Introduction

African catfish species belonging to the genus Clarias (Scopoli, 1777) are native to the African continent (Ng 2020, Froese & Pauly 2021). The North African catfish Clarias gariepinus (Burchell, 1822) has a natural range from the River Nile to the River Orange (Cambray 2003), but has been widely introduced to Europe, Asia (Froese & Pauly 2021) and South America (Cambray 2005, Kubota et al. 2012, Rodríguez-Barreras & Zapata-Arroyo 2019), and translocated to South Africa in river basins where it was previously absent (Weyl et al. 2016). In countries with a temperate climate such as Brazil and Puerto Rico, C. gariepinus has established self-sustaining populations in inland waters (Weyl et al. 2016, Rodríguez-Barreras & Zapata-Arroyo 2019) and caused serious adverse impacts (Ng 2020, Froese & Pauly 2021). In inland waters of Europe, the first occurrences of this species were reported in 1985 from the Czech Republic (Lusk et al. 2004), in 1997 from gravel pits of Slovenia, likely as a result of sport fishing activities (Povž 2007), and in 2007 from the Ovcharitza Reservoir in Bulgaria (Uzunova & Zlatanova 2007). However, the reason for these introductions remains unknown.

Table 1.

Morphometric and meristic characters (min = minimum; max = maximum) of Clarias gariepinus × Heterobranchus sp. identified in this study, Clarias gariepinus (Froese & Pauly 2021), Heterobranchus bidorsalis (Teugels et al. 1990), Heterobranchus longifilis (Teugels et al. 1990), Clarias gariepinus × Heterobranchus longifilis and Heterobranchus longifilis × Clarias gariepinus (Legendre 1992). Head morphometry in % head length.

The genus Heterobranchus (Geoffroy St. Hilaire, 1809) is endemic to Africa and its native distribution is restricted to the Rivers Nile, Senegal, Gambia, Volta, Niger and Benue, as well as to Lake Chad (Fagbenro et al. 1993, Froese & Pauly 2021). Notably, no report on the introduction of any species belonging to this genus in inland waters outside its native range has so far been published. Yet, it seems that Heterobranchus sp. was introduced to Europe for hybrid catfish aquaculture production, and possibly identified as vundu Heterobranchus longifilis and African catfish Heterobranchus bidorsalis (Fleuren 2008, Popp et al. 2018).

Clarias gariepinus and Heterobranchus sp. are important in the aquaculture industry of African countries because they can achieve a large size within a short time (Olaniyi & Omitogun 2014, Ng 2020). Heterobranchus sp. is known to grow to a larger size than Clarias and can reach a weight of about 14 kg. In contrast, C. gariepinus grows faster, is more adaptable to environmental conditions, reaches sexual maturity earlier, and has a higher fecundity than Heterobranchus sp. (Solomon & Ezigbo 2010). Therefore, by combining the rapid growth of Heterobranchus sp. and the early maturation of C. gariepinus, intergeneric hybrids “Clariobranchus” and “Heteroclarias” have been developed and have become widespread aquaculture strains in African countries (Legendre et al. 1992, Owodeinde et al. 2012). Notably, “Heteroclarias” refers to crosses between female Heterobranchus sp. and male Clarias sp., whereas “Clariobranchus” is the reciprocal cross of the latter (Solomon & Ezigbo 2010). Only one record of escaped hybrid specimens from a fish farm is known for Lake Volta in Ghana (Solomon & Ezigbo 2010).

Fig. 1.

Captured specimen of hybrid African catfish “Clariobranchus” (Clarias gariepinus × Heterobranchus sp.) from the River Danube.

The first experimental farming of C. gariepinus in Europe started in the late 1970s (Welcomme 1988, Povž 2007), specifically in the Netherlands and later in Belgium, Czech Republic, Croatia, Germany, Greece, Hungary, Poland, Russia and Slovakia (Piria et al. 2018, Froese & Pauly 2021). However, because of its fast growth and short time to achieve market size, the “Clariobranchus” hybrid gained the attention of Hungarian and Dutch aquaculturists (Sattari et al. 2010, Popp et al. 2018). Production of this hybrid is based on flow-through systems using geothermal water (Feledi et al. 2013) and no escapes of “Clariobranchus” hybrids in inland waters of Europe have so far been reported. The aim of this paper is to provide a morphological and dietary examination of a “Clariobranchus” hybrid specimen found in the Croatian part of the River Danube.

Material and Methods

One specimen of a “Clariobranchus” hybrid was caught on 29 September 2016 by gillnet in the River Danube, Croatia (1,418 r.km, near Zmajevački dunavac: 45°47′15.15″ N, 18°51′29.28″ E), by a commercial fisher during routine fishing activities. Immediately after capture, the undetermined specimen was frozen at –10 °C and sent to the laboratory of the Faculty of Agrobiotechnical Sciences in Osijek, Croatia.

In the laboratory, the specimen was measured for total length (TL, mm) and weight (W, g). Following Kottelat & Freyhof (2007) and Legendre et al. (1992), 32 morphometric and seven meristic characters were measured and counted, respectively (Table 1; see also Tables S1 and S2). The specimen was identified based on Teugels (1990) and Legendre et al. (1992) (Table 1). Stomach contents were removed and prey items were determined to the lowest possible taxonomic category. Assessment of the fish's diet was based on frequency of occurrence (F%) and percentage number (N%). Condition factor (K) was calculated as W TL^–3.100 (Ricker 1975).

Results

The captured specimen was a male, TL = 634 mm and W = 2184 g, and confirmed to be “Clariobranchus”, i.e. a hybrid between C. gariepinus × Heterobranchus sp. (Fig. 1). Morphometric and meristic measurements are provided in Table 1.

