Greater predation by Double-crested Cormorants on cutthroat versus rainbowtrout fingerlings stocked in a Wyoming river

We studied the relative vulnerability of Yellowstone cutthroat trout (Oncor hynchus clarki bouvieri) versus rainbow trout (Oncorhynchus mykiss) stocked as fingerlings in the North Platte River, Wyoming, to Double-crested Cormo rant (Phalacrocorax auritus) predation. Cutthroat fingerlings decreased as a fraction of the population from stocking in late June to electrofishing s urveys in the following October and March. In contrast, the fraction of cut throat fingerlings among tagged fingerlings eaten by cormorants collected o n the river was significantly greater than that in the population when orig inally stocked. More limited data from pellets regurgitated by adult cormor ants at a nearby colony and in American White Pelicans (Pelecanus erythrorh ynchos) collected on the river showed the same trend toward greater percent ages of cutthroat trout being consumed than were present among trout stocke d. There were no differences in cormorant predation rates on the Eagle Lake strain of rainbow trout reared under shaded versus partially shaded condit ions, or between Auburn and Bar BC strains of Snake River (Yellowstone) cut throat trout. On the North Platte River, cutthroat trout fingerlings were m ore susceptible to cormorant predation than rainbow trout of similar size t hat were stocked simultaneously.