishes and therefore the connectivity of populations (Anderson et al. 2018), so C. leucas is probably affected by the negative ecological influence of this strong human impact.

No. 46: In the Amazon/Tocantins estuaries, juveniles of C. leucas are of commercial importance for artisanal fisheries. Juvenile C. leucas have been commercially targeted and tons of sharks are landed every year (Castro 2009; Karl et al. 2011). Results of the investigations by Souza-Araujo et al. (2021) suggest that the Amazon River mouth plays an important ecological role as a nursery area for this species in a region that is highly exploited by fisheries.

No. 47: Wosnick et al. (2021) reported C. leucas from São Luís (Maranhão) at the mouth of the Mearim River, which is part of the Brazilian Amazon coast.

Additions to Table 4

Thorson (1976a) reported, for the Pacific side of Central America, the occurrence of sharks (species not identified, but probably C. leucas) from further rivers not mentioned in Table 4, based on reports of fishers, local residents, and ichthyologists. These include Goascoran River (El Salvador/Honduras), Choluteca River (Honduras), and Grande de Térraba River (Costa Rica).

Bussing (1966) listed C. leucas as a component of the freshwater fishes of Costa Rica based on information by reliable observers, but without providing a certain locality. Gilbert et al. (2016) reported, for Costa Rica's Osa region, which includes the Sirena River and its estuary (Table 4, Nos. 6, 7), that estuaries are limited in extent there and under the strong tidal influence, and that although small, these estuaries are highly productive habitats, important as nursery areas and foraging habitats for C. leucas. Cooke & Jiménez (2008) reported that C. leucas travels considerable distances inland in the Tropical Eastern Pacific, including the biogeographical province of Santa Maria, Panama. Lasso et al. (2011b) reported C. leucas from continental waters of the Pacific coast of Colombia, but without naming any precise locality.

Nos. 11, 12: Nowadays, dam buildings prevent the migration of sharks into Lake Bayano, an artificial impoundment founded in the 1970s (Montoya & Thorson 1982). Specimens of C. leucas were trapped in the lake after the dam was built (Montoya & Thorson 1982), with the long-term perspective of extinction of this local population. However, Montoya & Thorson (1982) showed that C. leucas can live in freshwater for extended periods, as they found dead specimens (mature females) four and five years after the closure of the dam wall. Currently, however, it is supposed that there are no longer sharks in Lake Bayano as the entry to this lake is presently closed by a dam wall. The former presence of mature female C. leucas in this lake may indicate that it once was a nursery (Montoya & Thorson 1982). The upper stretch of the Chepo River, the drainage of Lake Bayano, is named Bayano River, and drains into the lake. Thorson (1976a) listed the Bayano River as a locality with sharks, but it is unclear if he was referring to the Chepo River or to the upper reaches of the Bayano River, farther inland.

No. 13: Additionally, Orcés (1959) reported C. azureus (= C. leucas) from Puná Island, at the mouth of the Guayas River.

Additions to Table 5

There is contradictory information regarding the occurrence of C. leucas in some freshwater localities in Benin. In the dataset provided by the GBIF (2018a), there is an entry with a record for the Pendjari River at Porga (Benin, Burkina Faso), which is a tributary to the upper reaches of the Volta River (Ghana). The Volta river system includes Lake Volta (Ghana), which has a dam at the outlet of the lake into the lower Volta River. This impediment offers a barrier, which excludes the migration of anadromous fish species into the upper reaches of the Volta system. Therefore, a migration of C. leucas into the Pendjari River would appear to be impossible and a plausible explanation would be required for this entry, otherwise it should be considered an erroneous database entry.

Table 5.

Occurrences of Carcharhinus leucas in African rivers, lakes, estuaries, and lagoons: Atlantic Ocean coast. Abbreviations: WC = water conditions, F = freshwater, B = brackish water up to hypersaline conditions, F/B = salinity gradient from fresh to brackish, LHS = life history stage, Ad = adult, Sub = subadult, Juv = juvenile, Y-O-Y = young-of-the-year, N = neonate, U = unknown.

For C. leucas in Gabon, Ogandagas (2003: 77) noted: “This species has been captured in the Lambaréné lakes.” This information is vague as the Lambaréné lakes in the Ogooué river system include Lake Zilé, Lake Azingo, and further small lakes like Lake Nkonié and Lake Ouambé, so the exact location of the report by Ogandagas (2003) cannot be localized. However, it is almost certain that C. leucas occurs also in additional lakes associated with the Ogooué river system, which is rich in tributary waters. Additionally, Whitfield (2005a) reported C. leucas in the species inventory of western and central African tropical estuaries.

Nos. 3, 4: At the river mouth of the Saloum River in the Sine-Saloum Estuary, the estuary is divided into numerous small sea arms, so-called “Bolongs”, with ranging and strongly varying water conditions from hypersaline to salty and brackish as an effect of high evaporation and the mix of tidal and fresh water that flows toward the ocean from the river. In this suitable habitat for C. leucas, there is evidence of its presence from a locality named Bolong Bamboung, by Simier (2013), Ecoutin et al. (2014), and Simier et al. (2017).

No. 12: There are reports of a shark attack in the Forcados River at Burutu (Information Nigeria 2012), even though the involved species of shark remains unclear. Furthermore, a photo-documented catch at Port Harcourt in the Niger Delta exists (Nairaland Forum 2017), which very likely illustrates a C. leucas (diagnostic features: small eyes, blunt and rounded snout). It is unclear if these incidents took place in pure freshwater or brackish water, because the Niger Delta is an ecocline between riverine and marine ecosystems.

Table 6.

Occurrences of Carcharhinus leucas in African rivers, lakes, estuaries, and lagoons: Indian Ocean coast, including Madagascar and Réunion Island. Abbreviations: WC = water conditions, F = freshwater, B = brackish water up to hypersaline conditions, F/B = salinity gradient from fresh to brackish, LHS = life history stage, Ad = adult, Sub = subadult, Juv = juvenile, Y-O-Y = young-of-the-year, N = neonate, U = unknown.

No. 15: Loubens (1964: 11) reported occasional catches by fisher nets of unspecified sharks (Carcharhinus sp.) from the Ogooué river basin, at the town of Lambaréné, and the adjacent southern lakes, which he suspected “to be Carcharhinus leucas Müller Henle”. Cutler et al. (2020) considered C. leucas as a species that will be negative affected by habitat loss and limited in its distribution due to future dam development in rivers of the Ogooué basin.

No. 19: Although there exist a few reports of C. leucas for this major African river (Keller 1987; Lamar University 2018), there are neither voucher specimens collected from this river nor photo material that could verifiably confirm the presence of C. leucas in this river. Information on extent of freshwater incursions is missing too. However, a verified record of a juvenile C. leucas collected by I. MARÉE at the mouth of the Congo River at Banana Point in 1953 (catalog no.: MRAC 37417) was investigated and verified as C. leucas by Garrick (1982). Due to the environmental conditions of the Congo River and to the ecological behavior and distribution of C. leucas along the West African coast, the occurrence of C. leucas in this river system can be considered very likely.

Additions to Table 6

On Réunion Island, there are stories of local fishers catching juvenile C. leucas on the east coast at Rivière du Mât (a perennial river), and the presence of individuals in Ravine Blanche (a temporal creek) at St. Pierre on the south coast of the island. Juveniles, subadults, and adults were regularly observed together in marine habitats at Reunion Island off Etang du Gol and off Etang de St. Paul (S. Jaquemet 2018, pers. comm.). On Réunion Island, even small creeks and temporary water-filled ravines are utilized by juvenile C. leucas as breeding areas. Froese & Pauly (2019b) published a picture of a neonate C. leucas that was captured at Baie du Cap, Mauritius, near a small freshwater outlet (Rivière du Cap). Possibly, the small creeks and ravines of Mauritius also function as nursery areas for juvenile C. leucas in the Mascarene Islands.

Kiilu et al. (2019) reported the capture of a 1.5 m TL C. leucas in the vicinity of the Tana River Estuary in Kenya, so possibly this estuary/river system is also utilized by C. leucas as a nursery ground. Carcharhinus leucas is also mentioned by Eccles (1992: 26) for Tanzania, from “large coastal rivers”, but without naming the particular river system; possibly, the Pangani, the Rovuma, and also the Rufiji rivers are meant. There are more reports for the Rovuma River (Tanzania) from local fishers, who have reported that Zambezi sharks (= bull sharks) come way up the river (Holgate 2006). Hughes & Hughes (1992) reported that C. leucas is present in most of the large rivers of Mozambique. There are unconfirmed reports by Murray (2016) of Zambezi sharks from Mozambique's and southeastern Zimbabwe's Save River and Runde River (the latter is a tributary of the first at the Mozambique/Zimbabwe border), approximately 300 km from the ocean but only prevented from migrating farther upriver by the Chivirira and Chitove Falls. Murray (2016) further reported that these sharks are bound to river pools on which the sharks rely and that the process of silting has reduced their depth, with the consequence that sharks are not seen there for years.

Bass (1978) reported that C. leucas has been recorded from most of the river and lake systems of the East African coast from the Zambezi River to Durban Bay. Later on, for South African C. leucas, Bass et al. (1986: 73) stated: “The young often going into rivers, sometimes many kilometers from the sea.” Whitfield (2005a) reported C. leucas in the species inventories of both tropical East African and subtropical South-East African estuaries. Even Skelton (1994) listed C. leucas in a list of fishes associated with southern African estuaries in tropical to warm-temperate climates. Additionally, Perera et al. (2011) and Perera (2013) mentioned C. leucas as a breeding resident that inhabits freshwaters of the Maputaland-Pondoland-Albany region of South Africa. Pienaar (1971) reported C. leucas from freshwater systems of Kruger National Park in northeastern South Africa. Even Compagno et al. (1989) mentioned C. leucas for rivers of the Kruger National Park. Furthermore, Russell (2011) reported C. leucas as a primary marine and estuarine species that occasionally penetrates the freshwater systems of the Kruger National Park as a transient. Van Niekerk & Turpie (2012) presumed that additional river systems in South Africa, not listed in Table 6, may offer suitable habitats for C. leucas, i.e., Gouritz River, Gamtoos River, Sundays River, and Mngazana River. For some rivers of the east South African coast, there are anecdotal reports and observational evidence of shark occurences, presumably of C. leucas, such as Great Kei River, Mtentu River, and other rivers (R. DALY, pers. comm. 2021).

No. 10: The occurrence of C. leucas in the Galana-Sabaki river system probably needs verification, as Seegers et al. (2003: 20) noted, about the evidence of C. leucas in this river system, that “...records of Carcharhinus leucas (Müller & Henle, 1839)… by Okeyo (1998) are unsubstantiated and need confirmation.”

No. 12: Selous (1893) reported the catch of a small-sized freshwater shark of three and a half feet (= 1.07 m TL) near the junction of the Ruenya and the Mazowe rivers, with a detailed description of the specimen but without a species determination or diagnostic features. However, the circumstances of a shark at this location in inland waters far away from the ocean, together with the size, are good arguments that this catch was a juvenile C. leucas. Furthermore, Selous (1893) discussed this catch with a native who told him that he knew this fish well from the Zambezi River at Tete. Selous added that there are no barriers from the ocean to the Lower Ruenya River that could prevent this shark from swimming upriver. Moreover, Selous didn't believe that C. leucas occurs in the Zambezi River above the Victoria Falls, a natural impediment that prevents fishes from swimming upriver (today, the Victoria Falls lie behind two man-made impediments such as the Cabora Bassa Dam wall and the Kariba Dam wall, which prevent migratory fishes from moving up the river).

