Size‐Selective Predation on Gammarus Pseudolimnaeus by Trout and Sculpins

Length distributions of the stream amphipod Gammarus pseudolimnaeus were obtained from analysis of the stomach contents of age I and older brook (Salvelinus fontinalis), brown (Salmo trutta), and rainbow trout (S. gairdneri) collected from July 1979 to May 1980, and of sculpins (Cottus cognatus) and young—of—year trout. Length distributions of Gammarus in benthic and drift samples were also analyzed for comparison with stomach—content data. For all sample dates, length of Gammarus in the stomachs was significantly greater than in the benthic samples. Mean and median sizes in the drift were usually smaller than in the benthos, indicating that length distributions in the drift did not directly account for the length distributions of Gammarus in the stomachs. Age I and older trout generally consumed more Gammarus in the day than at night, and those consumed at night tended to be larger. The occurrence of larger Gammarus in the night drift than in the day drift and an observed lower exploitation rate at night support Allan's drift/predation risk hypothesis. Size selective predation by sculpins and young—of—year trout was significant but less pronounced than in age I and older trout. Although there was seasonal differences in the lengths of Gamarus in fish stomachs as well as in the drift and benthos, size—selective predation occurred throughout the year. Size—selective predation may have its greatest impact on Gammarus populations in the winter, when density and growth rates are low.