The population dynamics of fish in northern lakes is strongly influenced by climatic factors. In this study, we investigated whether there is a link between the late 1980s climate regime shift in Europe and the collapse of vendace (Coregonus albula) population at the same time in Lake Peipsi. Until the end of the 1980s, vendace was very abundant in the lake, but then its catches sharply declined. This decline inspired investigations into the extreme weather events preceding the vendace collapse using data on daily water temperatures and ice phenology together with commercial fishery statistics since 1931 and test catch data since 1986. We identified using advanced statistical methods that the hot summer of 1988, which was accompanied by a severe cyanobacterial bloom and extensive fish kill, and the subsequent non-permanent ice cover and early ice-offs in 1989 and 1990 in Lake Peipsi were the main reasons for the disappearance of vendace from catches in 1991. Moreover, a negative correlation appeared between catches of the predatory pikeperch (Sander lucioperca) and vendace. Predation pressure as well as fish habitat degradation caused by lake eutrophication may contribute to the instability of the vendace population too. Our study showed that extreme weather events such as heat waves in summer and non-permanent ice-cover in winter in consecutive years may have long-lasting harmful effects on the population abundance of cool-water fish species such as vendace whose eggs usually develop under an ice cover in north-temperate lakes.

Although massive fish kills are wide-spread and can be economically devastating, there is little information on exact causal mechanisms of fish kills in nature. In large shallow Lake Vortsjarv, sporadic fish kills have been registered mainly in cold winters, yet in 2013, an unexpected fish kill occurred beginning mid-June. At the time of the fish kill, an investigation was conducted to determine species composition, number, and sizes of dead fish along the lake shore. To determine possible causes of the fish kill, we analysed the dynamics of key physical and chemical parameters of lake water, including diurnal fluctuations of water temperature (WT), pH, dissolved oxygen (DO), ammonium ion concentrations (NH4-N), and the development of water stratification, during the growing season of 2013 using high-frequency water quality monitoring buoy and monthly manual monitoring data. Environmental data between 2010 and 2012 were used as a reference because no fish kill occurred. The results suggest that the fish kill was induced by a combination of successive and co-occurring extreme water parameters such as high WT (up to 24.5 degrees C), pH (up to 9.2), and NH4-N (up to 0.13 mg L-1), short-term stratification, and low DO concentration in the bottom water (0.49 mg L-1, saturation 5.4%) induced by quick warming of this shallow lake after a long ice-covered period and leading to a likely ammonia poisoning and hypoxia. The main target species was the bottom-dwelling ruffe (Gymnocephalus cernuus), indicating that the summer kill started at the bottom of the lake. The event highlights the significance of short-term disturbances on fish populations, which can be detected only using high-frequency monitoring data.

Under the prevailing warming and eutrophic conditions, the yields of the pikeperch Sander lucioperca have increased in Lake Peipsi and Lake Võrtsjärv in Estonia. Yet, some decrease in the catches has been observed in recent years. We explored whether long-lasting size-selective harvesting of pikeperch together with changing climate factors might have triggered a truncation of the size-structure of the pikeperch population and resulted in the weakening of the population. Thus, we analysed long-term commercial fishery statistics (since 1931) and trawl sample data (since 1990) from the Peipsi and Võrtsjärv lakes in terms of climate change and fishing pressure. Our study suggests that fisheries had direct negative effects on the size composition of the pikeperch population due to the nearly complete removal of the larger specimens. The truncation of the size-structure of the pikeperch population refers that cascading effect through the food web may occur as the abundance of inferior small fish has increased accordingly. Moreover, we found that climate changes influenced the pikeperch population in the same direction; large cohorts have become more frequent in the Peipsi and Võrtsjärv lakes, and 0+ pikeperch are able to survive milder winters even without undergoing ontogenetic diet shifts. Consequently, overdensity of small fish have occurred, these effects might have serious implications for the economies of the poor rural communities that surround the lakes. The present fishing regulations seem to be inappropriate and this may lead to further population weakening and thus should be revised to provide effective protection for large specimens. However, continued use of active (towed) gear (e.g., Danish seines) and the difficulties in enforcing fishery regulations under transboundary conditions complicate the implementation of policies that might result in more stable pikeperch populations in large Estonian lakes.

