We observed, photographed, and recorded (video and audio) spawning of five whitefish species — Coregonus migratorius (Baikal basin), C. peled, C. tugun, C. pidschian, C. muksun (the Ob basin) — in the spawning devices. All species spawned in autumn in the dark and the spawning period lasted 10–30 days. Individual fish spawned several times during the night and the spawning behaviour consisted of three, cyclically repeated phases: courtship, mating and recovery. Spawning occurred in male–female (with males to be initiators) or male–male combination, and took place either in the water column or near the water surface. A mating act consisted of a rhythmic parallel movement of the fish swimming side by side with the synchronous release of the gametes. Three types of mating acts were observed: vertical (from bottom to top), horizontal and combined including both vertical and horizontal movements. The male in the spawning pair moved forward towards the female by the length of its head; it rhythmically struck the back of the female's abdomen by bending the caudal peduncle. Depending on the species and size of the fish, the mating act lasted for 0.3–3 s within 0.3–2 m. The frequency of rhythmic body collisions was on average 17 Hz for C. migratorius and C. pidschian, while for C. tugun and C. peled it was 25–27 Hz. An egg batch released during the mating act amounted to about 90 eggs for C. tugun and 290–370 for C. peled, which corresponded to 1%–6% of the total fecundity of a female. A female participated in 20–100 mating acts during 1–3 nights. Eggs cannibalism was also observed. During spawning, eggs were widely dispersed across the spawning area, which may be regarded as an adaptation aimed at increasing survival rate during embryogenesis.

Depending on their reproductive strategy, different fish species either aggregate or disperse eggs and larvae in their reproductive habitat. Because yolk-sac larvae of vendace (Coregonus albula) disperse widely across the littoral and pelagic zones of boreal lakes, it is unclear where the exact spawning and egg incubation locations are. Vendace egg and larvae densities were studied in Lake Southern Konnevesi to clarify its spawning strategy. In autumn 2019, 1–2 weeks prior to spawning, 500 egg samplers were installed in five depth zones in 20 sampling plots. Fertilized eggs were found in 18 plots. The mean density of eggs was 74 eggs m^–2 and the mean fertilization rate 85%. During spawning, vendace dispersed their offspring throughout the lake. The sampling-plot-specific egg density in autumn 2019 did not correlate with larval density in the spring next year. The reproduction strategy of vendace reduces the effects of high spatial and temporal fluctuation in their reproduction and nursery habitats.

The yolk syncytial layer (YSL) is a multifunctional transient structure of teleost embryos and larvae. The YSL performs nutritional, morphogenetic and immune functions. However, the data on the YSL structure in different teleost species is fragmented. We aimed to characterize YSL of the postembryonic inconnu (Stenodus leucichthys nelma) along with the depletion of maternal reserves by analyzing serial histological sections. At hatching, the yolk complex was located anterior to the liver and contained a large oil globule (OG) in the front region, 1–2 additional OG, and the yolk itself. The YSL region around the OG was striated, the region surrounding the yolk was regionalized in an apico-basal direction. The yolk syncytial nuclei (YSN) were large and complex-shaped. The yolk was exhausted by approximately the 17th day post hatching (dph). One–two large OG remained in the yolk complex until the programmed death of the YSL at 31 dph. The organization of the larval yolk complex of the inconnu was typical for Coregonidae. The abundance of comet-shaped YSN was its peculiarity.

To determine if a survival bottleneck occurs in Lake Erie's lake whitefish (Coregonus clupeaformis) population and explore possible mechanisms responsible, we examined contemporary and historical dynamics of lake whitefish eggs, larvae and juveniles. Widespread spawning and low overwinter egg retention were observed in 2016–2018, however subsequent larval CPUE remained consistent with historical observations when regular recruitment occurred. Highest larval CPUE was consistently observed in nearshore areas 3–11 km from mid-lake spawning locations. Fall age-1 juvenile presence was predicted by fall age-0 catches, indicating the bottleneck occurs during the first growing season. Our results suggest the following: (1) factors limiting recruitment affect survival during or after the pelagic larval stage to fall age-0, and (2) physical and biological processes underlying connectivity between spawning and nearshore nursery habitats may be limiting recruitment. Future research focusing on larval nursery habitat characterization and lake whitefish growth and survival may reveal mechanisms affecting recruitment.

