Consequences of size-selective harvesting and changing climate on the pikeperch Sander lucioperca in two large shallow north temperate lakes

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.