This article focuses on the socioeconomic aspects of recreational ice fishing and on the related pressures on the fish stocks of Lake Peipsi in changing ice conditions. Structured interviews conducted with anglers were combined with the assessment of the landings, the numbers of fisher- men, and fluctuations in ice-cover periods. At weekends, up to 3000 anglers were observed on the Estonian side of the ice-covered lake. Respondents defined ice fishing as a hobby; however, financial gain from selling caught fish was an important incentive for retired or unemployed persons. Eurasian perch, Perca fluviatilis L., was the most important target fish. Catches amounted from 0 to 20 kg (mean ~ 4 kg) per day per angler. Depending on the length of the ice-cover period, which varied from a few days to four months, the total catch may differ about ten times. In the case of long ice-cover periods, anglers may fish out approximately 40% of the total catch of perch, roach, Rutilus rutilus (L.), and ruffe, Gymnocephalus cernuus (L.) in the lake. However, favouring recreational fisheries would help to maintain the traditional fisheries-dependent lifestyle and benefit socioeconomically less secured people around Lake Peipsi and beyond.

The fish kill in lake Peipsi (Estonia/Russia) during the extraordinarily hot summer of 2010 evoked an investigation into the effects of environmental extremes and long-term eutrophication on the fish community of the lake. Current data on lake Peipsi indicate that temperature extremes and synergistic interactions with eutrophication have led to a radical restructuring of the fish community. Commercial landings of lake smelt, Osmerus eperlanus eperlanus m. spirinchus (Pallas), the previous dominant species of the fish community, have decreased dramatically since the 1930s, these declines being coupled with summer heat waves coinciding with low water levels. Gradual decline in smelt stock and catches was significantly related to a decline of near-bottom oxygen conditions and to a decrease in water transparency. The first documented fish kill in 1959 occurred only in the southern, most shallow and eutrophic lake (lake Pihkva). Recently, summer fish kill have become more frequent, involving larger areas of the lake. In addition to the cold-water species, e.g. smelt and vendace Coregonus albula (L.), the abundance of bottom-dwelling fishes such as ruffe Gymnocephalus cernuus (L.) and juvenile fish have significantly decreased after the 2010 heat wave probably due to hypoxia and warm water temperatures. This study showed that fish community structure in large shallow lakes may be very vulnerable to water temperature increases, especially temperature extremes in combination with eutrophication.

Analysis of historical and recent data is essential to understand how eutrophication and/or climate change might trigger shifts in the feeding mode of fish and trophic dynamics of shallow lakes. To assess long-term changes in the diet and growth of juvenile pikeperch (Sander lucioperca), the prey selection and growth of pikeperch fry from Lake Võrtsjärv was investigated in 2007 - 2010 and compared with data from 1920 to 1970. Over the observed period, larger cladocerans have become less frequent in the diet as eutrophication has altered the zooplankton community. Furthermore, climate change has triggered a mismatch between the predator and its prey: the smelt population has collapsed but other fish fries are too large for YOY (young-of-the-year) pikeperch. However, the mean length of fish has decreased mainly due to the postponed diet shift.

In Lake Vortsjarv pikeperch was observed not to shift to piscivory in their first autumn of life, although juvenile stages of a variety of fish species were abundant in the lake. It was hypothesized that the diets of predator and prey fish fry overlap and that coarse fish species are important food competitors for juvenile piscivores and thus, pikeperch and perch fry do not shift to piscivory during their first growing season. To discover the possible linkages in this pattern, in 2009 the feeding relationships of pikeperch, perch, ruffe and roach fry were analysed. The stomach content analyses showed that in the summer period, Mesocyclops leuckarti was the most frequent prey for perch and ruffe, pikeperch consumed Leptodora kindti in large quantities, and roach ate mostly plant material. Towards autumn, M. leuckarti was the most abundant prey for all percids. However, average stomach content weight and the number of prey items eaten by ruffe were considerably higher than for other fish fry. Since the feeding opportunities of fish fry are considered poor in the examined lake, the prey has the potential to restrict the recruitment to piscivory of their predators, as prey fish seem to have better abilities to persist in this ecosystem. Furthermore, supposed competition in the juvenile stage may result in a reduced top-down effect on coarse fish.