No. 14: Mepham (1987a) reported C. leucas as common in the Shire Swamps, in the floodplain of the Shire River. There exist anecdotal reports suggesting that C. leucas may have once been present as a marine vagrant in the Elephant Marsh in the floodplain of the Shire River (Tweddle & Willoughby 1979), although there is no evidence that this species has been observed in the lifetime of the current generation of fishers (Turpie et al. 2016). The absence of C. leucas from the Elephant Marsh is most likely due to overfishing or other factors downstream (e.g., barriers), rather than to unsustainable harvesting in the Elephant Marsh itself (Turpie et al. 2016).

No. 15: Peters (1852) described (in Latin) Carcharias (Prionodon) zambezensis from this river. Later, Peters (1868) produced a more detailed description of the species based on a juvenile male specimen caught in 1845 in the Zambezi River at Tete. Peters (1868) underlined that the presence of this species in freshwater was remarkable. Moreover, Peters (1852, 1868) recognized that the collected specimen from this river was closely related to Carcharhinus leucas, which was first described by VALENCIENNES in Müller & Henle (1841) based on specimens collected in the Antilles. Garrick (1982) examined the 760 mm TL specimen collected by Peters from the Zambezi River and determined that it fully agrees with C. leucas (Fig. 1). Barnard (1925), presumably referring to Peters (1852), reported C. leucas under the name C. zambesensis in a monograph of the marine fishes of South Africa, also from Tete on the Zambezi River. Interestingly, Barnard (1925: 25) named it “River Shark” and gave further information of the size of this species as up to 760 mm TL, which indicates that Barnard was in all likelihood referring to the previous record by Peters. Current scientific investigations and records of C. leucas in the Zambezi River are scarce, and most reports refer to old records.

There is contrasting information regarding the reach of freshwater incursions by C. leucas in the Zambezi River, especially in historical times before regulation of the river. There are reports of C. leucas traveling distances of 1,000 km and more up the Zambezi by Bass (1978) and Daget (1984), and 1,120 km by Bass et al. (1973). These authors were presumably referring to reports of the species at Chirundu (Zambia). D'Aubrey (1964: 39) reported, for C. leucas in southern Africa, that “Small specimens have been caught over 300 miles [= 482 km] from the sea in the Zambezi River.” Bell-Cross & Minshull (1988) reported that prior to the building of the Cabora (Cahora) Bassa Dam (= Cabora Bassa Gorge), C. leucas occurred up the Zambezi River at least as far as the Kariba Gorge. The authors mentioned that C. leucas had been caught at Chirundu (Bell-Cross & Minshull 1988), beyond the Cabora Bassa Gorge before it was finished, more than 1,000 km from the ocean, but this record was not listed in earlier publications about the freshwater fish fauna of southern Africa (Jackson 1961; Jubb 1961, 1967). Nowadays, in the Zambezi River, the Cabora Bassa Dam wall and the Kariba Dam wall prevent C. leucas from migrating in the upper reaches of the river. The closure of the Kariba Dam in 1959 on the middle Zambezi and of the Cabora Bassa Dam in 1974 allows migratory fishes to travel only approximately 640 km upriver in the Lower Zambezi only. This was confirmed by Hughes & Hughes (1992), who reported that C. leucas penetrates the Zambezi River as far as Cabora Bassa. However, C. leucas may never have penetrated the Zambezi River beyond the Cabora Bassa Gorge sinces its completion (Marshall 2000).

The Cabora Bassa Gorge is conventionally regarded as a boundary for migrating fish species, particularly primary marine species like C. leucas, which may occur inland as far as the gorge but not beyond it (Marshall 2000; The World Bank 2010). Probably, migrations of C. leucas up the Zambezi River extended farther in historical times than today. Marshall (2000: 471) further noted, for C. leucas in the Zambezi River, that “Several recent sightings ranging from the mouth of the Micelo River to up stream of Morromeu were reported to me during the expedition. Although not positively identified as C. leucas (Zambezi or bull shark) this is the most likely species to enter estuarine and riverine environments.” About occurrences of C. leucas in the Zambezi River in the recent past, Timberlake (2000: 14) noted: “The lungfish, eels and Zambezi shark are all found only in the Lower and Middle Zambezi.” Furthermore, Coetzer (2017) provided photo material of a juvenile C. leucas captured at Tete in 2010, which is evidence that C. leucas still reaches as far up as the Lower Zambezi River. Jackson (1986) listed the family Carcharhinidae in his work dealing with the fish fauna of the Zambezi River; although he did not explicitly mention C. leucas, he was presumably referring to this species.

No. 18: For the Limpopo River, a number of shark attacks on swimmers and canoes have been reported at locations far inland and at considerable distances from the ocean, which can be attributed to C. leucas. Even when a species determination was not mentioned, it is very likely that specimens of C. leucas were involved. The Shark Research Institute (2018d) reported three incidents in 1970 (all on the same day!) at Gijana, 150 km inland, and one incident in 1961 at an undefined location at approximately 190–240 km from the ocean. In 1963 a shark, presumably C. leucas, bit a canoe and several other sharks bumped two other canoes at a location approximately 200 km from the ocean (Davies 1964).

No. 25: This estuarine lake system includes hypersaline (salinities of > 50‰, induced by drought) and brackish water conditions near the mouth/drainage into the Indian Ocean and freshwater conditions in regions far away from the ocean (Bass et al. 1973). Carcharhinus leucas has been regularly netted in this lake system at salinities up to 47‰ (Bass et al. 1973). Even Whitfield (1996) reported for the St. Lucia lake system of South Africa that specimens of C. leucas were regularly netted at salinities up to 47‰. Bass (1978) reported that sharks captured in the lake during times with salinities above 50‰ were in noticeably poor condition, even though food was not scarce. In African rivers and lakes, as opportunistic feeders, the food spectrum of large C. leucas may include young hippopotamuses. There are only a few reports of encounters of bull sharks with hippopotamuses. Green (2018) reported a rare encounter of a C. leucas with hippopotamuses in the iSimangaliso Wetland Park (KwaZulu-Natal; former Greater St. Lucia Wetland Park), which is a big complex of wetlands, swamps, and lakes that are connected to Lake St. Lucia in South Africa. Filming material from an encounter between a single C. leucas and a group of hippopotamuses exists on the internet (Internet Reference 3). Otherwise, there is little information on shark/hippopotamus interactions. In contrast, in Lake St. Lucia, pups of C. leucas are prey of another apex predator, the large Nile crocodile (Crocodylus niloticus Laurenti, 1768), which also occurs in the lake (Whitfield & Blaber 1979; Perissinotto et al. 2013; Daly et al. 2021).

The “Global Shark Attack File” (Shark Research Institute 2018a) included a couple of shark attacks that occurred in South African freshwater rivers not mentioned in Table 6. Some of the attacks happened not only in the estuaries but also inland, so specimens of C. leucas were probably involved in these incidents. For completeness, the rivers are named here: Bilanhlolo, Little Brak, Groot, MaKakatana, Umgeni, Kowie, Riet, and the Umlaas Canal.

Additions to Table 7

Possibly, C. leucas also occurs in the Indus River (Pakistan) and the Brahmaputra River (Bangladesh), two major rivers in Asia located within the coastal range of C. leucas, but this needs verification. Belcher (2003, 2018) discovered teeth of C. leucas in an ancient settlement in Pakistan's Indus River Valley at Bakalot dated ∼3000–1700BC, which could be an archaeological indication of the utilization of C. leucas specimens from the Indus River as a nursery area and/or as a foraging habitat. Barreiros & Gadig (2011) and Moazzam & Osmany (2021) mentioned C. leucas for the Indus River and its estuary, but the source of these records remains unclear. Sajid (1962), and subsequently Mirza (1975), reported Pristis microdon Latham, 1794 (the largetooth sawfish) as the only freshwater elasmobranch species from the Indus River near Hyderabad, at about 293 km from the ocean. Considering this record of a further euryhaline elasmobranch in this river and the fact that the Indus Delta is located inside the marine and coastal range of C. leucas, its past or present occurrence in this river is quite imaginable.

Day (1878) reported that he caught a specimen of “Carcharias gangeticus” at Cuttack along India's Mahanadi River, but it remains unclear if this catch was Glyphis gangeticus or C. leucas. Mohapatra et al. (1954) reported Carcharhinus gangeticus from the Mahanadi River 60 miles (= 97 km) upstream, at the Zobra Barrage. This is here considered unusable information, as both C. leucas and G. gangeticus probably occur in this river and Carcharhinus gangeticus is an early name that was used for both taxa (see Methods). Mohapatra et al. (1954) gave no further information allowing a clear identification, nor did they deposit a voucher specimen in a scientific collection. Thus, the true identity of the sharks reported from the Mahanadi River by Mohapatra et al. (1954) needs clarifying. Until today, there are no confirmed reports of C. leucas from the Mahanadi River, but a presence cannot be excluded due to its location inside the coastal range of C. leucas and the preference of this species for low salinity habitats.

The Shark Research Institute (2018b) also reported shark incidents at the mouth of the Devi River (an outlet of the Mahanadi River) and in the “Cochin River” (which is quite imprecise because the town of Cochin includes numerous river outlets, canals, and small river systems); although these reports do not include remarks on the involved species, they may be an indication of the use of these freshwater habitats by C. leucas.

Table 7.

Occurrences of Carcharhinus leucas in Asian rivers, lakes, estuaries, and lagoons: Indian Ocean coast incl. Persian Gulf and Pacific Ocean coast. Abbreviations: WC = water conditions, F = freshwater, B = brackish water up to hypersaline conditions, F/B = salinity gradient from fresh to brackish, LHS = life history stage, Ad = adult, Sub = subadult, Juv = juvenile, Y-O-Y = young-of-the-year, N = neonate, U = unknown.

In the Asian region and especially in India, the name Carcharhinus gangeticus was presumably used for records of C. leucas at least until the mid 1980s (see Methods). For the inland waters of the Philippines, Herre (1958: 88) reported, for Carcharias gangeticus: “It enters all the rivers of Mindanao except those too small or too steep, and ascends the Agusan to Monkayo and beyond.” Presumably, Herre was also referring to C. leucas.

There is only semi-reliable information from the Mekong River (= Mae Khong River) (Cambodia, Viet Nam, Thailand), by Fernicola (2016); the presence of this species in this river needs clarifying and investigating further. Carcharhinus leucas is also mentioned for this river in the checklist by Rainboth (1996: 51): “Expected, but not yet recorded from the Mekong.” Later, Rainboth et al. (2012) presented a photograph of a juvenile C. leucas (940 mm TL) from a fish market of Kien Giang Province in the Mekong Delta, with the statement that the photo is cited as evidence that this species occurs in the Mekong. Rainboth et al. (2012) mentioned that this species had been sighted by the main author in Mekong Delta markets, but it remains unclear whether the specimens were taken from marine, estuarine, or riverine habitats. Poulsen et al. (2004) listed C. leucas in a list of Mekong River fishes, but they didn't provide data allowing validation of this record. The bull shark was also listed in the checklist of freshwater fishes of Viet Nam by Froese & Pauly (2018a). The occurrence of C. leucas in the Mekong river system seems very likely, as this major river system provides suitable habitat conditions for the species; however, there are no precise records or locations for C. leucas from within the Mekong system. Vidthayanon (2002) reported that C. leucas has never been seen in Thai rivers, but that either Glyphis cf. gangeticus or C. leucas were anecdotally reported by the Karen people along the Salween River of the Tak-Mae Hong-son Province, northern Thailand. Vidthayanon & Premcharoen (2002) reported nine species of elasmobranchs from the middle reaches of Thailand rivers, but without information on these species.