The main character of the captured specimen that distinguishes it from the parent species is the presence of an adipose fin that is shorter than in Heterobranchus sp. but absent in Clarias sp. (Table 1, Fig. 2; Tables S1 and S2). The captured specimen had a longer dorsal fin and higher number of rays in the dorsal fin than in Heterobranchus sp., but the dorsal fin was shorter and had a lower number of rays than in Clarias sp. All measurements of the head and body length corresponded with the hybrid described by Legendre et al. (1992) (Table 1; Tables S1 and S2). Identification was additionally confirmed by A. Horváth and B. Kovács.

Fig. 2.

Captured “Clariobranchus” specimen (A) left side of the head; (B) front aspect of the specimen; (C) tail and view of the adipose fin.

The stomach of the specimen was full, with the dominant portion consisting of macrophytes (F% = 25), gastropods (N% = 33, F% = 25), crayfish (N% = 67, F% = 25) and detritus (F% = 25). K was estimated to be 0.73.

Discussion

“Clariobranchus” hybrids between C. gariepinus × Heterobranchus sp. are fertile and can produce offspring (Solomon & Ezigbo 2010). The presence of an adipose fin is the main morphological feature for differentiation between “Clariobranchus” hybrids and C. gariepinus (Teugels et al. 1990, Legendre et al. 1992, Froese & Pauly 2021). “Clariobranchus” hybrids possess a short adipose fin that is absent in C. gariepinus (Iswanto et al. 2015). The adipose fin of Heterobranchus sp. is considerably longer than that of “Clariobranchus” hybrids (Legendre et al. 1992). Also, “Clariobranchus” hybrids differ from Heterobranchus sp. by a longer anterior dorsal fin (Popoola et al. 2008). Identification of the specimen in this study was supported by the recorded presence of hybrids between C. gariepinus × Heterobranchus sp. at fish farms in countries with access to the River Danube (Feledi et al. 2013, Popp et al. 2018).

Although the literature reports that in Europe farming is based on a female C. gariepinus × male H. bidorsalis cross (Popp et al. 2018), the exact species of introduced Heterobranchus sp. in Europe is still debatable, as it could also be H. longifilis (B. Kovács, pers. comm). Indeed, H. longifilis exists in Europe and is used in Dutch hybrid catfish aquaculture production (Fleuren 2008).

The introduction pathway of “Clariobranchus” remains unclear. However, it is suspected that the specimen caught in this study was an escapee from fish farms located in the River Danube basin of Croatia, or from farms in neighbouring countries (i.e. Hungary). Hungary is the biggest producer of C. gariepinus in Europe, where this species and its hybrid “Clariobranchus” have been farmed for the last 30 years (Feledi et al. 2013, Popp et al. 2018) and where cultivation is mainly conducted in open ponds (Popp et al. 2018), which may increase the likelihood of escape. The captured specimen's TL was nearly that of market size (Feledi et al. 2013) and the fish possessed a relatively high condition factor (Keyombe et al. 2015, Olopade et al. 2015, Getso et al. 2017). Therefore, aquaculture is the most probable source of introduction. A second possibility for the introduction may be from accidental release from a public freshwater fish aquarium or a home aquarium.

Clarias gariepinus, Heterobranchus sp. and, consequently, “Clariobranchus” are omnivorous and their presence in inland waters may cause negative impacts on native fish species, biodiversity and the aquatic environment (Cambray & van der Waal 2006, Solomon & Ezigbo 2010, Weyl et al. 2016, Ng 2020). The captured specimen's diet consisted chiefly of macrophytes and gastropods, similar to items consumed by C. gariepinus (Tesfahun 2018). It also contained an undetermined crayfish specimen representing an atypical prey item. It has previously been noted that “Clariobranchus” feeds on insects and fish material (Solomon & Ezigbo 2010). Nevertheless, it has been assumed that “Clariobranchus” cannot survive the low winter temperatures of continental Europe because the temperature range for African catfish is generally between 8 °C and 35 °C (Britz & Hecht 1989). However, a risk of establishment of a population still exists due to the presence of thermal spring habitats, as demonstrated by the adaptation of Nile tilapia Oreochromis niloticus in Slovenia (Povž et al. 2018), which is supported by its ability to colonise a variety of habitats (Solomon & Ezigbo 2010). Further, the Adriatic basin of Croatia could potentially offer a suitable climate for the establishment of the “Clariobranchus” hybrid, particularly in habitats with standing water or artificial water bodies (Piria et al. 2016).

The “Clariobranchus” hybrid is not listed in the European Council Regulation (EC) No 708/2007 of 11 June 2007 concerning the use of alien and locally absent species in aquaculture (EU 2007). According to this directive, closed aquaculture facilities are recommended for farming locally absent species to prevent their escape into inland waters. “Clariobranchus” hybrid escapees can contribute to native biodiversity loss, particularly through their diverse feeding habits and the possibility of introduction of new pathogens into novel environments (Weyl et al. 2016, Pofuk 2021).

The present finding suggests that the “Clariobranchus” hybrid escaped from an unprotected system or open pond farm. In this regard, in case of additional escapes, the threat of establishment of a population into suitable environments in Croatia and surrounding countries cannot be excluded since the species has been ranked as posing a high risk of invasiveness for the region (Radočaj et al. 2021). However, there have been no further reports of catches of this species since 2016, hence it can be assumed that the occurrence of this hybrid specimen may have been incidental, although increased control on fish farms is needed in order to avoid additional cases of such escapes in the future.