Eel fishery in Estonian inland waters depends entirely on the stocking of glass eels or pre-grown (farmed) eels. Via importation of live eels of 20-30 cm length the non-indigenous swimbladder nematode Anguillicoloides crassus was probably introduced via Germany into Lake Vortsjarv in 1988, and has since spread to many inland waters of Estonia. In 1992, the parasite was found in eel caught from Lake Vortsjarv. Between 1992 and 2002 and additionally in 2008, we examined in total 870 eels from Lake Vortsjarv (270 km(2)) and in 2008, 63 eels from three small lakes for adult A. crassus. The aim of the study was to obtain information on the variation of A. crassus infection in eels in Estonian lakes, to determine the temporal dynamics of prevalence and intensity of infection, and to establish a relationship between the length of host and intensity of infection in the eels in Lake Vortsjarv. There appeared to be a pronounced variation in prevalences of infected eels (from 3.7% to 100%) between the four investigated lakes. However, in Lake Vortsjarv, the prevalence of adult A. crassus infection remained stable (mean about 65%) for many years. The average number of nematode per infected eel (mean intensity) ranged from 12.6 +/- 2.5 in 1993 to 4.0 +/- 0.6 in 1999 in Lake Vortsjarv, while it was significantly higher (P < 0.0001) in the period 1992-1998 compared to 1999-2002 and 2008. The mean number of parasites per swimbladder was not related to eel length and no statistical difference was found in the condition factor of infected and non-infected eels. Although under normal environmental conditions A. crassus has not caused serious disease problems to eels in the study area, high intensity of parasite infection may contribute to eel kills due to oxygen deficiency in winter under the ice in Lake Vortsjarv.

Pikeperch is a major predator in the pelagic zone of eutrophic waters, such as the large north-temperate lowland lakes Võrtsjärv (Estonia) and Peipsi (Estonia/Russia). The size and structure of the pikeperch population is strongly influenced by their success at the juvenile stage. Therefore, we investigated the diet and prey selection of pikeperch fry caught in the ice-free period in lakes Peipsi and Võrtsjärv in 2007 and 2008. We analysed the stomach contents of 635 pikeperch from Lake Peipsi and 202 pikeperch from Lake Võrtsjärv, and compared our findings with similar data from the 1950s (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), 1961). Analysing 4–20 cm long fry, we studied differences in prey size, seasonal diet patterns and the ontogenetic diet shift. In both lakes, 0+ pikeperch feed mostly on large predatory zooplankters. However, in Lake Peipsi the stomach content weight and the average number of food items in stomach were higher, and the food spectrum was wider than in Lake Võrtsjärv. There was also a difference in the type of food that dominated fry’s stomach content (calculated by weight) in the two lakes. In Lake Peipsi, chironomids larvae, as well as zooplankters Daphnia galeata and Bythotrephes longimanus dominated, while in Lake Võrtsjärv zooplankters Mesocyclops leuckarti and Leptodora kindti. Seasonal analysis showed that cladocerans dominated in pikeperch fry stomach content in summer and at the beginning of September, but copepods were dominant in autumn and spring. In contrast to the studies carried out from 1952 to 1958 (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), 1961), the shift from planktivory to piscivory at the end of the first growing season was hardly ever observed during our investigation. We believe this is due to the lack of suitable prey fish as there was a collapse of the smelt, Osmerus eperlanus (L.), population in both lakes. The transition of pikeperch from planktivory to piscivory was delayed till the next summer.