We analyzed the variability in vendace population parameters in the context of a life table. Parameters related to growth, fecundity and survival were estimated for post-recruitment (age > 1 growing season) life stages of Lake Southern Konnevesi vendace in both very high and low population abundance regimes. Pre-recruitment survival producing population stability was then determined. We found a very strong compensatory density dependence in growth, fecundity and survival: during low abundance, a lifetime reproductive output of a female was almost 20 times that of the abundant regime. To maintain the low abundance regime, pre-recruitment survival must counterbalance it by decreasing to a very low level. Potential drivers of high variability in pre-recruit survival and potential for depensation, Allee effect, are discussed, as well as the implications of the results on fisheries management and risk of extinction due to anthropogenic stressors such as global warming or extensive predator stocking.

Long-term monitoring of the planktivorous vendace (Coregonus albula) in Pyhäjärvi, a lake in SW Finland, revealed periods characterised by different types of population regulation. In 1971–1989, the vendace stock was strong and exhibited a two-year cycle. In 1990–1999, extreme weather conditions and predation resulted in recruitment failures, after which overfishing kept year-classes small. From 2000 onwards, the two-year oscillations were re-established at a lower level. Here, we show that the two-year cyclicity prevalent in the 1980s was consistent with the hypothesis of asymmetric competition between adults and juveniles. Food consumption by juveniles of strong year-classes retarded growth and weakened condition of the co-occurring adults, resulting in less abundant year-classes. In 2000–2018, the role of intraspecific competition diminished due to interspecific competition from increased populations of other planktivorous fish, preventing vendace from attaining higher abundance. Elevated temperature probably confounded the effects of competition, but its direct role was masked by simultaneous gradual eutrophication and fish assemblage changes.

The tendency towards two-year cyclicity is considered typical of many Fennoscandian vendace populations, especially in fluctuation of recruitment, based on time series of individual lakes. We used two robust indicators to identify and quantify two-year cycles in vendace population proxy time series at different life-stages — spawning stock biomass (SB), density of newly hatched larvae (LD) and recruitment (REC) — from 22 Finnish lakes. Then we applied Fisher's meta-analytical test to assess the adequacy of the evidence to support the hypothesis that vendace population dynamics include two-year cyclicity. The results supported this hypothesis for REC but not for SB or LD. Yet, the indicators and test are conservative and time-series of SB and LD are shorter than those for REC. The appearance of cycles in REC is associated with high post-recruitment mortality, consequently practically only one spawning per cohort. Cycles may be typical for the recovery period from low abundance period also. Still, some populations with moderate post-REC mortality and non-cyclic SB abundance exhibited cycles in REC. Such dynamics presuppose the existence of more complex regulation based on the interaction of different life stages.

Re-establishment of a declined apex predator fish species in a lake ecosystem may have dramatic effects on other fish and plankton community already inhabiting the ecosystem. We studied mechanistically potential impacts of re-establishment of the brown trout (Salmo trutta) in the west-central European Lake Constance focusing on two commercially important fish species: whitefish and Eurasian perch. We compared simulation model outputs from two versions of an allometric trophic network model for Lake Constance, one with and one without the trout as the apex predator. The re-establishment of the declined brown trout reduced the perch population directly by predation and indirectly by increased resource competition, whereas the whitefish population was directly affected by increased predation. The decrease in fish biomass densities was highly sensitive to trout larval survivability. Both species showed strong compensatory behaviour by achieving higher per capita resource consumption when the populations decreased in the presence of brown trout.

Lake Inari is a subarctic, oligotrophic, regulated lake in northern Finland, connected via the Paatsjoki (Pasvik) River to the Arctic Ocean. A new coregonid species, the vendace (Coregonus albula), was introduced into the watershed in the 1960s and gradually established a population in the lake becoming part of its ecosystem. The annual vendace catch in 1989 reached 300 tonnes, while during the following 25 years the vendace catches declined stabilizing at a very low level. Vendace introduction radically changed the fishing practices within a short time, and its population became firmly established in the lake. It benefited both fisheries and predatory fishes, especially salmonids. The vendace year-class strength varied greatly between 1983 and 2019. According to winter seine CPUE data, the strongest year-classes were up to 100 times greater than the weakest ones. Winter seine CPUE of one-year-old fish correlated positively with catches in the coming years. The growth data for the period 2015–2021 revealed a decrease in the youngest vendace age-groups.

