Andu Kangur

The role of temperature in the population dynamics of smelt Osmerus eperlanus eperlanus m. spirinchus Pallas in Lake Peipsi (Estonia/Russia)

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.

Condition and growth of ruffe Gymnocephalus cernuus (L.) in two large shallow lakes with different fish fauna and food recourse

Ruffe is one of the most numerous fishes in Estonian large Lakes Peipsi (area 3555 km2, mean depth 7.1 m) and Võrtsjärv (270 km2, 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 ruffes condition factor than in its linear growth rate.

Diet composition and size‐related changes in the feeding of pikeperch, Stizostedion lucioperca (Percidae) and pike, Esox lucius (Esocidae) in the Lake Peipsi (Estonia)

Abstract The aim of this study was to estimate the composition of the diet and size‐related changes in the prey selection of pikeperch and pike in L. Peipsi. A total of 591 pikeperches with a standard length (Sl) of 9–73 cm and 254 pikes (Sl = 25–105 cm) were sampled with a bottom seine and an experimental trawl in 1994–1996 and their stomach content analysed. The frequency of occurrence and number of prey fish per individual were used for the description of the diet. The recent diet of pikeperch and pike in the Lake Peipsi were not exactly the same, although smelt, ruffe and perch were the main prey fishes of both. Smelt was the commonest in the diet of pikeperch in contrast to ruffe that was mainly taken by pike.

A comparative study on the feeding of eel, Anguilla anguilla (L.), bream, Abramis brama (L.) and ruffe, Gymnocephalus cernuus (L.) in Lake Võrtsjärv, Estonia

The stomach contents of eel, bream and ruffe were examined in order to detect the diet overlap and possible food competition between these benthophagous fishes in the eutrophic Lake Võrtsjärv. The frequency of occurrence, mean number and mean restored weight of food items per individual were used for the description of their diet. Overlap in diet of bream (standard length >15 cm), eel (SL>30 cm) and ruffe (SL>4.5 cm) appeared mostly in respect of Chironomus plumosus(L.) larvae and pupae, which may lead to food competition between these fishes. However, these fishes can eat different chironomid instars. Unlike eel, who takes mainly large fourth-instar larvae and pupae of C. plumosus, ruffe and bream eat, in addition, small third- and second-instar larvae of this species as well as other small-sized chironomids, mainly Einfeldia carbonaria(Meig.), Microchironomus tener(Kieff.) and Procladiusspp. Obviously, even if the overlap was pronounced, serious competitive interactions would arise only in case of limited food resources in the years with a low biomass of C. plumosus. A significant correlation (r= 0.88; P< 0.05) was found between the mean biomass of fourth-instar larvae of C. plumosusin the lake sediment and in the stomach of ruffe in different years. The diet composition of ruffe was more diverse (including a wide range of invertebrates, fish ova and detritus) than that of eel and bream. Evidently, omnivorous mode of feeding provides ruffe with an advantage over other benthophagous fishes living in the lake. Unlike eel and bream, ruffe continued to take food in late autumn and winter when water temperature in L. Võrtsjärv decreased to 2–0.2°C.

Long-term trends in the fish communities of Lakes Peipsi and Võrtsjärv (Estonia)

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.