PLANKTIVOROUS BALTIC HERRING (CLUPEA-HARENGUS) PREY SELECTIVELY ON REPRODUCING COPEPODS AND CLADOCERANS

Plankton and Baltic herring (Clupea harengus) were sampled simultaneously at nine sites in the northern Baltic Sea and Gulf of Bothnia. The stomachs from 45 herring and 54 plankton samples were analysed. The prey consisted mainly of adult mesozooplankters: copepods and cladocerans. Larger zooplankters (mysids and amphipods) were very rare in the stomachs as were the younger stages (copepodids) of copepods. The stomach contents of the fish changed from brackish water and neritic plankton species in the northern Baltic Proper to limnic species in the northern part of the Bothnian Sea. The diversity of plankton species decreased along with the salinity from south to north. Comparison of the plankton samples and herring stomach contents showed that prey with a large body size was selected as food. Generally this resulted in female copepods being chosen, since they are larger than males of the same species. Especially preferred food items were species and stages which carried conspicuous egg sacs (Eurytemora affinis females) or pigmented eggs and embryos (Bosmina longispina, Podon spp). Our results suggest that the Baltic herring is capable of exerting a regulative effect on the prey population comparable with that found in freshwater planktivores.