Selective predation on Cladocera by brown trout Salmo trutta L.

The size and species composition of zooplankton ingested by brown trout in Lough Sheelin was studied. Zooplankton and fish samples were taken simultaneously. The zooplankton samples consisted of Daphnia hyalina var. galeata , Copepoda and Bythotrephes longimanus in the proportion of 246: 10: 1. The entire stomach contents of the fish were examined with particular attention paid to the zooplankton component. The results indicated a high degree of selection for B. longimanus .