Carcharhinus leucas was mentioned by Kottelat (1989) for the inland waters of Indochina, Southeast Asia (Laos, Cambodia, Viet Nam, Thailand, Myanmar), but without naming any precise locality or river system. Later, Kottelat (2013) summarized eight records of C. leucas from inland waters in Southeast Asia in a literature review. White et al. (2006) reported that C. leucas occurs in Indonesian freshwaters, but without naming a particular river. Parenti & Lim (2005) expected sharks of the family Carcharhinidae for the rivers of the Rajang Basin, Sarawak, Borneo (Malaysia). The Department of Fisheries Malaysia (2006) gave the information that C. leucas is found in the rivers of Sabah, Borneo. Possibly, C. leucas also inhabits the Yangtze River (China), as Garrick (1982) examined a single juvenile specimen (♂, 729 mm TL; catalog no.: BMNH 74.1.16.63) collected from Shanghai, China, which is situated on the estuary of this major Chinese river.

Nos. 1, 2, 3, 9: Verified occurrences of C. leucas exist, at least in historical times, from north of Baghdad, 850 km from the sea. For the waters of Iraq, Kennedy (1937: 746) reported: “Sharks are not frequent visitors so high up the Tigris as Baghdad, but isolated ones are heard of every year. In the river at Basrah they are more common.” Coad (2010) and Moore (2018) delivered detailed synopses of freshwater occurrences of C. leucas and localities of shark attacks for Iraq in the Tigris/Euphrat and Shatt Al-Arab systems. Furthermore, Moore (2018) reported unconfirmed, anecdotal records of juvenile C. leucas from the Iraqi Marshes, from north of Ahwaz on the Karun River, and from Abadan on the Shatt Al-Arab River. In the Middle East, the occurrence of sharks in the Tigris/Euphrat system is well-known since antiquity (Moore & Mcdavitt 2009). Already in the early historical work “The Wonders of Creation” by Qazvini (1263), the author reported sharks as powerful and dangerous fishes that were known from Basrah. The work of Qazvini may represent one of the earliest distributional records of C. leucas in the Euphrat-Tigris-Shatt Al-Arab system (Moradi 2017).

More modern reports of sharks in the Mesopotamian rivers are mainly focused on attacks that occurred inland, far from the coast (e.g., Hunt 1951; Thesiger 1964). Carcharhinus leucas frequently enters numerous rivers, canals, and creeks in the Tigris/Euphrat Basin of Iran/Iraq, where attacks have also been continuously reported (Armantrout 1980; Coad & Papahn 1988; Coad & Al-Hassan 1989; Coad 2015). In the Tigris/Shatt Al-Arab system and the Karun River (Iran), there are freshwater reports of sharks under different names, such as Carcharias gangeticus, C. lamia, and C. menisorrah (Günther 1874; Kennedy 1937; Khalaf 1961; Mahdi 1962). Even when the specific identity of these large sharks is disputed (Coad 1979), their occurrence in inland waters, far from the coast at Ahwaz (Iran) and farther inland than Baghdad (Iraq) exclude other carcharhinids and leave the euryhaline C. leucas as the most plausible species. Jawad (2012) critically discussed the validity of shark reports by numerous authors from the inland waters of Iraq and assigned Günther's (1874) Carcharias gangeticus and Kennedy's (1937) Carcharias lamia to Carcharhinus leucas. Coad (2010) pointed out that studies on carcharhinid sharks and museum specimens indicate that only C. leucas occurs in freshwaters of the Tigris/Euphrat Basin. Moreover, Coad (2010) stated that C. leucas was the only shark species commonly encountered in inland Iraqi freshwaters in the past. However, the influence of the tide in the Shatt Al-Arab River (200 km in total length) is felt about 140 km inland, with penetration of marine organisms upstream (Rzoska 1980). Besides C. leucas, further carcharhinids were reported from the Shatt Al-Arab River. Mohamed & Abood (2017) also reported Rhizoprionodon acutus Rüppel, 1837 (milk shark) from the Shatt Al-Arab River. This is a representative of a genus whose members utilize low salinity habitats and that has been reported multiple times from estuaries, river mouths, and the lower parts of certain rivers worldwide (Compagno 1984). Nevertheless, these small members of the family Carcharhinidae are not known for attacks on humans or for penetrating rivers for great distances (Compagno 1984). Since human impediments in the Tigris River prevent sharks from migrating upriver, reports of sharks from or north of above Baghdad have declined. Young (1977) reported that local people spotted sharks at Baghdad frequently but only on rare occasions. According to Coad (2010), C. leucas occurred regularly as far upriver as Baghdad before river regulation and building of barrages and dams took place. Jouladeh-Roudbar et al. (2020) reported that since the construction of various dams on the Tigris and Karun rivers, C. leucas is found primarily in the Shatt Al-Arab River estuary (= Arvand River estuary). Freyhof et al. (2021) reported, also for the Euphrat and Tigris rivers, that nowadays dam construction terminates the migrations of fishes that started their migrations upriver from the ocean, such as long-distance migrating species like C. leucas. For freshwaters of Iraq and particularly the Tigris River, Freyhof et al. (2021) mentioned that bull sharks once traveled up to Baghdad, but that they nowadays only reach as far as Basrah on the Shatt Al-Arab due to dams.

Moore (2018) outlines the Tigris/Euphrat river system as an important nursery area for C. leucas due to its rank as one of the few and largest freshwater inflows in the Persian Gulf. In this context, Jawad (2021) underlined the important role of Iraq's southern marshes for threatened species such as C. leucas. Young (1977) recorded reports by the native people of Iraq that small sharks use the Iraqi marshes during flooding. Al-Daham (1982) postulated that sharks regularly ascend the Shatt Al-Arab River, also reaching the southern marshes. Garstecki & Amr (2011) reported C. leucas from the freshwaters of the Hammar marsh (Iraq). The Mesopotamian marshlands (Hammar marsh, Chybayisch marsh, Hawizeh marsh), part of the Tigris-Euphrates Basin, and the numerous irrigation canals included there offer suitable habitats for C. leucas. Coad (2010) reported the occurrence of C. leucas from Hammar marsh in the Mesopotamian marshlands. The Al-Ahwar marshland (East Hammar marsh, West Hammar marsh, Huweizah marsh) in southern Iraq, which is irrigated by discharges of branches of the Tigris/Euphrat river system, was mentioned as a critical habitat for C. leucas by Al-Lami et al. (2014), and the authors highlighted C. leucas as a key locally migrating species for this region. With regard to the high importance of the Tigris/Euphrat river system as a nursery area for Persian Gulf C. leucas, Esmaeili et al. (2010b) reported that dam construction, pollution, drought, overfishing, and habitat destruction are the main threats to the ichthyofauna of the Tigris River Basin. Therefore, conservation efforts in this region are highly demanded.

For Iranian waters, Armantrout (1980) reported “Carcharias lamia” for the Tigris River and “Carcharias gangeticus” for the Tigris and the Karun rivers, but he was referring to older literature (Gunther 1874; Kennedy 1937) and these are undoubtedly records of C. leucas. Armantrout (1980) reported shark attacks in the Karun River near Dezful, which is puzzling as this locality is on the Dez River, a confluent of the Karun. Esmaeili et al. (2010a) gave the information that C. leucas occurs in the Tigris River Basin, which include, besides the Tigris River, also the Karun River. Carcharhinus leucas probably also occurs in the Dez and Gargar rivers, two side branches of the Karun River. Aberoumand (2010, 2011) reported that he obtained fresh skin of C. leucas for pharmaceutical investigations from a local fish market in Ahwaz, Iran, which is located on the Karun River at 275 km from the Persian Gulf. Owfi (2015) reported C. leucas for Chuzestan (Iran) and the Karun River Basin. Coad (1999) mentioned that C. leucas occurs in rivers of the Iranian province of Chuzestan, up to 420 km from the coast, which presumably refers to records of C. leucas in the Karun River, from Shushtar.

No. 10: This toponym is quite imprecise because there are numerous rivers in Mumbai (the former Bombay), like for example the Dahisar, Mithi, Chorna, Oshiwara, Poisar, Tansa, and Tasso rivers. Day (1878) reported the collecting of a juvenile (18 inches = 45.72 cm TL) of Carcharhinus gangeticus in Bombay, which may indicate an occurrence of C. leucas in this region, although the size of this juvenile specimen seems to be very small for a newborn C. leucas, thus his record possibly belongs to Glyphis gangeticus.

No. 15: Chaudhuri (1916) reported a catch of a juvenile (747 mm TL) of “Carcharinus gangeticus” [sic] in the Chilika Lagoon. Carcharhinus gangeticus was mentioned for this lagoon also by Jones & Sujansingani (1954) and Misra (1962), and more recently by Rao & Shibananda Rath (2014). It is unknown whether both C. leucas and G. gangeticus occur together in this lagoon or if these literature records represent misidentifications of C. leucas (see Methods). The description by Menon (1961: 68) of Carcharhinus gangeticus from the Chilika Lagoon seem to agree with the diagnostic features of Glyphis gangeticus, and therefore a co-occurrence of both C. leucas and G. gangeticus in the Chilika Lagoon cannot be excluded.

No. 18: Hamilton (1822: 3) commented: “In the mouths of the Ganges sharks are exceedingly numerous, and occasionally, but rarely, come up as far as Calcutta.” Blyth (1860) reported an examined specimen of “Squalus (Carcharinus) gangeticus” [sic] from the fish market of Calcutta, but this specimen was probably Glyphis gangeticus and the precise location of this catch remains unclear. The verified record of C. leucas from the Hooghly River is based on a ♂ 650 mm TL fetus or newborn (catalog no.: ZSI 10250) collected in April 1867 by J. Anderson and misidentified by the collector as “Squalus gangeticus” (Compagno 1984; Talwar 1991). Day (1878: 710), reporting on sharks in Indian rivers, wrote: “The most savage species appear to be the ground sharks of the rivers, as Carcharias Gangeticus, which seldom loses an opportunity of attacking the bather.” Day (1878: 715) further remarked: “This is one of the most ferocious of Indian sharks, and frequently attacks bathers even in the Hooghly at Calcutta, where it is so dreaded that a reward is offered for each that is captured. I have taken it at Cuttack.” This description of a ferocious character does not agree with Glyphis gangeticus, which feeds primarily on fish (Compagno 1984) and doesn't normally attack humans; therefore, Day (1878) was likely referring to C. leucas. Even the description by Day (1878: 711): “Snout obtuse. Teeth in both jaws serrated. Seas of India to Japan; it ascends rivers.” seems to be more suitable for C. leucas than G. gangeticus. Mcculloch (1922: 5) reported Carcharias gangeticus from the Australian waters of New South Wales with the comment: “A ferocious species in Indian estuaries.”, which probably also refers to C. leucas.

No. 19: Hamilton (1822: 4) reported sharks in the Ganges River and distinguished different species of shark (“merely sharks” and “ground sharks”) occurring in the river; however, it is not possible to make a clear identification of the recorded species, even though it is almost certain that the information refers to C. leucas and/or G. gangeticus. To bring clarification into the distribution of Australian sharks, Whitley (1940: 105) reported, for “Platyodon gangeticus”: “This shark, which is much feared in the River Ganges, India, has been recorded doubtfully from North-western Australia, New South Wales and South Australia.”, a description that likely refers to C. leucas. Venkateswarlu & Menon (1979) reported Carcharhinus gangeticus in a taxonomic checklist of the fish fauna of the Ganges River and its branches, but the authors were just referring to the old reports of Hamilton (1822) and Day (1878). The publication by Roberts (2007), which had the aim of clarifying the distribution of Glyphis gangeticus as the “Gangetic freshwater shark” of India and Bangladesh was not very helpful at all, as the author was not able to distinguish Carcharhinus leucas from Glyphis gangeticus. The photographs that were presented by Roberts (2007: 269) of “Glyphis gangeticus” specimens, which were obtained from Sittway markets and were caught in the marine waters of the Bay of Bengal, are undoubtedly C. leucas (the height of the second dorsal fin in sharks of the genus Glyphis Agassiz, 1843 is about three quarters of the height of the first dorsal fin, whereas it is less than three quarters of the height of the first dorsal fin in C. leucas). Already Compagno et al. (2010) pointed out that images of juvenile specimens of G. gangeticus in Roberts (2007) were misidentifications of C. leucas.

