PREDATION AT INTERMOUNTAIN WEST FISH HATCHERIES

During 1993-94, we observed predators and surveyed managers at Intermo untain West fish hatcheries to quantify the extent and species respons ible for depredation losses. Our field study confirmed hatchery manage rs' impressions that great blue herons (Ardea herodins), black-crowned night herons (Nycticorax nycticorax), ospreys (Pandion haliaetus), an d California gulls (Larus californicus) were the most important predat ors. At 2 intensively monitored hatcheries, we calculated losses to av ian predation to be 7.0 and 0.5% of annual production, in contrast to the managers' estimates of 15% at each hatchery. Feeding mostly on dea d or moribund fish, feral cats (Felis catus), raccoons (Procyon lotor) , and striped skunks (Mephitis mephitis) had little effect on fish pro duction. Private hatchery managers estimated higher (13%) total depred ation losses than did managers of state owned hatcheries (5%). Hatcher y managers viewed netting to be the only effective nonlethal control m easure.