CONTENTS & ABSTRACTS
In English. Summaries in Estonian
Proceedings of the Estonian Academy of Sciences.
Volume 51 No. 1 March 2002
Special issue on the ecosystem of Lake Võrtsjärv
Commented list of macrophyte taxa of Lake Võrtsjärv; 5–25
Abstract. A summary list of data on 114 taxa of macroflora, found by different researchers in L. Võrtsjärv (area 270 km2) in 1912–2001, and a brief characterization of the habitats of the majority of the species and their frequency are presented. All these taxa are contained also in the floristic list of the largest lake of Estonia – L. Peipsi, connected with L. Võrtsjärv, which differs from that of L. Võrtsjärv mainly in having more rare species. Changes in the course of eutrophication are quite similar for both lakes. Expansion of reeds and decline of shallow-water and several submerged species have taken place.
Key words: changes during 20th century, parallelism with L. Peipsi, anthropogenic eutrophication.
Mean zooplankter weight as a characteristic feature of an aquatic ecosystem; 26–44
Juta Haberman and Helen Künnap
Abstract. The mean zooplankter weight is largely shaped by the trophic state of the water body. At the same time, it reflects even relatively slight differences in the trophy of lakes. In moderately eutrophic Lake Peipsi the mean zooplankter weight is 4.4 µg and in strongly eutrophic Lake Võrtsjärv, 2.7 µg. The mean cladoceran weight is 28 and 10 µg, the mean copepod weight 10 and 6.7 µg, the mean rotifer weight 0.9 and 0.6 µg, respectively. For individuals of the gen. Daphnia the mean weight is 52 and 30 µg, and for individuals of the gen. Bosmina, 21 and 7 µg, respectively. The average zooplankter of moderately eutrophic L. Peipsi is considerably larger compared with the average zooplankter of strongly eutrophic L. Võrtsjärv. Therefore, the zooplankton in L. Peipsi is in a far better condition than in L. Võrtsjärv and it can play its role in the transfer of energy from the algae to the fish more efficiently. Correlation analysis shows that the mean zooplankter weight is mainly built by the cladocerans and rotifers. The relationship between the mean weight of the individual and water temperature is more pronounced for L. Võrtsjärv and less pronounced for L. Peipsi where the amount of coexisting planktonic animals with different temperature requirements is larger. Also, the relationship between the mean weight and water biogen content is more clearly expressed in strongly eutrophic L. Võrtsjärv than in L. Peipsi, which has still retained some characteristics of a mesotrophic lake. Mean zooplankter weight characterizes both the zooplankton community and the whole ecosystem of the water body. Indirectly, it characterizes the animal groups dominating in zooplankton, feeding relationships between phyto- and zooplankton and between zooplankton and fish, as well as the pressure of fish on zooplankton and the trophy of the water body. Mean zooplankter weight can be used as a marker characteristic in the qualification of the ecosystem of the water body.
Key words: mean zooplankter weight, moderately eutrophic lake, strongly eutrophic lake.
The stock and yield of the European eel, Anguilla anguilla (L.), in large lakes of Estonia; 45–61
Andu Kangur, Peeter Kangur, and Külli Kangur
Abstract. As a result of stocking since 1956, eel has become the most important commercial fish in L. Võrtsjärv (270 km2) today. Downstream migration of eel from L. Võrtsjärv supports eel fishery in L. Peipsi (3555 km2). 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 five-year 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.
Key words: European eel, stocking rate, stocking frequency, effectiveness of stocking, composition of catches.
Prevalence and intensity of Anguillicola crassus infection of the European eel, Anguilla anguilla (L.), in Lake Võrtsjärv (Estonia); 62–73
Andu Kangur, Peeter Kangur, and Külli Kangur
Abstract. 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.
Key words: Anguillicola crassus, prevalence and intensity of infection, Lake Võrtsjärv.
Influence of water level on fish stocks and catches in Lake Võrtsjärv; 74–84
Ain Järvalt and Ervin Pihu
Abstract. Lake Võrtsjärv is a large (area 270 km2) shallow (maximum depth 6.0 m) eutrophic lake. Eel, pikeperch, bream, and pike are the main commercial fishes in the lake, while perch, roach, ruffe, and burbot are of secondary economic importance. High water level in the lake in spring expands the spawning areas of pike and has a strong positive influence on the abundance of a new pike generation. Bream catches are larger at high water level, when traps can be placed nearer to the spawning places of this fish. High water favours formation of strong generations of burbot, which prefers cold water. Abundant generations of pikeperch originate from the years of low water level in the lake, when spawning occurs at an earlier time and pikeperch fingerlings grow more rapidly and pass over to feeding on prey already in their first summer of life. With low water level and severe winter fish kills may occur in the lake.
Key words: Lake Võrtsjärv, water level, pike, bream, burbot, pikeperch, winter kills.
Instructions to authors; 85–87
Copyright Transfer Agreement; 88