FISH PRODUCTION AND CLIMATE: SPRAT IN THE BALTIC SEA

Processes controlling the production of new fish (recruitment) are poorly understood and therefore challenge population ecologists and resource managers. Sprat in the Baltic Sea is no exception: recruitment varies widely between years and is virtually independent of the biomass of mature sprat. Sprat is a key prey and predator species in the Baltic ecosystem and is commercially exploited (1.86 × 10⁸ kg/yr since 1974). The population and fishery must therefore be managed sustainably and if necessary accommodate environmental effects on population dynamics. We demonstrate using 45 years of data that recruitment depends on temperature conditions during the months when sprat gonads, eggs, and larvae are developing. We also show that recruitment can be predicted before adults spawn (and fully 15 months earlier than using present technology) by using linkages between recruitment, large-scale climate variability (North Atlantic Oscillation), Baltic Sea ice coverage, and water temperature. These relationships increase our understanding of sprat population dynamics and enable a desirable integration of fisheries ecology and management with climatology and oceanography.