Nevertheless, the recent status of C. leucas in the Ganges river system remains uncertain, as since the early records by Günther (1870) and Day (1878), no specimens of C. leucas were collected and no further reports regarding the occurrence of C. leucas in this river system were noted. Compagno (1984) reported in the middle of the 1980s that although sharks are currently caught in the Ganges system, it is not known how common the true Ganges shark (Glyphus gangeticus) is relative to C. leucas. It can be estimated that both taxa are nowadays rare in Indian rivers due to high fishing pressure and to the intensive pollution of India's inland waters. However, Mitra (2014: 20) reported about the distribution of C. leucas from the mouth of the Ganges, the Sunderbans, and the adjacent Bay of Bengal, as follows: “Entire stretch of Indian Sundarbans and aquatic phase of Bay of Bengal. Throughout the year.”

There are some reports from pearl fisheries in the Gulf of Mannar on the east coast of the Indian subcontinent about the risk of a shark attack while harvesting, from species such as Galeocerdo cuvier Péron & Lesueur, 1822 (tiger shark) and presumably C. leucas. James (1973: 493) reported: “….danger from ferocious sharks like C. gangeticus and the tiger shark during pearl fisheries operations in the Gulf of Mannar.” Furthermore, James (1973: 488) stated: “Ascends rivers even beyond tidal influence. Known to be one of the most ferocious sharks.” There are numerous reports listed in the “Global Shark Attack File” (Shark Research Institute 2018b) of shark attacks on bathers and pilgrims along the Hooghly River at Calcutta, Dakshineshwar, Barrackpore, and Chitpur and along the Ganges River, especially from the end of the 19th century. Even if the involved species cannot be clearly identified and considering the sympatric occurrence with G. gangeticus in Indian rivers, these historical attacks can likely be attributed to the opportunistic and more powerful C. leucas (Habegger et al. 2012).

No. 20: Carcharhinus leucas and other elasmobranchs were not included in the fish checklist of the Perak River by Hashim et al. (2012). Evidence of further carcharhinids besides C. leucas was provided for this river. Smith (1931) made investigations on four freshwater elasmobranchs in the Perak River, which also included Carcharhinus melanopterus Quoy & Gaimard, 1824 (blacktip reef shark). Besides C. leucas and C. melanopterus, one further freshwater tolerating elasmobranch Scoliodon lati-caudus Müller & Henle, 1838 (spadenose shark), was recorded for this river, from the pure freshwaters at Telok Anson, 70 km upstream from the coast (Teshima et al. 1978).

Table 8.

Occurrences of Carcharhinus leucas in Australian rivers, lakes, estuaries, and lagoons: Indian Ocean coast. Abbreviations: WC = water conditions, F = freshwater, B = brackish water up to hypersaline conditions, F/B = salinity gradient from fresh to brackish, LHS = life history stage, Ad = adult, Sub = subadult, Juv = juvenile, Y-O-Y = young-of-the-year, N = neonate, U = unknown. Abbrevations of Australian Territories: N.T. = Northern Territory; QLD = Queensland; W.A. = Western Australia.

No. 29: A single juvenile specimen of C. leucas was taken by a villager in the Kinabatangan River close to the Malbumi Estate in 2010, which is approximately 40 km from the river mouth (Min 2013). This river location is also illustrated in a distribution map of C. leucas for Borneo provided by Last et al. (2010).

No. 32: There are existing early reports about the occurrence of sharks in Laguna de Bay, but without identification of the involved species. De La Gironière (1855: 102) narrated: “Deux poissons de mer se sont acclimatés dans le eaux douces du lac: le requin et la scie. Le premier est heureusement assez rare, mais le second est très-abondant [“Two sea fishes have acclimated to the freshwater of the lake: the shark and the sawfish. Fortunately, the first is quite rare, but the second is very common.”]. Later, Wood (1875) reported on the occurrence of sawfishes (genus Pristis Latham [J. F.], 1794) and small sharks from Laguna de Bay, located near the city of Manila. Harting (1876: 62), who was referring to Wood (1875), reported in a short account the occurrence of sharks in the freshwater of Laguna de Bay: “…zaagvisschen (Pristis) en nog een andere soort van kleine haaien.” [“…sawfish (Pristis) and a further sort of small shark.”]. These small sharks, which were observed by Wood (1875) and subsequently by Harting (1876), were in all likelihood juvenile specimens of C. leucas, even though the authors made no species determination.

Nos. 39, 40: Papa & Mamaril (2011) reported that sharks were already observed in Lake Taal (Philippines) by Spanish and American explorers in the late 16th Century. Hargrove & Medina (1988) reported that in 1754 the waters of Lake Taal threw up dead alligators and fish, including sharks. Maybe this event was the result of volcanic activity in and around the lake. Today, it is unclear if C. leucas still occurs in Lake Taal and its drainage, the Pansipit River. Fishery management of the lake was unsustainable and combined with overexploitation since the 1930s, which has led to the extirpation of sharks in the lake (Hargrove & Medina 1988); it remains unclear if there is still a local population of C. leucas there.

Additions to Table 8

Munro (1961: 20) mentioned C. leucas under the common name “Swan River whaler shark” for fresh and brackish water habitats of Western Australia: “Bays and rivers, W.A.”. Bishop et al. (1990) reported C. leucas for freshwaters of the Alligator Rivers Region (Northern Territory). Herbert et al. (1995) sampled the freshwater ichthyofauna of the Cape York Peninsula during the period 1992–1993 and mentioned that local residents reported occurrences of sharks in the Edward River (Queensland), Coleman River (Queensland) at Blazeaway Hole, and from King Junction Hole on the Palmer River, which is a tributary of the Mitchell River (Table 8, No. 27), nearly 300 km from the ocean.

There were unconfirmed reports of C. leucas from the Jim Jim Creek drainage, which is a tributary of the South Alligator River, and the Yellow Water Billabong, a side-water of the South Alligator River in Kakadu National Park (Bishop et al. 2001). For the Northern Territory of Australia, and especially for the Kakadu National Park, Thorburn et al. (2004a) and Larson et al. (2013) gave a detailed overview and extended listing of freshwater records of C. leucas, based on a literature review. Carcharhinus leucas is an inhabitant of the wetlands of the Kakadu Region, northern Australia, where it occurs in estuaries and enters nontidal waters (Larson 1999; Finlayson et al. 2006). Walden & Pidgeon (1998) also mentioned C. leucas as a marine species in freshwaters of the Kakadu National Park. Kyne (2014) reported, for Kakadu National Park, that juvenile C. leucas were abundant in both the South and East Alligator rivers (and probably elsewhere in Kakadu National Park). Morgan et al. (2014) listed C. leucas in an overview of freshwater fishes of Western Australia, with occurrences in the Southwestern and Pilbara Province and the Kimberley Region. Furthermore, Morgan et al. (2014) mentioned several reports of C. leucas from the estuaries of the Swan, Canning, Blackwood, and Collie rivers, but none appear to make the transition into freshwaters, possibly due to the seasonality of parturition in this species, which doesn't agree with the defined high flow regimes of the rivers resulting from the Mediterranean climate of the Southwestern Province.

No. 3: Whitley (1940: 101) reported, for the Swan River: “There has been some discussion concerning the identity of the small shark common in the Swan River near Perth. Stead calls it the whaler.” Presumably, he was referring to juvenile C. leucas, and the Swan River is probably an important nursery area for the species.

No. 8: Although the survey of Allen (1975) dealt with the freshwater fish fauna of the Prince Regent River Reserve, Allen's report of C. leucas from the location of his catches at King Cascade indicate tidal influence causes brackish water conditions in this part of the lower reaches of the Prince Regent River.

No. 10 & 11: In the regulated Ord river system, dams now prevent marine vagrant fishes from moving farther upstream. Traveling of sharks is now limited by the dams to about one-quarter of the former range (Storey & Trayler 2007), and the distribution of C. leucas in this river system is restricted to the Lower Ord River. The dam wall of Lake Kununurra (Kununurra Diversion Dam) provides an insurmountable barrier to the movements of C. leucas in the Ord system (Gill et al. 2006), as does the second dam wall of Lake Argyle (Ord River Dam). Buckle et al. (2010) speculated that amphidromous fish species like C. leucas would have traveled farther upstream, but that they are now limited by the dams. Although there are no data to demonstrate that the fauna of Lake Kununurra and Lake Argyle has changed since the building of the dams in 1990, C. leucas is now likely to have disappeared from these systems as a result of the river regulations (Hale & Morgan 2010).

Table 9.

Occurrences of Carcharhinus leucas in Australian rivers, lakes, estuaries, and lagoons: Pacific Ocean coast. Abbreviations: WC = water conditions, F = freshwater, B = brackish water up to hypersaline conditions, F/B = salinity gradient from fresh to brackish, LHS = life history stage, Ad = adult, Sub = subadult, Juv = juvenile, Y-O-Y = young-of-the-year, N = neonate, U = unknown. Abbrevations of Australian Territories: NSW = New South Wales; QLD = Queensland.

No. 13: Based on indigenous sources, there have been reports of bull sharks from the Daly river system (Northern Territory) far upstream from the King River, which is a tributary of the Katherine River and secondary to the Daly River, which in turn drains into the Timor Sea (Jackson et al. 2014).

Additions to Table 9

The analysis of shark catches in northeastern Australia by Harry et al. (2011) impressively showed the preference of the euryhaline C. leucas for riverine environments, as C. leucas was the only species of shark regularly captured in river zones. Herbert & Peeters (1995) stated that C. leucas is distributed throughout all coastal rivers of the Cape York Peninsula and is known to penetrate great distances into freshwater. Last (2002) gave information that small specimens of C. leucas less than 1 m TL have reportedly nipped at the ankles of swimmers more than 100 km up rivers of Cape York, northern Queensland. Gehrke (1997) reported records of C. leucas from unregulated lowland river sites on the north coast of New South Wales, but without naming any precise locations.

The “Global Shark Attack File” (Shark Research Institute 2018c) included a couple of attacks having occurred in Australian rivers and lakes that are not mentioned in Tables 8 and 9. Unfortunately, the involved species could not be identified in these cases. Some of the attacks happened far inland and far up rivers and an associated lake distant from the sea, so the incidents were probably caused by specimens of C. leucas. For completeness, these rivers and lake are named here: Tweed River, Cataract River, Maria River (Port Macquarie), and Watson Taylors Lake. Even Whitley (1940) reported shark attacks and shark incidents in Australian rivers from the Sydney area of New South Wales, which presumably can be attributed to C. leucas; these occurred in the Lane Cove River, a side river of the Parramatta River (Tab. 9, No. 42), and in Cabramatta Creek, which is a tributary of the Georges River (Table 9, No. 44).

Table 10.

Occurrences of Carcharhinus leucas in Melanesian Island rivers, lakes, estuaries, and lagoons: Indian Ocean coast, including the Timor Sea and Arafura Sea coasts and the South Pacific Ocean coast. Abbreviations: WC = water conditions, F = freshwater, B = brackish water up to hypersaline conditions, F/B = salinity gradient from fresh to brackish, LHS = life history stage, Ad = adult, Sub = subadult, Juv = juvenile, Y-O-Y = young-of-the-year, N = neonate, U = unknown.

No. 5: The distribution of C. leucas in the Burdekin River is nowadays limited to a maximum distance of freshwater penetration of approximately 150 km by the impediment of the Burdekin Falls Dam, which prevents marine vagrants from moving up the river (Pusey et al. 2003).

No. 17: Whitley (1940) reported, from the Brisbane River, a shark attack on the Queensland State Champion outrigger. The Brisbane River is a documented nursery area for C. leucas (Last & Stevens 1994), and Pillans (2006) reported juvenile C. leucas in high numbers in this river.

