Long-term monitoring of pikeperch (Sander lucioperca) populations under increasing temperatures and predator abundances in the Finnish coastal waters of the Baltic Sea

The Baltic Sea has been under intense environmental changes in the recent decades, such as climate change, eutrophication and increasing abundance of top-predators, which pose serious challenges to its aquatic life. For informed conservation measures and sustainable yields, we need to understand how the populations are being affected. Accordingly, we used long-term data series (covering the period between 1980 and 2021) to assess how these changes have affected populations of an ecologically and economically important predatory fish, the pikeperch (Sander lucioperca), in the coastal waters of Finland in the northern Baltic Sea. We investigated the estimates of abundance and recruitment, commercial and recreational catch statistics and growth and mortality rates. We found a clear increase in the total catches in the northernmost part of the Finnish coast (Bothnian Bay) that were not explained by changes in fishing effort, indicating increased abundance, most likely due to higher water temperature. In the southern part of the study area (Archipelago Sea), density-dependent factors prevented the development of particularly strong year classes, despite the beneficial conditions of warming seawater and consecutive warm summers. Individual growth has increased in younger age groups, contributing to an upward trend in the spawning population biomass. We also uncovered a declining trend in the total mortality in the southern area, despite increased abundances of cormorants and seals, explained by reduced total fishing mortality. These results show that the pikeperch is one of the species that has, thus far, benefited from the environmental change in the northern Baltic Sea, strengthening its role in the ecosystem.