Reconstructing the population dynamics of sprat (Sprattus sprattus balticus) in the Baltic Sea in the 20th century

Long time-series of population dynamics are increasingly needed in order to understand human impacts on marine ecosystems and support their sustainable management. In this study, the estimates of sprat (Sprattus sprattus balticus) biomass in the Baltic Sea were extended back from the beginning of ICES stock assessments in 1974 to the early 1900s. The analyses identified peaks in sprat spawner biomass in the beginning of the 1930s, 1960s, and 1970s at similar to 900 kt. Only a half of that biomass was estimated for the late 1930s, for the period from the late 1940s to the mid-1950s, and for the mid-1960s. For the 1900s, fisheries landings suggest a relatively high biomass, similar to the early 1930s. The exploitation rate of sprat was low until the development of pelagic fisheries in the 1960s. Spatially resolved analyses from the 1960s onwards demonstrate changes in the distribution of sprat biomass over time. The average body weight of sprat by age in the 1950s to 1970s was higher than at present, but lower than during the 1980s to 1990s. The results of this study facilitate new analyses of the effects of climate, predation, and anthropogenic drivers on sprat, and contribute to setting long-term management strategies for the Baltic Sea.