Additions to Table 10

For completeness, Allen (1996), presumably referring to Lake Jamoer (Table 10, No. 5), included C. leucas in a list of freshwater fishes that occur in coastal streams of Irian Jaya (= West Papua, Indonesian New Guinea). Gehrke et al. (2011) reported the harvesting of C. leucas in lowland rivers and estuaries in Papua New Guinea, but without naming a precise locality.

For New Caledonia, there are further notes of C. leucas in inland waters (freshwater) by Marquet et al. (1997), but without a precise locality. Acoustic tagging of C. leucas in the southern province of New Caledonia by Werry et al. (2010) revealed migrations of C. leucas into Prony Bay, in which some small creeks (Rivière Bleue, Ruisseau de la Bergerie) drain. Possibly, this is a further breeding place for C. leucas in New Caledonia.

The first mention of occurrences of C. leucas in freshwaters of Fiji was by Günther (1870: 368), who described a 30 inch (= 76.2 cm TL) juvenile, but with imprecise locality data: “In fresh waters of the island of Viti-Levu.” Günther (1870: 368) examined one collected specimen in the Britsh Museum that was identified as Carcharias gangeticus and taken at Viti Levu, and which he re-identified as C. leucas: “Our examples agree perfectly with Müller and Henle's description of C. leucas.” However, he later (Günther 1874) made the interesting statement that Carcharias gangeticus inhabits the freshwaters of Viti Levu, in a lake separated from the sea by a cataract. Unfortunately, it is not possible to identify the locality referred to by Günther. Nevertheless, several authors reported C. leucas from Fiji under the name Carcharias gangeticus. Engelhardt (1913: 43) reported Carcharias gangeticus “… in süßen Gewässern der Fidschiinseln” [= “in fresh waters of the Fiji Islands”]. Carcharhinus leucas was later generically reported from Fiji by Fowler (1928, 1959) under the names “Eulamia gangeticus” and “Eulamia gangetica” and by Whitley (1927) under the name Carcharinus gangeticus [sic]. Ryan (1980: 59) listed Carcharhinus gangeticus in the checklist of the brackish and freshwater fishes of Fiji, but he commented: “….it is likely that this shark is C. leucas.” He (Ryan 1980: 59) further remarked: “Reported from a long way up a number of Fiji rivers.” Boseto & Jenkins (2006) compiled the results of earlier works about the fish fauna of Fiji and mentioned C. leucas for fresh and brackish water habitats too, but without naming a precise locality. Also Boseto (2006) and Jenkins et al. (2009) mentioned C. leucas in listings of freshwater fishes of Fiji, but they didn't provide any localities. Possibly, C. leucas occurs in further major rivers in Fiji (see Table 10), as indicated by the anecdotal reports mentioned by Rasalato et al. (2010). Jenkins & Jupiter (2011) listed C. leucas as a marine migrant in the freshwaters of Vanua Levu (Kubulau district), again without naming a precise locality. Glaus et al. (2015) reported catches of sharks in Fijian rivers by artisanal fisheries—probably all juvenile C. leucas.

No. 1: There is a further indication of the presence of sharks (and sawfishes) in Lake Sentani in Van Pel (1958: 29): “While populated mainly with fresh-water species, it is remarkable that some sea fish are also found in its waters.”

No. 5: The first, and so far only, scientific report of sharks in Lake Jamoer, and an early indication of C. leucas's occurrence there, was by Boeseman (1956a: 24): “The most spectacular fish collected here was, beyond doubt, a kind of shark probably confined to fresh water.” Boeseman and his team collected three voucher specimens of C. leucas from Lake Jamoer, which are nowadays part of the fish collection of the Naturalis Biodiversity Centerin Leiden (catalog nos.: RMNH 24699, ♂ 146 cm TL; RMNH 24611, ♀ 73 cm TL; RMNH 24697, ♀ 125 cm TL; see Allen & Boeseman 1982). Polhemus et al. (2004) concluded that the population of C. leucas in Lake Jamoer, which may or may not still be present, was representative of a remarkably isolated inland population. However, there is no reason to believe that these sharks were isolated, as this lake is connected with the Arafura Sea by the Omba River (Table 10, No. 6). On the other hand, Polhemus et al. (2004) were almost certainly correct in their statement that modern fishing methods (gillnetting) may have played a major role in the presumed demise of C. leucas in Lake Jamoer. Allen (1991) reported that C. leucas was very common in Lake Jamoer in the 1950s, but its current population size is unknown.

No. 12: On the occurrence of C. leucas in the Fly River, Roberts (1978: 10) wrote: “…this species is to be expected in the Lower Fly and might occur in the Middle and Upper Fly, although there is no evidence that it does.”

No. 24: Copeland (2013) also reported that the villagers of Viria, located on the Rewa River approximately 50 km upstream from the coast, in the province of Naitasiri, tell anecdotes about the presence of sharks in the river.

Conclusive remarks

The present account of low salinity habitats with occurrences of C. leucas shows that the Atlantic Ocean side of the North American continent has the highest number of habitats, followed by the Atlantic side of Central/South America and the Indian Ocean side of the African continent (Fig. 3). The number of identified habitats was lowest for the Atlantic part of Africa and the Pacific part of the American continent.

Fig. 3.

Absolute numbers of fresh and brackish water localities with verified records of Carcharhinus leucas (n = 415) per continent/region, ordered from highest to lowest. Abbreviations: AOC = Atlantic Ocean coast; IOC = Indian Ocean coast; POC = Pacific Ocean coast.

4.1 Distribution in the Atlantic Ocean

4.2 Distribution in the Indian Ocean

From a biogeographical point of view, one important question concerning the range of Carcharhinus leucas in the Indian Ocean is whether its distribution is continuous from the South African coast to the Indian coast and farther to the Southeast Asian coast. Already Fowler (1941) delivered a detailed listing of reports of C. leucas (and its numerous synonyms) from the Indo-Pacific region and an intensive study of the available references and literature available at that time. The listing of Fowler (1941) includes numerous doubtful records and errors regarding the distribution of C. leucas due to the confusion with Carcharhinus gangeticus, a name repeatedly used in studies from this region (see Methods).

4.3. Distribution in the Pacific Ocean

Carcharhinus leucas is wide-ranging on both sides of the Pacific Ocean (Compagno 1984), including in its marginal seas. This large ocean basin represents a major geographical barrier that has an enormous impact on the migration of non-pelagic fishes, including coastal sharks. The vast size of this ocean, which is poor in oceanic islands and “stepping stones”, successfully prevents transoceanic migrations and gene flow of coastal sharks, including C. leucas.

5.1 Influence of salinity

Elasmobranchs are essentially marine, but ∼15% of species occur in brackish or freshwater (Wosnick & Freire 2013). Carcharhinus leucas is considered the best known of the 43 species of elasmobranch, in ten genera and four families, to have been reported in freshwater (Compagno & Cook 1995). Carcharhinus leucas has a life cycle closely linked to the freshwater-estuarine-marine continuum (Werry et al. 2018), which provides a salinity gradient from 0 up to ∼35‰. Although C. leucas is not the only euryhaline carcharhinid shark, and river sharks of the genus Glyphis also occupy habitats with low salinities in southeast Asia and northern Australia, C. leucas is unique in enduring water conditions with nearly no salinity and pure freshwater. This enables this species to enter low salinity environments where no other primarily marine sharks can follow it. Numerous authors have reported the frequency of C. leucas in low salinity habitats, and some of them outlined the dependency of this species on these habitats during certain stages of its life history. Moreover, C. leucas exhibits salinity preferences (Blackburn et al. 2007; Froeschke et al. 2010a; Drymon et al. 2014) that may regulate its abundance in certain habitats.

Simpfendorfer et al. (2005) reported, from the inland waters of Florida, that juvenile C. leucas displayed spatial segregation by body size, thus partitioning available food resources and reducing competition between size classes. This partitioning by juvenile C. leucas appears to be driven by temperature and salinity gradients, along with varying preferences for these parameters between size classes (Simpfendorfer et al. 2005; Heupel & Simpfendorfer 2008). In the hot tropical and subtropical river systems of northern and eastern Australia, neonate C. leucas travel upstream from estuaries after birth and undertake extensive movements into the upper reaches of rivers, where they can remain in purely fresh water for up to four or five years (Pillans 2006; Thorburn 2006; Thorburn & Rowland 2008; Last & Stevens 2009). Here, they are safe from predation from other sharks. However, in river systems of temperate latitudes, the low water temperatures of the upper reaches of rivers in winter causes the sharks to migrate into environments with higher salinities closer to the river mouth, and their residence in freshwater environments is thus time-restricted. Moreover, tidal influences in estuaries play an essential role in the distribution of juvenile C. leucas in estuarine environments, where movements and travel directions of immature C. leucas have been positively correlated with different tidal stages due to changes in salinity (Ortega et al. 2009). Pillans et al. (2020) observed movements of juvenile C. leucas in two Australian rivers that were correlated to both flow and salinity, with sharks moving downstream in response to increasing flow/declining salinity and upstream during low flow/increasing salinity.

Already Springer (1950: 6) noted, for North America and regarding the distribution and habitat use of C. leucas, especially during times of breeding: “Carcharhinus leucas…reaches peak abundance near the mouth of large rivers during its summer breeding season. The young frequent bays and are more common where the water is slightly brackish.” In this context, Mccord & Lamberth (2009) reported that a single pregnant female C. leucas that was tracked in South Africa's Breede River Estuary remained within the 15–35‰ salinity ranges and in the lower 20 km of the estuary. This may indicate that adult females of C. leucas are partially estuarine-dependent and utilize estuaries as pupping and nursery grounds.

On the other hand, immature C. leucas favor lower salinities, which suggests that a change in physiological tolerances with age contributes to niche separation (Simpfendorfer et al. 2005; Wiley & Simpfendorfer 2007; Heupel & Simpfendorfer 2008). Heupel & Simpfendorfer (2008) found out that juvenile C. leucasleave estuaries when salinity declines, which is astonishing for a fully euryhaline shark that is tolerant of a broad salinity amplitude, able to adapt rapidly to salinity changes, and that actively seeks low salinity habitats during the early stages of its lifetime. a study conducted by Pillans et al. (2020) in Australia's Logan and Albert Rivers revealed that despite fluctuations in environmental salinity (0–32‰) in these rivers and a strong declining gradient in salinity with increasing distance upstream, neonate and juvenile C. leucas (74–102 cm TL) remained within a narrow band of salinity (6–10‰) throughout the tracking period (30 months). a study by Drymon et al. (2014) showed that juvenile C. leucas had the highest affinity for moderate salinities (10–11‰) in Alabama's Mobile Bay. These results support the idea that juvenile C. leucas could have a preferred salinity range, or perhaps an ecological optimum salinity range, despite the fact that this shark species can survive in a wide range of salinity values (Ballantyne & Fraser 2013), and this since its earliest life stages (Pillans et al. 2005a) and for extended periods. Considering the energetic costs of osmoregulation in C. leucas, the observation made by Heupel & Simpfendorfer (2008) is comprehensible.

Alford (2012) reported the highest abundance of C. leucas in Louisiana's Barataria Estuary at salinity ranges between 12 and 23‰, and postulated a significant positive relationship between abundance and salinity (the size of the sharks was not reported). Investigations conducted by Ortega et al. (2009) in Florida's Caloosahatchee River Estuary revealed that juveniles of C. leucas (77–104 cm TL) occupied a salinity range between 2.4 and 12.8‰. Streich & Peterson (2011) reported salinities in Georgia's Altamaha River Estuary, at sites 14–18 km upstream, varying from 10.4 to 12.4‰, in which neonates and young-of-the-year C. leucas occurred. Tinari & Hammerschlag (2021) listed occurrences of the 142–300 cm TL size-class C. leucas in a salinity range of 24–45‰ in waters off South Florida (including the Miami and Keys regions), in a spectrum below and above the mean salinity of seawater (∼35‰). Before, Loftus & Kushlan (1987) reported two newborn specimens of C. leucas from Florida's Shark River at 0.8‰ salinity, just downstream from the freshwater section of this creek. Also Pillans (2006) revealed, in Australia's Brisbane River, that juvenile C. leucas showed a strong preference for the upper freshwater reaches of this river in environments with extremely low salinities. Hueter & Tyminski (2007) recognized for Florida estuaries that although older juvenile C. leucas utilize estuarine nursery areas (1.7–41.1‰ salinity), they do not appear to venture as far into freshwater as the neonates and young-of-the-year do.