Catches of vendace (Coregonus albula) from Pyhäjärvi, a boreal lake in SW Finland, were exceptionally high for decades. Gradually, however, eutrophication and climate warming significantly changed the lake environment. From the 1970s to the 2010s, total phosphorus and chlorophyll a levels increased two- and threefold, respectively, while the average June–September surface temperature increased by 0.34 °C decade^–1. The highest population biomass of the young-of-the-year vendace in autumn, 6–18 (mean = 14) kg ha^–1, was recorded in 1973–1989, a period of sustainable fishery. Overfishing in 1990–1999 reduced autumn biomass of young-of-the-year vendace biomass to 3–11 (mean = 6) kg ha^–1, allowing the competing planktivores perch (Perca fluviatilis), roach (Rutilus rutilus), and smelt (Osmerus eperlanus) to increase and consume a larger part of the plankton resources. Eutrophication and climate warming appear to have favoured these species, and the new resource division persisted even after the vendace population recovered.

Vendace is commonly assumed at low risk to recruitment overfishing. This assumption has been confirmed for boreal stocks but might not apply at lower latitudes. We evaluated the risk of recruitment overfishing at the southernmost extent of vendace populations by comparing fecundity, natural mortality, and fishing mortality of two stocks differing in growth with those of a population not subject to fishing. Although in one study lake vendace grew faster owing to high lake productivity and low stock density, and had substantially higher age-specific fecundity, intense harvest before first spawning negatively affected the stock's recruitment potential by reducing spawner abundance. In contrast, in the slow-growing stock, vendace entered the fishery after first spawning, and egg production per recruit was similar to that of the population not subject to fishing. We conclude that vendace stocks characterized by fast somatic growth may be at higher risk of recruitment overfishing, which can be reduced by protecting first-time spawners.

I conducted a population-genetic study of the Eurasian cisco complex in an area extending from the Baltic Sea to the East-Siberian Sea using 30 enzyme loci. The results indicate that the least cisco (Coregonus sardinella) populations from most rivers of the Kara, Laptev and East-Siberian sea basins are genetically fairly homogeneous, and that the cluster they form is different from the one for the vendace (C. albula) populations from the waterbodies of the Baltic and White seas (Nei's genetic distance (D_N) = 0.076). The least cisco and vendace originated from two major phylogenetic lineages of the species occurring as the purest form in the above regions. As a result of the evolution and complex interactions among local populations within the two phylogenetic lineages of cisco at the various stages of Middle and Late Quaternary glaciations, most extant modern populations are hybrid in origin. For example, the Barents Sea populations and the easternmost population from the East-Siberian Sea basin (Kolyma River) emerged as a result of hybridization during the last colonization wave by the least cisco (East Siberian) along the Arctic coast upon cisco's interaction with local ciscoes in the west and east. The White and Kovzhskoye lake populations from the Caspian Sea basin, which form a joint cluster with them, are the hybrids of the local vendace and the descendants of the first wave of least cisco's (West Siberian) expansion to Europe. In this paper, I also discuss the most probable scenarios of cisco's colonization of the study area using paleolimnological reconstructions.

Whitefish is well-known for its delicious meat and consumers are willing to pay high prices for it, but local supply is often limited due to fluctuating or declining catches. It is thus expected that whitefish aquaculture production should increase to help meet the demand, provided that insufficient knowledge or other roadblocks do not interfere. Here we use a literature review, expert interviews, and case-study approach to understand the existing knowledge, current production levels, and limitations of whitefish aquaculture in individual countries. As shown through the literature review, there is sufficient technical knowledge to scale-up whitefish production. However, interviews with several experts point out different factors hindering aquaculture production: strict environmental regulations, established production of other freshwater species, and possible competition with fish imports. The failure to establish whitefish aquaculture in Lake Constance underlines this outcome, as there is nearly no chance to establish whitefish aquaculture in this area despite very high demand paired with drastically decreasing supply from capture-fisheries. As a result, at least 50% of whitefish consumed at Lake Constance are imported. This case highlights a common pitfall in environmental management: decisions to forego or reduce local production often results in replacing a product of exceptionally high sustainability (locally produced fish) with imports from other countries.