Patterning long-term changes of fish community in large shallow Lake Peipsi

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.