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We investigated prey selection by the 2 most important planktivores in the northern Baltic, the Baltic herring Clupea harengus membras L. and the mysid shrimp Mysis mixta Lilljeborg. We hypothesised that the intensity of prey selection by herring is positively related to prey size and that deviations from this relationship can be explained by prey escape capabilities. For the non-visu- ally hunting mysids we assumed that prey size is less important than in herring and that escape capa- bilities of the prey explain a larger fraction of selectivities. To evaluate these hypotheses, we obtained selectivity indices for C. harengus membras and M. mixta from previous studies made either in the field (herring) or in the laboratory (mysids). We then quantified the escape performances of the 5 most abundant crustacean zooplankton species in the Baltic Sea by videofilming the plankters escap- ing from an artificially created water flow (siphon). As hypothesized, herring behaved as size-selec- tive predators: prey selectivity rank was Temora longicornis > Eurytemora affinis > Pseudocalanus elongatus > Acartia sp. > Bosmina longispina maritima. In M. mixta, the selectivity rank was surpris- ingly similar, only the cladoceran B. longispina maritima was relatively more positively selected than in herring. Comparing the observed predator selectivities with the results of the siphon experiments allowed us to identify cases where the predators appeared to show 'true' positive selection for a cer- tain 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 important 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.

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