A self-organising map (SOM) was applied to extract long-term dynamic patterns of fish community in shallow eutrophic Lake Peipsi using the commercial fishery statistics recorded for 64 years from 1931 to 2002 (excluding 1940–1949 due to the World War II). In the dataset of fishery statistics, mainly 11 taxa were recognised according to their commercial importance. Therefore, we used the dataset consisting of 64 sample units (i.e., year) with 11 taxa for patterning the fish community by the SOM. Our results suggest that the fish community of Lake Peipsi was mostly gradually changed in long-term scale, but some abrupt changes were also noticeable. Despite the same species composition, the total annual catch has declined indicating changes of fish community. The fish community of Lake Peipsi has shifted in long-term scale from clean- and cold-water species like vendace Coregonus albula (L.), whitefish C. lavaretus L. and burbot Lota lota (L.) to more pikeperch Sander lucioperca (L.) preferring productive warm and turbid waters. Both, the deterioration of aquatic environment and predatory effect of pikeperch prevent recovery of vendace population. Gradual reduction of the stock of smelt Osmerus eperlanus L. is also a sign of ongoing eutrophication, while its disappearance from catches for 3 years (1973–1975) was the result of summer fish-kill. The effect of fishing is the most important human impact on the fish community in Lake Peipsi. Extensive use of fine-meshed towed fishing gear (e.g., trawls replaced later by bottom seines) affected mostly recruitment of pelagic predator, pikeperch, killing young specimens of this fish in large quantities. Even though the fishery methods have changed, the fish stocks of the lake have repeatedly suffered from the over-fishing.

We analysed lake smelt (Osmerus eperlanus eperlanus m. spirinchus Pallas.) population dynamics in relation to water level and temperature in Lake Peipsi, Estonia/Russia, using commercial fishery statistics from 1931 to 2004 (excluding 1940–1945). Over this period, smelt provided the greatest catch of commercial fish although its stock and catches have gradually decreased. At times, catches of smelt were quite variable with a cyclic character. Disappearance of smelt from catches in years 1973–1975 was the result of summer fish kill. Regression analysis revealed a significant negative effect of high temperature on the abundance of smelt stock, while the effect of water level was not significant. Our results suggest that critical factors for the smelt population are the absolute value of water temperature in the hottest period (≥20°C) of summer and the duration of this period. These weather parameters have increased in synchrony with smelt decline during the last 7 decades. There appeared to be a significant negative effect of hot summers on the abundance of smelt operating with a lag of one and 2 years, which can be explained by the short life cycle (mainly 1–2 years) of this species.

The present study examines the composition of the diet and size-related as well as seasonal changes in the prey selection of pikeperch in large eutrophic L. Võrtsjärv.The total stomach contents of 325 pikeperch with a standard length of 5-76 cm were analysed in 1994-2005 with the goal to assess how the predation pressure of pikeperch is distributed over various prey species.The frequency of occurrence (FO), the number and restored weight of prey fish as well as the index of relative importance (IRI) and Ivlev's selectivity index (E) were used for the description of pikeperch diet.The results of our study showed that the first prey fish for young pikeperch were mainly congeners and smelt Osmerus eperlanus (L.).With increasing body size the diet of pikeperch was enriched with ruffe Gymnocephalus cernuus (L.), perch Perca fluviatilis L., and roach Rutilus rutilus (L.).According to IRI, ruffe (IRI = 56%) was the most important prey for pikeperch in L. Võrtsjärv, while roach and perch were of second-rank importance.Bream Abramis brama (L.) was a rejected prey fish (E = -0.9)for pikeperch, while smelt (E = 0.8) was actively selected.In comparison with data from the 1950s (Erm, V. 1961.Eesti riim-ja magevete kohade bioloogilistest ja morfoloogilistest erinevustest.In Hüdrobioloogilised uurimused, Vol. 2, Tartu), the main long-term changes in the diet of pikeperch are related to shifts in the fish community structure.Vendace Coregonus albula (L.) was the main prey (FO = 54%) for pikeperch in the 1950s, but it is absent from the recent diet.At present smelt replaces vendace in the diet of young pikeperch.Cannibalism was not observed half a century ago due to the small number of pikeperch in the lake at that time.