A study conducted by Dwyer et al. (2020) in a north Australian river found that Glyphis glyphis Müller & Henle, 1839 (speartooth shark) used higher salinity environments (mean salinity = 19.22‰) located between 30 and 70 km from the mouth of the river, whereas C. leucas occupied freshwater reaches (mean salinity = 1.98‰) between 60 and 110 km upstream. Moreover, this study revealed that climate change plays a role in the behavior of freshwater tolerating sharks. At the onset of the wet season, both C. leucas and G. glyphis undertook a coordinated downstream migration towards the lower estuary before returning upstream (Dwyer et al. 2020). This spatial segregation could be interpreted as a niche partitioning behavior between river shark species, driven by seasonal fluctuations in environmental salinity. However, juveniles of C. leucas were reported from purely freshwater (estimated 0‰ salinity) from numerous locations worldwide (Tables 1–11).

As a euryhaline shark species with a wide amplitude of salinity tolerance, C. leucas not only occurs in low salinity habitats but also in hypersaline environments, like some lakes and saltwater lagoons in southern Africa (Lake St. Lucia) and eastern Australia (Lake Macquarie) (Compagno 1984; Last & Stevens 2009). Additionally, occurrences were reported from hypersaline bays like Mexico's La Paz Bay (Abitia-Cárdenas et al. 1994) and hypersaline estuaries like the Sine-Saloum Estuary in Senegal (Diouf 1996). However, sharks in these environments are sometimes found in poor conditions, and these habitats can be considered as suboptimal (Compagno 1984). Therefore, even for a euryhaline species like C. leucas, salinity is an environmental limiting its distribution.

Based on a comprehensive assessment of the published literature regarding the osmoregulation competencies of C. leucas (e.g., Thorson & Gerst 1972; Thorson et al. 1973; Pillans & Franklin 2004; Anderson et al. 2005a; Pillans et al. 2005a, 2006, 2008), there is no support for a shift in salinity preference based on its physiology; as specimens of C. leucas grow, their surface area/volume decreases, which reduces osmotic stress induced by long-time use of low salinity waters. As such, the use of low salinity environments by C. leucas is most likely due to biotic factors, particularly predation risk in marine environments, rather than physiological preferences (see also section 5.9). In this context, the results of Pillans et al. (2020) on juvenile C. leucas in two Australian River systems indicate that habitat choice by juvenile C. leucas is a complex tradeoff between hydro-graphic factors, physiology, food availability, and predator avoidance, resulting in large differences between adjacent systems and more broadly across the species' range. Notwithstanding this, the utilization of low salinity habitats by immature C. leucas throughout its whole geographic range reinforces the thesis that this behavior is mainly driven by instinct and/or inherited behavior.

5.2 Influence of water temperature

The distribution of aquatic animals such as fish is highly affected by parameters of the surrounding element. Water parameters like temperature, salinity, and dissolved oxygen are regulating factors that influence the occurrences of sharks in general and especially of Carcharhinus leucas in lacustrine, riverine, estuarine, and also marine environments, during all stages of the species' life history. This greatly influences the distribution of C. leucas. In the literature, C. leucas is mostly considered a warm-water species with a tropical stronghold. Bass (1978) reported that the distribution of C. leucas is basically tropical, and Schwartz & Burgess (1975) stated that C. leucas is primarily tropical. As is the case for all biota, also the distribution of C. leucas is temperature-restricted, especially in the marginal areas of its range. Water temperature can be estimated as the main factor limiting the range of C. leucas, not only in coastal marine habitats but even in freshwater habitats. Carcharhinus leucas usually inhabits the continental coast of all tropical to subtropical seas, but it undertakes seasonal migrations into warm-temperate regions with a favorable increase in water temperatures. Adult C. leucas can undertake long migrations in marine environments, depending on seasonal warming/cooling of the waters.

The study by Blackburn et al. (2007) revealed occurrences of C. leucas in Louisiana's coastal waters between March and September, with a temperature range from 15 to 37 °C, even when occurrences in waters below 20 °C may be an exception (Froeschke et al. 2010a). Results of a study by Drymon et al. (2014) from Alabama's Mobile Bay demonstrated that juvenile C. leucas showed the highest affinities for warm water (29–32 °C). Curtis et al. (2007) reported catches of C. leucas in Florida's Indian River Lagoon system in a temperature range between 18.5 °C and 37 °C. Hueter & Tyminski (2007) mentioned that young-of-the-year C. leucas have been documented in Florida estuaries at temperatures as low as 16.4 °C, but most individuals only remain in these nurseries until as late as November or until water temperatures fall to about 21 °C, at which point they leave the estuaries. Tinari & Hammerschlag (2021) reported occurrences of C. leucas in the coastal region of southern Florida in a temperature range between 19 and 33 °C, with a mean temperature of 26.18 °C. Lear et al. (2021) observed movements of subadult and adult C. leucas (1.81–2.69m TL) in waters off the west coast of Florida, northern Gulf of Mexico, during the winter months (November to April) in a water temperature range between 19.8 and 26 °C. Carlson et al. (2010) found, for U.S. waters of the northern Gulf of Mexico, that tagged subadult C. leucas (1.5–2.0 m FL [fork length]) occupied temperatures primarily between 30.5 and 32 °C, with individuals occupying most temperature between 26 and 32 °C. Specimens of C. leucas in Carlson et al.'s (2010) study area were rarely found at temperatures < 20 °C, which agrees with the results of Froeschke et al. (2010a). Ortega et al. (2009) recorded, in Florida's Caloosahatchee River Estuary, movements of juvenile C. leucas (77–104 cm TL) in a surface water temperature that ranged between 27 °C and 37.3 °C. Streich & Peterson (2011) reported temperatures during June and July in Georgia's Altamaha River Estuary at sites 14–18 km upstream, varying from 28.8 to 31.4 °C, in which neonates and young-of-the-year C. leucas occurred.

Lee et al. (2019) found that on the east coast of Australia, C. leucas was present in the study area when the sea surface temperature was between 20 °C and 26 °C, with a peak abundance of sharks at 24 °C. The results of Niella et al. (2020a) also revealed that C. leucas's abundance in southeast Australia was highest at a sea surface temperature above 22 °C. Investigations by Brunnschweiler (2007) on C. leucas in the Fiji Islands showed that most time was spent by the sharks in water with temperatures between 26 and 27 °C. Interestingly, the analysis of acoustic tracking data of C. leucas tagged in Florida's Biscayne Bay revealed that temperatures above 27 °C had a negative impact on the presence of C. leucas in this area (Rider et al. 2021).

Carey et al. (1971) measured the body temperatures of carcharhinid sharks in comparison to the surrounding medium and found that the body temperature of C. leucas was just beneath the water temperature. In contrast to some of the thermoregulated mackerel sharks (Lamnidae), in carcharhinid sharks the body temperature depends on the temperature of the aquatic environment in which they stay. However, Hueter & Tyminski (2002) reported, from the inshore waters of Florida, that young-of-the-year and juvenile C. leucas have been found in the warm water effluents of the Tampa Bay and Yankeetown power plants during the winter months. It is believed that these sharks become trapped within these warm water plumes when the temperature of the surrounding water falls below the sharks' tolerance level (Hueter & Tyminski 2002). As a result of the ecological behavior of C. leucas and its preference for warm water, the water temperature of rivers and lakes may also have a selecting effect for occurrences in freshwater. Thomerson et al. (1977) reported the catch of a single specimen of C. leucas in the Mississippi River at Alton (Illinois) in September 1937, with water temperatures of the river of ∼27 °C at the location of the catch and ∼24 °C at the river mouth, where the freshwater starts penetrating. The authors suggested that the most effective limiting factor for the movement of sharks in this river was water temperature, withtemperatures below 24 °C (i.e., the temperature at the river mouth) limiting the movement of sharks in the Mississippi River. In a spatio-temporal context, Springer (1950) reported that adults of C. leucas appear in great numbers near the mouth of the Mississippi River from May through July and produce their young there. Further, Springer (1950) mentioned that this species disappears from inshore waters of the northern Gulf of Mexico with the onset of cold weather and becomes relatively more abundant than along the Florida coast in the vicinity of the Florida Keys, which indicates a seasonally induced migratory behavior of C. leucas in the Gulf.

In all likelihood, also the distribution and occurrence of C. leucas in freshwaters of the subtropical Persian Gulf region (Iraq, Iran) are influenced by seasonality, and penetration into freshwater systems here may depend on changes in water temperature. Hussain et al. (1995) reported, for the Shatt Al-Arab River, a water temperature range between a minimum of 11.5 °C in February and a maximum of 30 °C in July, and that water temperatures higher than 20 °C encourage the migration of marine species from the Persian Gulf into this river. In this context, Mohamed et al. (2015) reported an increasing number of marine species in the Shatt Al-Arab River during summer and autumn and a sharp decrease in winter. Possibly, the Tigris-Euphrat-Karun system is only utilized by C. leucas during the summer months in the northern hemisphere, as the sharks leave the system in October when water temperatures drop below 14 °C (Bishop et al. 2016). As an example of a record of C. leucas in a warm-temperate estuary, Mccord & Lamberth (2009) measured a temperature range of 20–24 °C in South Africa's Breede River Estuary in January 2009.

Investigations on the thermal behavior of sharks (Wheeler et al. 2020) revealed that young elasmobranchs are forced to endure suboptimal, local conditions as they arise, and that they experience a broader thermal environment compared to adults. Lear et al. (2019) found that free-ranging juvenile C. leucas experienced a 16 °C temperature range (19–35 °C) in a freshwater environment, nearly double that of adults (23–31 °C) in a marine environment. Despite the circumstance that most of the habitats that are utilized by C. leucas exhibit variability in water temperature, residential behavior was observed in C. leucas in some regions. Seasonal cooling and warming of water bodies affect the distribution of C. leucas in both hemispheres and can be understood as one of the drivers of large-scale migrations. For example, in the tropical waters of Florida in the northern West Atlantic, C. leucas is a year-round resident (see section 4.1.1), with northward-directed movements during the summer months. North of Florida, at nearly 30.00°N, C. leucas changes from being a year-round resident to a summer vagrant that also occurs in other states of the east coast of the United States, including Georgia, South Carolina, North Carolina, Virginia, Maryland, Delaware, New Jersey, New York, Connecticut, Rhode Island, and Massachusetts, at least up to 41.53°N.

5.3 Influence of water depth

Carcharhinus leucas is considered as a coastal, estuarine, riverine, and lacustrine species with a primarily neritic distribution, usually found in water less than 30 m deep; however, on the shelf, it can descend to the shelf edge to a depth of 152 m (Compagno 1984, 2016; White et al. 2006). This shark species shows a preference for shallow waters of the continental shelf with a main accumulation in waters of less than 30 m (Compagno 1984) and was commonly recorded in coastal Florida in shallow waters of 1–2 m depth (Heupel et al. 2006; Wiley & Simpfendorfer 2007). Investigations that were conducted by Carlson et al. (2010) in the northern Gulf of Mexico using pop-up satellite archive tags revealed that subadult C. leucas (1.5–2.0m FL) spent the majority of their time in waters less than 20 m deep. Tagged specimens in the study by Carlson et al. (2010) exhibited significant differences with regard to depth behavior, but this was not correlated to time of day. In contrast to these results by Carlson et al., Ortega et al. (2009) found, by using acoustic telemetry, that juvenile C. leucas in the estuary of Florida's Caloosahatchee River swam significantly closer to the surface during the night (mean = 0.6 m depth) and remained deeper in the water column during the day (mean = 1.5 m depth).

