Estimation of the total fish biomass in the Baltic Sea during the 20th century

Catch composition and yield data are used to estimate the evolution of fish biomass. These results are compared with the catch rates of Polish and Swedish fisheries. Both series, estimated biomasses and catch rates, suggest that biomass was low during the first half of this century. Subsequently, it increased to over 8 million tonnes in the 1970s. The yield shows essentially the same picture. Peak catches of almost 1 million tonnes were achieved at the end of the 1970s. Fishing mortality increased between 1900 and 1945. After this time, yield increased because of growing biomass (1940–1950, 1965–1970) or because of growing fishing effort (1950–1965, 1970–1975). There is evidence to indicate that eutrophication has caused the increase in biomass of Baltic fish stocks.