Long-term changes in fish community structure revealed by gillnet monitoring in a shallow, lowland reservoir
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Background. Fish communities can support or hinder water quality management. Sulejów Reservoir, which is situated on the Pilica River in Central Poland, serves as a strategic reserve of drinking water for the Łódź Agglomeration. Precise knowledge of the long-term dynamics of the structure of the fish community, what was the aim of this study, is important for proper management to slow the eutrophication of the reservoir and prevent toxic algal blooms. Materials and methods. To determine fish species composition, multimesh gill nets with mesh sizes ranging from 11 to 80 mm were used. From 1993 to 2015, gillnetting was conducted 1–2 times per month from June through November in two representative parts of the reservoir, and principal component analysis (PCA) was applied to explore the temporal variation in fish community structure expressed as the relative contribution of each species to the total fish biomass. Results. Dominance fluctuated among four species in the fish community: common bream, Abramis brama (Linnaeus, 1758); roach, Rutilus rutilus (Linnaeus, 1758); white bream, Blicca bjoerkna (Linnaeus, 1758); and pike-perch, Sander lucioperca (Linnaeus, 1758). PCA revealed three principal components with eigenvalues larger than one that explained 73.3% of the observed variance in the temporal changes in fish community composition. A negative correlation was found between pike-perch biomass and roach biomass (r = 0.82), while a positive correlation was observed between the biomass of pike-perch and that of common bream (r = 0.73). Consequently, there was a highly negative correlation between the biomasses of common bream and roach (r = 0.80). Furthermore, an interesting temporal pattern was observed; from 1993 to 2000, the fish community was dominated by common bream and pike-perch but then shifted towards domination by roach and white bream, despite declining phosphorous and carbon concentrations. Recently, this trend has been reversed, and a more balanced fish community structure has been established. Conclusion. Fish community succession, which is usually reported to be a one-directional process in reservoirs, i.e., from a riverine fish or percid-dominated to a cyprinid (bream)-dominated state, was more ambiguous in Sulejów Reservoir. This was likely a consequence of both restoration issues related to decreasing nutrient loads and the influence of zebra mussel, Dreissena polymorpha invasion on the trophic relations in the reservoir.
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