Diet composition of herring (Clupea harengus L.) and cod (Gadus morhua L.) in the southern Baltic Sea in 2007 and 2008

Diet composition of two commercial fish species, herring and cod, were studied in some regions (mainly Gdansk and Bornholm Basins, and the Polish coast) of the southern Baltic Sea in 2007 and 2008. Herring is the dominant zooplanktivorous species in the ecosystem of the Baltic Sea, but apart from mesoplanktonic organisms it also eats macroplanktonic and benthic species in considerable amount. The diet of cod consists of fish and crustaceans from pelagic, hyperbenthic and benthic habitats. The feeding preferences of fish indirectly reflect changes in the whole food chain in the Baltic Sea. This research focuses specifically on these invertebrate species, which are eliminated from the environment by most of the ichthyofauna of this region. The aim of this research is to examine the role of invertebrate organisms belonging to Crustacea in the diet of herring and adult cod to supply updated results about feeding of these fish as little data have been collected since the 1990s. The present study is a preliminary survey and results can not be considered conclusive. The restricted numbers of analyzed stomachs of fish and selected seasons of the year addressed in this paper are a starting point for further studies with a larger scope. In this study, 20 to 90% of herring had empty stomachs. Mesozooplankton dominated the diets of small and large herring. Mysidacea, which were historically important prey for herring, are now scarce and have been replaced by planktonic Amphipoda. In the case of cod, consumption of Mysidacea has never been as low as in this study. As for other invertebrate prey, the benthic isopod Saduria and Crangon shrimp achieved the highest amount by number and weight. These results show distinct changes in diet when compared to previous investigations and require verification at a larger spatial scale.