Evaluating the Potential Impacts of American White Pelican Predation on the Fishery in Strawberry Reservoir, Utah

Fisheries managers often feel pressure to control highly visible avian predators, but whether birds affect game fish populations is not always clear. We addressed this question at Strawberry Reservoir, UT, by estimating the numeric and behavioral impact of American white pelican (Pelecanus erythrorhynchos) on Bonneville cutthroat trout (Oncorhynchus clarkii utah) and other fishes. We tracked PIT-tagged trout movements into and out of two spawning tributaries, while manipulating the distribution of pelicans via hazing techniques. We determined pelican-related trout mortality based on pelican stomach contents and PIT tags recovered from pelican loafing areas. We found that >95% of the fish consumed during the study period were Utah sucker (Catostomus ardens), while <4% consisted of trout.  Using a bioenergetics model constructed from the diet analysis and counts of the pelican population between 30 May and 30 August, we estimated that pelicans consumed 46 tonnes of Utah sucker and 0.33 tonnes of Bonneville cutthroat trout.  This constitutes 50,223 Utah sucker and 378 trout consumed.  To determine if this level of trout predation is significant relative to other sources of mortality we will ultimately develop a population model.  However, these preliminary results suggest that pelicans are feeding primarily on non-game fish in Strawberry Reservoir.