Brunnschweiler (2007) equipped specimens of C. leucas around the Fiji Islands with pop-up satellite archival tags and reported a maximum depth of 204 m for this species during occasional deep-diving vertical movements, which is the greatest depth ever directly measured for C. leucas; however, most of the time was spent by the sharks in waters less than 50 m deep, and they remained deeper during the day than at night. Curtis et al. (2007) reported catches of C. leucas in Florida's Indian River Lagoon system at depths between 0.2 and 4 m. Tinari & Hammerschlag (2021) reported, for waters off South Florida (including the Miami and Keys regions), occurrences of the 142–300 cm TL size-class of C. leucas in a depth spectrum between 2.16 and 44.77m and at a mean depth of 12.79 m.

Depending on the age of individuals, the habitat used by adult C. leucas may include also offshore environments during open-ocean migrations (Drymon et al. 2010; Love et al. 2013; Lea et al. 2015); however, sightings of adult C. leucas in open-ocean waters are not commonly recorded (Kohler et al. 1998).

5.4 Influence of dissolved oxygen

Besides salinity, additional water parameters such as dissolved oxygen may influence the distribution of Carcharhinus leucas in low salinity environments. Heithaus et al. (2009) reported that dissolved oxygen had a greater influence on the distribution of juvenile C. leucas in a Florida estuary than salinity, and that the number of individuals was high when dissolved oxygen levels were high. Pillans et al. (2020) observed, in two Australian rivers, that during periods of negligible flow and stable salinity, juvenile C. leucas moved upstream and downstream in response to increasing/decreasing dissolved oxygen. However, it should be considered that the upper stretches of river systems can exhibit limited tidal exchange together with high levels of microbial degradation of organic material, which lead to low oxygen conditions in these river portions. This may decrease the suitability of upper river portions for C. leucas.

Individuals of C. leucas were observed in a fish kill that occurred in March 1978 in the Belmore River (Macleay river system, northern New South Wales) during rapid deoxygenation of floodwaters (Bishop et al. 2001). Although the exact number and percentage of dead individuals of C. leucas were not reported, numerous specimens died during this event, when dissolved oxygen levels dropped below 20% saturation at temperatures ranging between 22 °C and 25 °C (Bishop et al. 2001). Thus, the completely euryhaline C. leucas probably depends also on suitable dissolved oxygen levels as well as suitable salinity levels. Ortega et al. (2009) reported, for a Florida estuary, that juveniles of C. leucas were observed in a dissolved oxygen range at the water surface of 3.6–9.4 mg/L. For the subadult and adult size-classes of C. leucas (142–300 cm TL), Tinari & Hammerschlag (2021) reported the species in a dissolved oxygen range between 1.46–12.00 mg/L (mean dissolved oxygen = 7.01mg/L) in waters off South Florida (including the Miami and Keys regions).

5.5 Influence of underwater visibility

Carcharhinus leucas exhibits a preference for turbid waters (Ellis 1989), as these conditions exist especially in estuaries and river mouths. Already Davies (1962) recognized the affinity of C. leucas for the freshwaters and estuaries of South Africa, and that these sharks were attracted by floodwater from rivers; according to this author, this preference is due to an increased likelihood of finding prey organisms in waters with turbidity. Also Compagno (1984) pointed out that C. leucas is often found in muddy areas and in the inshore waters of estuaries and river mouths, where few other shark competitors occur. Catch rates of C. leucas from South African waters revealed that the number of caught specimens was highest in underwater visibility below 1 m and decreased with increased visibility (Cliff & Dudley 1991; Wintner & Kerwath 2018). However, this information should be used with caution, as visibility may influence the catch rate of C. leucas but does not really provide any evidence for the habitat preferences of this species. Blackburn et al. (2007) reported occurrences of C. leucas in Louisiana waters with turbidity ranging from 0.1 to 2 m underwater visibility.

Compagno (1984) concluded that the very small eyes of C. leucas may have evolved as a result of the species' adaptation to estuarine, riverine, and lacustrine life habitats, where locating prey relies on other senses due to local turbidity. However, C. leucas also uses marine coastal waters with high underwater visibility, like reef ecosystems, and is therefore also subject to intensive dive tourism operations worldwide, e.g., in Fiji (Brunnschweiler 2010; Ward-Paige et al. 2020; Bouveroux et al. 2021).

5.6 Influence of sea bottom type

Carcharhinus leucas is both a marine and an estuarine/riverine apex (top) predator (O'Connell et al. 2007; Navia et al. 2010) and has adapted to life in a wide variety of environments, from freshwater rivers to offshore habitats (Love et al. 2013). In marine waters and coastal areas, adult and subadult C. leucas inhabit a variety of different benthic habitats from soft-bottom, sand-dominated habitats, including seagrass meadows, to rocky bottoms and coral reefs (Gilmore Jr. 1977; Ceccarelli et al. 2014). a study by Hueter & Manire (1994) in coastal waters off the west coast of Florida revealed that specimens of C. leucas showed no clear bottom preference, and were found over sand or mud bottoms as well as over seagrass.

In tropical to subtropical estuaries with brackish water conditions, mangrove forests with halophytic tree and shrub species are the dominating vegetation type of shoreline habitats. In estuarine ecosystems, juveniles of C. leucas can be found in mangrove estuaries and even in adjacent wetland marshes (Thollot 1996b; Ley et al. 2002; Matich et al. 2011; Kottelat 2013; Tuiwawa et al. 2013; Gaskins et al. 2020). Gonzáles-Acosta et al. (2015) reported C. leucas from the flooded mangrove forests of southern Baja California and Vega & Villarreal (2003) reported it from a mangrove estuary of Panama's Coiba Island, both on the eastern Pacific coast. Mangrove forests function as nurseries for a high number of marine and estuarine fish species (Laegdsgaard & Johnson 1995; Faunce & Serafy 2006) as well as for a high number of elasmobranchs (Nagelkerken et al. 2008), as they provide shelter from larger predators and high amounts of accessible prey. In these environments, as an estuarine top predator, juvenile C. leucas are niched in these tidally influenced ecosystems and are part of the estuarine food web, feeding on accessible prey like small bony fishes, elasmobranchs, and crustaceans. Estuaries with mangrove forests represent an important habitat type for the early life stages of C. leucas (Heithaus et al. 2009). As strong predators on other elasmobranch species, adult C. leucas may move inshore for foraging on other juvenile elasmobranchs in mangrove estuaries.

5.7 Influence of extreme climate events

Extreme climate events can affect the presence and abundance of Carcharhinus leucas in riverine and estuarine systems. In rare cases, the occurrence of extreme climate events can have impacts on the small-scale distribution and habitat use of C. leucas, especially when they affect the temperature of the water. Even in the tropics, and in regions known to be residential areas for C. leucas during the winter such as Florida in the western Atlantic, extreme climate events can have a disastrous effect on C. leucas populations in inland waters. Snelson & Bradley (1978), Gilmore Jr. et al. (1978), and Snelson (1979) reported several fatalities in C. leucas in the Indian River Lagoon system, caused by the extremely cold winter of 1976–1977 and hypothermal water conditions down to 4 °C. These extreme climatic conditions, which are unsuitable for tropical and subtropical fishes, led to a concentration of high numbers of C. leucas around the heated effluents of electricity generating stations (Snelson & Bradley 1978; Snelson 1979). Matich & Heithaus (2012) also reported the effects on juvenile C. leucas of an extreme winter weather phenomenon in the Shark River Estuary in Florida, a “cold snap” of nearly two weeks in January 2010 with a water temperature minimum of 9.1 °C at the peak of the event. This extreme climatic event resulted either in the death of sharks or in sharks permanently leaving the estuary system (Matich & Heithaus 2012). As results of long-term monitoring of habitat use by juvenile C. leucas in the Shark River Estuary have shown, the recovery of shark abundances and population structure in the river after such events can take up to seven years (Matich et al. 2020a).

Sometimes, natural disasters have led to spectacular findings of C. leucas, like after the tropical cyclone “Debbie” in northeastern Australia in March 2017, which washed C. leucas specimens out of the Burdekin River onto a nearby street (Clamann 2017; Sandeman 2017). One specimen was seen swimming in the flooded streets of Brisbane (Queensland, Australia) during the Queensland floods in 2010–2011 (BBC 2011). Several bull sharks were sighted in one of the main streets of Goodna (Queensland, Australia) shortly after the peak of of the Brisbane River flood in January 2011 (Garry 2011). a spectacular habitat is the golf course lake at Carbook, Logan City (Queensland, Australia), which is home to several C. leucas and has been for more than 20 years. These specimens were trapped in the golf course's lake following a flood of the Logan and Albert rivers in 1996 (Boswell 2013). Some of the sharks inhabiting the lake were found dead after a second flood in 2013 (Boswell 2013). According to Stevens et al. (2005) and Pillans et al. (2009), in rivers of the Northern Territory of Australia specimens of C. leucas are often captured in freshwater billabongs or sections of rivers isolated from the main tidal stream during the dry period, when the water level of these rivers decreases.

Some research has focused on the response of C. leucas to incoming hurricane events. Investigations on the behavior and spatial distribution of C. leucas using acoustic telemetry, conducted by Gutowsky et al. (2021) in the subtropical Biscayne Bay, Florida (USA), showed that most of the tagged C. leucas were no longer detected after Hurricane “Irma” in 2017, and that the number of sharks in the study area declined after the hurricane. Presumably, the sharks left the area as a response to the storm. In this context, Strickland et al. (2020) also investigated the effects of Hurricane “Irma” on the behavior and survival of juvenile C. leucas that inhabit Florida's Shark River Estuary. They found that most of fourteen tagged sharks attempted to leave the shallow waters of the estuary before the hurricane strike: eight specimens left within days or hours before the hurricane, whereas three left more than a week in before; finally, three specimens supposedly died as a result of the hurricane.

On the other hand, an increase in the number of C. leucas recognized in Lake Pontchartrain (Louisiana) was documented after Hurricane “Katrina” in August 2005 (Internet Reference 2). This was possibly a response to low oxygen levels in coastal rivers after the hurricane, which may have reduced the access of sharks to the adjacent rivers (Hoffmayer et al. 2006). However, it may also have been the result of higher amounts of food caused by the flushing of flotsam into the lake. In this context, Van Vrancken & O'Connell (2010) found that Hurricane “Katrina” has an influence on dissolved oxygen as well as salinity and water temperatures in Bayou Lacombe, a small tributary of Lake Pontchartrain. Perret et al. (2010) investigated the effects of Hurricanes “Katrina” and “Rita” in August and September 2005 on the sport fish fauna in the Atchafalaya River Basin and suggested that the loss of sport fish in the basin was the result of either a temporary event such as a precipitous drop in dissolved oxygen levels, or release of hydrogen sulfide causing more subtle changes in habitat.

