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 Poli sh and Swedish fisheries. Both series, estimated biomasses and catch r ates, suggest that biomass was low during the first half of this centu ry. 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 mortali ty increased between 1900 and 1945. After this time, yield increased b ecause 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. (C) 1997 International Council for the Exploration of the Sea.