Estimating Rates of Pelican Predation on Yellowstone Cutthroat Trout

Recent increases in piscivorous migratory bird populations and their impacts on fish populations have created conservation concerns for fish populations. These impacts can be especially dramatic in situations where native fish are exposed to increasing bird populations. Such is the case with American white pelicans preying on Yellowstone cutthroat trout in the upper Blackfoot River system of southeast Idaho. In this study we used both radio telemetry and passive integrated transponder (PIT) tagged fish to estimate total pelican predation on Yellowstone cutthroat trout within the Blackfoot River system. In the fall of 2009 and the spring of 2010, 57 cutthroat trout were telemetry tagged via boat electrofishing in the Blackfoot Reservoir. With tracking and tag recoveries we determined that 14 percent (n=8) of the fish were confirmed to have been consumed by pelicans. Another 12 fish mortalities were tracked to areas that also indicated predation by pelicans; providing an upper estimate of pelican predation of 36 percent. We also PIT tagged 1,648 cutthroat trout during the spring and summer of 2010. Scanning the nesting islands in the fall of 2010, after the majority of pelicans had migrated out of the area, resulted in the recovery of 44 PIT tags, or an estimated 2.7 percent rate of predation on cutthroat trout. After correcting this number of found tags on the island with tags deposited away from the islands, this predation estimate is increased to 13 percent. These results indicate between two and 36 percent of cutthroat trout are consumed by pelican predation. We plan to use these data to model how such rates of predation may influence the population dynamics of cutthroat trout in the upper Blackfoot River.