Cormorant and Grebe Predation on Rainbow Trout Stocked in a Southern Utah Reservoir

Abstract

We investigated poststocking losses of rainbow trout Oncorhynchus mykiss to avian predation in Minersville Reservoir, Utah. Double-crested cormorants Phalacrocorax auritus and western grebes Aechmophorus occidentalis consumed 31.3% and 8.8% of the planted fingerlings in 2-week periods following stocking in April and June 1988, respectively. Piscivorous birds were associated both spatially and chronologically with fingerling trout in the reservoir. The birds consumed hatchery trout far more frequently than they did Utah chub Gila atraria, which were abundant in the reservoir. Subadult rainbow trout and cutthroat trout O. clarki composed 75.0% of the diet, by biomass, of double-crested cormorants sampled. Double-crested cormorants removed more trout from Minersville Reservoir than did anglers.