Trophodynamic control by clupeid predators on recruitment success in Baltic cod?

Throughout the 1980s, reproductive success of the top-predator cod declined and stock sizes of the main prey species herring and especially sprat, imp ortant planktivorous predators in the system, increased substantially. Alth ough the hydrographic conditions conducive for survival of early life stage s improved during the 1990s, recruitment success of cod remained far below average. As clupeids have been identified as major predators on cod eggs an d larvae in the Baltic, increased predation may be an important factor hamp ering stock recovery. Results from stomach content analysis of herring and sprat during the spawning season of 1988-1995 and ichthyoplankton surveys i n the Bornholm Basin (the only important spawning area of cod in the Centra l Baltic in this period) allow a comparison of estimated consumption rates by the predator populations with standing stocks and production rates of co d eggs and larvae. Despite uncertainties in the estimation procedure, the f indings confirm substantial predation on cod eggs by both clupeid species. Especially at the beginning of the cod-spawning season, characterized by lo w zooplankton availability, sprat consumed a considerable proportion of the eggs produced. In 1993, the relative importance of zooplankton as prey inc reased, while fish eggs were encountered more rarely. In contrast, predatio n by herring remained on the same level or even increased, especially late in the spawning season. The larval stage of cod is not substantially affect ed by predation owing to limited spatial overlap between prey and predator. (C) 2000 International Council for the Exploration of the Sea.