Prey selection by juvenile cyprinids from running water

SUMMARY. 1. The juveniles of dace ( Leuciscus leuciscus (L.)) and roach ( Rtuihts rtuihis (L.)) and yearling minnows ( Phoxtnus phoxinus (L.)) exhibited generally similar patterns of feeding behaviour and preferences for particular prey types. 2. Factors affecting prey selection included predator and prey size, prey activity and environmental conditions. 3. All three species preferred prey sizes approximately 0.6x their maximum gape. 4. Prey motion was essential to induce attack, and prey were usually selected in proportion to their relative activities. 5. Specialization on one prey type was observed in still water, and was always directed at the most active prey type. In flowing water no specialization was observed, and size selection was suppressed in the juvenile fish. Yearling minnows, however, were size selective in both still and flowing water. 6. The presence of macrophyte cover did not significantly change the pattern of prey selection, but did reduce the predation rale. 7. These results are discussed in the context of previous studies of prey selection and optimal foraging.