. In Lake Peipsi (3550 km 2 , mean depth 7.1 m) sporadic fish kills have been registered repeatedly since 1959 during cyanobacterial blooms in warm summers. At the time of the excessive fish kill in August 2002 comprehensive investigations of phytoplankton abundance, key physical and chemical parameters of lake water (including diurnal fluctuations of the dissolved oxygen and ammonium ion concentrations and pH) as well as species composition and number of dead fish were carried out along the Estonian shore. The aim of the study was to assess how many fish were killed and to determine ecological conditions in the lake that led to the fish kill. The results suggest that the fish kill was induced by the synergistic effect of several unfavourable conditions, which appeared in the lake during the strong bloom of the cyanobacterium Gloeotrichia echinulata accompanied with continuous hot weather and decreasing water level. A combination of factors such as high water temperature (up to 26 °C), low water level (70 cm below the long-term average), great spatial and diurnal variations in the dissolved oxygen (saturation 25–165%) and ammonium ion (up to 0.33 mg N L –1 ) concentrations as well as in the pH (7.7–9.5) were responsible for the mass mortality of fish. The influence of cyanotoxins on the condition of fish cannot be neglected either. Ruffe, a bottom dwelling fish, suffered the most severely.

Ruffe is one of the most numerous fishes in Estonian large Lakes Peipsi (area 3555 km 2 , mean depth 7.1 m) and Võrtsjärv (270 km 2 , 2.8 m). Owing to its small size ruffe has little value for commercial or recreational fisheries today. However, due to their high abundance, ruffe play an important role in the food webs of both lakes. The aim of the investigation was to establish the proportion of ruffe in the fish community and to assess the wellbeing of this fish in the two lakes on the basis of growth rate and condition factor. According to experimental catches made in autumn 1998–2002, the average total catch of fishes per trawl-hour (CPUE) in L. Peipsi was about two times higher than in L. Võrtsjärv (204 kg and 105 kg, respectively). CPUE of ruffe made up 16 kg in L. Peipsi and 7 kg in L. Võrtsjärv. In these lakes the fish community composition and food resource appear to be different. Benthivorous fishes were dominating in L. Võrtsjärv and the biomass of macrozoobenthos was significantly lower in this lake than in L. Peipsi. In the first year, the growth rate of ruffe was higher in L. Peipsi where the state of zooplankton was better compared with L. Võrtsjärv. Differences in the linear growth rate of older fishes in L. Peipsi and L. Võrtsjärv were not significant. The lower condition factor of ruffe in L. Võrtsjärv reflects the limited food resource of abundant benthivorous fishes in this lake. Different trophic relationships in Estonian large lakes are better reflected in the ruffe's condition factor than in its linear growth rate.

As a result of stocking since 1956, eel has become the most important commercial fish in L. Võrtsjärv (270 km 2 ) today.Downstream migration of eel from L. Võrtsjärv supports eel fishery in L. Peipsi (3555 km 2 ).The aim of this study was to assess the effectiveness of eel stocking in L. Võrtsjärv.The annual mean catch of eel and stocking rate and frequency were analysed by fiveyear periods.The stocking of eel in L. Võrtsjärv (annual average 35 ind.ha -1 in 1956-2001, maximum 84 ind.ha -1 in 1980-84) is below optimum rate, which explains the relatively low catches (annual average 1.2 kg ha -1 in 1965-2001, maximum 3.7 kg ha -1 in 1988, according to official statistics).The average number of glass eels required to produce 1 kg of eel catch (efectiveness of stocking) was about 32 in 1965-2001.A significant positive relationship (r = 0.41, p = 0.03, n = 28) was found between the stocking rate and the catches of eel in the sixth year after stocking.Eels caught from L. Võrtsjärv are larger (usually 60-80 cm) than those caught from other water bodies in Europe.Food competition between eel and the indigenous benthophagous fishes bream and ruffe is an important factor influencing the success of eel fishery in the lake.

