Temporal variability in functional responses and prey selectivity of the pelagic mysid, Mysis mixta, in natural prey assemblages

Predation rates and prey selection of the pelagic mysid shrimp, Mysis mixta , were studied experimentally in the northern Baltic Sea in 1998 during the ir most intensive growth period, from June to October. Functional responses during 5 months were determined by providing the mysids with a natural zoo plankton assemblage, diluted to several different concentrations. The resul ts show that ingestion rate increased, along with mysid growth, from early summer to autumn and that saturation level was reached between 400 and 500 mug C 1(-1). Ingestion rates increased with increasing prey concentration, and sigmoidal curves explained mostly the variation in ingestion rates (exp lanatory levels of 86-97%). Prey selection was evident in June, July and Au gust, though weaker during the latter 2 months. Selection differed between the studied months but, generally, copepods were more positively selected t han cladocerans. Rotifers were the main prey during June and July, when mys ids were small, while larger mysids fed on copepods and cladocerans. Of the copepods, Eurytemora affinis was a truly selected species. This study show s that mysids feed on many zooplankton taxa and that they undergo ontogenet ic diet shifts.