Zooplanktivory in the Baltic Sea: a comparison of prey selectivity by Clupea harengus and Mysis mixta, with reference to prey escape reactions

We investigated prey selection by the 2 most important planktivores in the northern Baltic, the Baltic herring Clupea harengus membras L. and the mysi d shrimp Mysis mixta Lilljeborg. We hypothesised that the intensity of prey selection by herring is positively related to prey size and that deviation s from this relationship can be explained by prey escape capabilities. For the non-visually hunting mysids we assumed that prey size is less important than in herring and that escape capabilities of the prey explain a larger fraction of selectivities. To evaluate these hypotheses, we obtained select ivity indices for C. harengus membras and M mixta from previous studies mad e either in the field (herring) or in the laboratory (mysids). We then quan tified the escape performances of the 5 most abundant crustacean zooplankto n species in the Baltic Sea by videofilming the plankters escaping from an artificially created water flow (siphon). As hypothesized, herring behaved as size-selective predators: prey selectivity rank was Temora longicornis > Eurytemora affinis > Pseudocalanus elongatus > Acartia sp, > Bosmina longi spina maritima, In M. mixta, the selectivity rank was surprisingly similar, only the cladoceran B. longispina maritima was relatively more positively selected than in herring. Comparing the observed predator selectivities wit h the results of the siphon experiments allowed us to identify cases where the predators appeared to show 'true' positive selection for a certain prey (E. affinis) and where the selection was 'truly' negative (P. elongatus). On the basis of these results we suggest that large-sized copepods are impo rtant food for both herring and mysids and that a hydrography-induced shift towards a cladoceran-dominated zooplankton community may decrease the food gain by Baltic herring.