This study was undertaken to determine the direction and extent of time related changes in the fish communities of the large shallow eutrophic lakes Peipsi and Võrtsjärv. The official statistics of commercial fisheries for the periods 1931–1940 and 1950–1999 were used for trend analysis. In the ichthyocoenosis of both lakes, several drastic and economically important changes have occurred during recent decades. Some trends in the population dynamics of fish (e.g., sharp decline of the stock of vendace Coregonus albula (L.) in Lake Peipsi) are probably irreversible owing to the eutrophication of the lake. A sharp decrease in the stock of smelt Osmerus eperlanus (L.), caused by nighttime water anoxia in the summer of several years, was followed by a quick recovery. Periodic changes in the population of pike Esox lucius L., are related to water level fluctuations. The population dynamics of several fishes, particularly pikeperch Stizostedion lucioperca (L.), are, to a large extent, controlled by fishery policy, although climatic factors are also of importance. The state of the stock of eel Anguilla anguilla (L.), depends entirely on introduction of young eels.

Nonindigenous swimbladder nematode Anguillicola crassus was probably introduced into Lake Võrtsjärv via importation of live young eels (standard length of 20-30 cm) from Germany in autumn 1988.Since 1992, the parasite has been found in eels that inhabit this shallow eutrophic lake.The aim of the investigation was to determine the prevalence and intensity of A. crassus infection in the eel population of L. Võrtsjärv in different years, and to assess the possible effect of the parasite on eel fishery.Since 1992, 623 eels were examined for Anguillicola.The prevalence of infected eels ranged from 51% to 86% in different years.The average number of worms per infected eel (mean intensity) ranged from 4.0 ± 0.6 in 1999 to 12.6 ± 2.5 in 1993, the maximum number of the parasite was 92 per eel.The mean number of parasites in a swimbladder of eel was not related with the length of the host fish.No statistical difference was found for the condition factor of infected and noninfected fish.Although under normal environmental conditions Anguillicola has not caused serious problems to eels in L. Võrtsjärv, high intensity of parasite infection together with other unfavourable factors (e.g., high temperature, oxygen deficiency, intensive algal bloom, etc.) may lead to mass eel kills.

The investigations of the biota in L. Peipsi, one of the largest lakes of Europe, started already one and a half centuries ago, complex studies were initiated in the 1930s. The paper gives a brief survey of research into the hydrochemistry, macrophytes, bacterio-, phyto- and zooplankton, zoobenthos, fishes, and fisheries management of the lake. Special attention is paid to the main findings. Intensive anthropogenic eutrophication of L. Peipsi started in the 1970s. The nutrient content reached its maximum in the 1980s and decreased thereafter. An expansion of reeds, increase in the similarity between different lake parts, and decline of rare macrophyte species have occurred. Regarding phytoplankton, L. Pihkva has become strongly eutrophic, whereas L. Peipsi s.s. has eutrophied to a smaller extent, its northern part has maintained a mesotrophic appearance. In 1965- 99, the number but not the biomass of zooplankton increased. The number of small-sized zoo- plankters (especially rotifers) increased. No essential changes in the composition and distribution of macrozoobenthos in connection with eutrophication of the lake were observed. Dreissena polymorpha, introduced accidentally in the lake in the 1930s, has become the most significant animal population in L. Peipsi. Considering annual fish catches L. Peipsi surpasses all large lakes in North Europe. The main commercial fishes are smelt, perch, pikeperch, roach, bream, pike, and until the 1990s also vendace. At present it is changing into a smelt-bream-pikeperch lake.

The distribution of the predatory pressure of pike over different prey species in large eutrophic L. Peipsi was studied.In 1995-98, 415 pike with a standard length of 21.5-105 cm were dissected and their stomach contents were analysed.Of the examined stomachs 42-45% were empty.The diet of pike included at least eight prey fish species, the most frequent being ruffe, smelt, and perch.The same species dominated in the diet numerically.Smelt was the commonest fish species in the diet of smaller pike (S 1 <4O cm).The proportion of smelt decreased in the food of pike with size, whereas that of ruffe increased.Cannibalism did not play an important role in the recent feeding of pike.In comparison with data from 1960-63 (Pihu, E. 1966.The importance of pike, perch, pikeperch and burbot as biological control in Lake Peipsi-Pskov.In Hydrobiological Researches, Vol. 4, pp.235-248.Valgus, Tallinn (in Russian)), the proportion of pikeperch in the food of pike was greater.