Trends in Vendace (Coregonus albula) Biomass in Pyhäjärvi (SW Finland) Relative to Trophic State, Climate Change, and Abundance of Other Fish Species

Catches of vendace (Coregonus albula) from Pyhäjärvi, a boreal lake in SW Finland, were exceptionally high for decades. Gradually, however, eutrophication and climate warming significantly changed the lake environment. From the 1970s to the 2010s, total phosphorus and chlorophyll a levels increased two- and threefold, respectively, while the average June–September surface temperature increased by 0.34 °C decade–1. The highest population biomass of the young-of-the-year vendace in autumn, 6–18 (mean = 14) kg ha–1, was recorded in 1973–1989, a period of sustainable fishery. Overfishing in 1990–1999 reduced autumn biomass of young-of-the-year vendace biomass to 3–11 (mean = 6) kg ha–1, allowing the competing planktivores perch (Perca fluviatilis), roach (Rutilus rutilus), and smelt (Osmerus eperlanus) to increase and consume a larger part of the plankton resources. Eutrophication and climate warming appear to have favoured these species, and the new resource division persisted even after the vendace population recovered.