5.8 Influence of rainfall

Besides seasonal warming of riverine and marine environments, which stimulates shark migrations, natural events such as rainfall can also influence the distribution and presence of Carcharhinus leucas in estuarine habitats, with a possible increase of abundance after rainfall due to higher amounts of food in these habitats. Moreover, increased and sustained rainfall/flooding will dramatically alter the salinity in estuarine environments and river mouths. Investigations by Werry et al. (2018) along the coastline of Queensland, Australia, suggest that the activity patterns of C. leucas are correlated with rainfall events, with an increased C. leucas catch (both juvenile and adult) from one to eight days after the rainfall—a relationship also confirmed by the movements of acoustically tagged sharks between estuarine and beach areas. In this context, Kiilu et al. (2019) reported, for Kenya's marine waters, that catch rates of C. leucas peaked during the months with the highest total rainfall. Werry et al. (2018) postulated two interacting mechanisms as drivers for an increased catch of C. leucas after rainfall. First, an increased movement of freshwater drains from a catchment into nearshore areas via rivers may physically push juveniles further towards marine waters. Second, the murky freshwater plumes interact with seawater to create localised in-water fronts. Such fronts aid plankton blooms, supporting the baitfish populations upon which juvenile and adult C. leucas feed. Storm events and intensive rainfall can change the salinity gradient in the transition zone of estuary systems dramatically, with a disruption of the normal spatial segregation of C. leucas and an abnormal and increased mixing of juveniles and adults (Werry et al. 2018). This may have negative impacts on local populations of C. leucas, as large specimens cannibalize smaller conspecifics, resulting in depletion of the juveniles.

5.9 Influence of predators

Habitat use by the euryhaline Carcharhinus leucas, especially when immature, is also driven by predation risk. Low salinity habitats and estuary nursery grounds seem to provide low-mortality environments for young C. leucas such as neonates, young-of-the-year, and juveniles (Heupel & Simpfendorfer 2011). The penetration of freshwater systems by juvenile C. leucas can be understood as an evolutionary strategy to decrease the predation risk of immature C. leucas by large coastal sharks in marine habitats, especially by apex predators that are known to be intensive elasmobranch consumers, e.g., Galeocerdo cuvier, Sphyrna mokarran Rüppell, 1837 (great hammerhead shark), and also adult C. leucas itself (Compagno 1984; Clua et al. 2014). Carcharodon carcharias and predatory marine mammals such as Orcinus orca Linnaeus, 1758 (orca or killer whale) may also include C. leucas in their diet (Compagno 2001). The strong predation pressure in coastal marine environments may have been an important driving force in the adaptation of C. leucas to use of low salinity environments. Physeter macrocephalus Linnaeus, 1758 (the sperm whale) has occasionally been observed to feed on sharks up to 3 m TL (Kawakami 1980), but this species may only represent a threat for adult C. leucas during their rare open ocean movements, as sperm whales seldomly move in coastal waters.

Considering the wide amplitude of environmental conditions C. leucas can endure, predation risk is likely the primary extrinsic factor shaping its habitat use, especially during the early stages of its lifetime. According to Sadowsky (1971) and Branstetter & Stiles (1987), at 1.24–1.30m TL, juveniles of C. leucas begin to occupy primarily continental shelf waters. At this length, they are large enough to avoid predation by larger sharks and further predators because of their size and speed. In contrast, C. leucas individuals of 1.5m TL and more have been involved in human-shark interactions in numerous freshwater locations around the world, e.g., in Lake Nicaragua and in the Euphrat-Tigris-Karun river system (Thorson 1976a; Coad & Papahn 1988; Moore 2018), which shows that they can remain in freshwater environments for longer than reported by Sadowsky (1971) and Branstetter & Stiles (1987).

Intraspecific predation is also a habitat-selecting factor for juvenile C. leucas. Thorson (1973) reported, about Nicaragua's San Juan River system, that juvenile C. leucas are concentrated in some of the side channels of the system, where they presumably are safer from predation by adults. In areas where C. leucas is sympatric with river sharks of the genus Glyphis, the larger river sharks may also prey on juvenile C. leucas in riverine habitats. Moreover, especially in freshwater ecosystems but also in brackish and close-to-shore marine ecosystems, C. leucas competes with other apex predators like alligators and crocodiles, which can also prey upon juveniles. In Florida's Everglades National Park and the southeastern United States, besides other freshwater elasmobranchs, C. leucas is sometimes prey to Alligator mississippiensis Daudin, 1802 (American alligator) (Nifong & Lowers 2017). In the riverine and estuarine habitats of tropical Australasia, C. leucas is sympatric with Crocodylus porosus, a further apex predator in fresh and brackish water environments. Reports of bull shark/crocodile encounters exist, often with sharks as prey of larger crocodiles (Messel et al. 1981; Doody 2009; Winchester 2014). Similar interactions between C. leucas and Crocodylus niloticus were reported by Whitfield & Blaber (1979) and Perissinotto et al. (2013) from the St. Lucia Lake system in South Africa. Recently, the occasional feeding behavior of a 2.5 m Nile crocodile consuming a juvenile C. leucas was observed in South Africa's St. Lucia Estuary (Jordan 2021). However, predation by reptiles as a natural population-limiting factor does not impact the local population sizes of C. leucas in river systems as much as fishing activities. In general, predation can be considered the main driving force regarding habitat selection, abundance, and distribution of juvenile C. leucas in low salinity environments.

5.10 Influence of human activities

Carcharhinus leucas has been reported from a wide range of riverine and estuarine environments, both natural and human-influenced to various degrees (West 2011; Werry et al. 2012; Smoothey et al. 2016) (see Tables 1–10). Estuary systems with a strong human impact that are utilized by C. leucas include fully artificial freshwater-ecosystems like the canals of Florida's Indian River Lagoon system (Gilmore Jr. 1977), Australia's Gold Coast canals (Werry et al. 2012), and, in historical times, the freshwater impoundment of Panama's Lake Bayano (Montoya & Thorson 1982). Carcharhinus leucas can also live in wastewater-influenced rivers, even though the effects and risks on the individuals of environmental pollution require further investigation (Gelsleichter & Szabo 2013).

Although C. leucas can adapt to a broad spectrum of highly human-influenced freshwater and brackish water environments, it is considered to suffer from habitat loss caused by modification of low salinity habitats (Simpfendorfer & Burgess 2009), due to its use of these habitats for reproduction. The location of nursery areas in estuarine and riverine systems and their vicinity to human settlements make this species vulnerable to anthropogenic pollution and habitat alteration. Nowadays, in some of the major rivers with at least historical records of C. leucas, technical constructions such as dams prevent the movements of C. leucas into the upper stretches of rivers, as these constructions represent barriers for migratory species into areas where they have formerly occurred. For example, nowadays, migrations of C. leucas to the upper reaches of the Tigris/Euphrat system and the Mississippi River, from where they were reported historically up to 850 kilometers and 2,800 kilometers upriver, respectively, are prevented by dams (Hussain et al. 2012; Coad 2015; Thomerson et al. 1977; Helfman & Burgess 2014). As a species that relies on low salinity habitats, the interruption of river systems and the degradation of estuaries make C. leucas vulnerable to a strong degree of habitat modification.

Overexploitation of stocks and local populations can also influence the small-scale distribution of C. leucas. Although not a target species of most fisheries, Kyne et al. (2012) reported that the strong fishing pressure in coastal waters of the western Central Atlantic has shifted the distribution of C. leucas to the outer barrier reef. Despite being a common species in many tropical to subtropical rivers, especially juvenile C. leucas are often subject to recreational fishing—like in many Australian rivers (West & Gordon 1994), where they are caught in high numbers—that can negatively affect the stability of local populations.

5.11 Influence of ontogeny on habitat shifts

Life history traits of Carcharhinus leucas in correlation to low salinity environments have been extensively documented (Simpfendorfer et al. 2005; Heupel & Simpfendorfer 2008), revealing a spatio-temporally dependent pattern of habitat use in this species throughout its life span. As the results of Simpfendorfer et al. (2005) have shown, the youngest age classes of C. leucas (young-of-the-year, neonates, juveniles) occur in riverine areas with freshwater (Fig. 2B), moving into coastal lagoons and, finally, offshore areas as they grow older. Chemical and isotope analysis of adult C. leucas teeth from the Shark Reef Marine Reserve in Fiji by Kocsis et al. (2015) revealed that in Fiji adults presumably do not return to freshwater habitats during their absence from the reef. This result supports the assumption that mature C. leucas progressively emancipate from low salinity habitats during their ontogenesis. However, the analysis revealed that at least adult female C. leucas periodically return to estuaries according to a one to two year cycle due to pupping related movements (Tillett et al. 2011b; Mcmillan et al. 2016). Conversely, the profiles of adult males suggested that they are less likely to return to estuaries throughout their lives (Mcmillan et al. 2016).

In large rivers, C. leucas is able to cover distances of many thousands of kilometers upriver, due to urea-based osmoregulation (Pang et al. 1977; Hazon et al. 2003; Anderson et al. 2005a; Hammerschlag 2006; Trischitta et al. 2012). This large-scale freshwater migration leads to extended periods in freshwater during the early stages of the natural life cycle of C. leucas (see Table 11). Already Günther (1910: 479) commented, for C. leucas: “This species rises far up in rivers.” The greatest penetration into freshwater by C. leucas was reported from the Ucayali River, the upper reach of the Amazon River at the locality of Pucallpa (Peru), more than 5,000 km from the mouth of the river and its estuary on the Atlantic coast (Thorson 1972a; Werder & Alhanati 1981; Soto 2001).

5.12 Dependence on low salinity habitat

It is well documented that many species of sharks use inshore protected water bodies (coastal lagoons, estuaries, and bays) as pupping and nursery areas (Snelson et al. 1984; Simpfendorfer & Milward 1993). The availability of freshwater locations seems to influence the distribution of Carcharhinus leucas, at least of females, and can be considered a motivation for migrations and therefore a driver shaping the geographical range of this species. Swift et al. (1993: 130) presumed, for C. leucas in the northeast Pacific: “Elsewhere in the world this species is closely tied to fresh and low salinity water, and the early demise of this habitat in southern California may have led to its disappearance here.” Carlson et al. (2010) found that the majority of subadult C. leucas observed in the U.S. waters of the northern Gulf of Mexico stayed in areas located near or adjacent to outflows of freshwater from the Apalachicola, Mississippi, and Caloosahatchee rivers, and the intercoastal waterway system from Lake Okeechobee. Most of the observed C. leucas specimens in that study spent most of their time in waters less than 20 m deep. These results by Carlson et al. (2010) emphasize the importance of the shallow coastal zone for this species as a potentially essential habitat, particularly in areas with high freshwater inflow.

An investigation of the shark composition of the Cayman Islands by Ormond et al. (2017) could not provide evidence of C. leucas for this remote island group in the Caribbean Sea. Ormond et al. (2017) suggested that this could be due to the absence of suitable breeding grounds for this species northwestern Caribbean Sea, although female C. leucas can travel long distances to find suitable pupping grounds (Lea et al. 2015). Probably, the nursery areas of C. leucas in the Caribbean region are around the Central American Isthmus and the north coast of South America (see Table 3). Fanovich et al. (2017) reported that C. leucas was observed only rarely (two times in a 14-month period) on the Atlantic side of Tobago (West Indies), possibly due to lack of suitable nursery grounds, but maybe also due to overfishing in this area.

Compagno (2016) pointed out that the littoral species C. leucas can also undertake long journeys to oceanic islands far from continental landmasses, which led to the conclusion that areas that do not provide any freshwater locations and which are located far from suitable nursery grounds can be settled by adult C. leucas. Furthermore, C. leucas has been collected (a single ♂, 225 cm TL) off the remote Quita Sueňo Bank (San Andrés Archipelago, Colombia) in the Caribbean Sea (Puentes et al. 2009). This leads to the conclusion that at least adult male C. leucas can move to small isolated oceanic islands without fresh and brackish water resources, using river-poor or river-less areas at great distances from low salinity ecosystems. These ocean movements are possibly motivated by foraging, although C. leucas seems to be rare or absent around most remote oceanic islands (Ballesteros 2007). In this context, Werry & Clua (2013) highlighted the fact that males and females of this species operate independently during certain periods, especially when parturition is not involved. However, it is still unknown whether there are fresh or brackish water habitats in the Greater Antilles and in the Caribbean that serve as nursery grounds for C. leucas (compare Lee et al. 1983), or whether breeding only takes place in continental waters of North, South, and Central America.