American white pelican predation of wild Rainbow Trout and Brown Trout in Silver Creek, Idaho

Objective Abundance of American white pelicans Pelecanus erythrohynchos has increased during the past several decades and has led to increased concern about potential impacts to recreational fisheries and conservation of native fish stocks. The primary objectives of this study were to quantify predation rates of Rainbow Trout Oncorhynchus mykiss and Brown Trout Salmo trutta by American white pelicans, especially magnitude, and determine whether pelicans preferentially selected trout by species or size. Methods Radio tags were surgically implanted into 150 Rainbow Trout or Brown Trout in both 2018 and 2019. Fish were selected to be representative of the trout community at large by systematically placing tags throughout the study area, by distributing tags amongst species in proportion to their relative abundance, and by tagging a wide range of sizes (range > 300 mm). Tagged fish were tracked by canoe, automobile, and fixed-wing airplane to determine fate. Results A total of 272 tags (91%) were detected at least once. Of those detected, 180 (66%) remained in Silver Creek, whereas 89 (33%) were detected at an American white pelican colony (i.e., nesting island) within Minidoka National Wildlife Refuge during 2018 (n = 37) and 2019 (n = 52). The unadjusted predation rate of tagged Rainbow Trout and Brown Trout in Silver Creek was 27% (n = 40) in 2018 and 35% (n = 52) in 2019. However, when detection efficiency was accounted for, adjusted predation was 29% (95% confidence limit [CL] = 7%) in 2018 and 39% (95% CL = 8%) in 2019. Pooled adjusted predation rate was 34% (95% CL = 5%). Conclusions Predation rates of this magnitude may be one factor contributing to decreased abundances of Rainbow Trout and Brown Trout in Silver Creek compared to historical estimates. We saw little evidence that American white pelicans disproportionally preyed upon either trout species; however, pelicans more often preyed upon smaller trout. Of note, predation occurred concurrently with an active hazing program, suggesting that staffing levels, budgetary investments, or implemented hazing and deterrence strategies were insufficient or ineffective. Alternative deterrence or other mitigation strategies may be needed to reduce American white pelican predation in Silver Creek and other portions of Idaho.