Size-selective predation on Gammarus pseudolimnaeus by trout and sculpins.

Length distributions of 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 and of sculpins Cottus cognatus and young-of-year trout. For all sample dates, length of Gammarus in the stomachs was significantly greater than in benthic samples. Age I and older trout generally consumed more Gammarus in the day than at night, and those consumed at night tended to be larger. 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. Size-selective predation occurred throughout the year, though this may have its greatest impact on Gammarus populations in winter, when density and growth rates are low. -from Authors