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 3 10, kg/yr since 1974). The population,and,fishery,must,therefore,be managed,sustainably,and,if necessary,accom- modate,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 under- standing of sprat population,dynamics,and enable a desirable integration of fisheries ecology and,management,with climatology,and,oceanography. Key words: Baltic Sea, food-web dynamics; climate change and fish populations; ecosystem management;,environmental,variability; fisheries ecology and management;,fisheries oceanography; population regulation; recruitment; sprat population dynamics.