Predation by American White Pelicans and Double-Crested Cormorants on Catchable-Sized Hatchery Rainbow Trout in Select Idaho Lentic Waters

Abstract

In southern Idaho, population growth of American white pelicans Pelecanus erythorhynchos at the Blackfoot Reservoir and Lake Walcott colonies since the early 1990s has generated concerns about whether pelican predation is impacting angler catch of hatchery trout stocked in Idaho waters. To evaluate this concern, we estimated rates of pelican predation (i.e., the proportion of fish consumed by pelicans) and angler catch (i.e., the proportion of fish caught by anglers) for 19 unique springtime fish stocking events over 3 years across 12 study waters; where feasible we also estimated double-crested cormorant Phalacrocorax auritus predation. Stocked Rainbow Trout Oncorhynchus mykiss averaged 247 mm in length and were internally PIT-tagged (to monitor bird predation) and externally anchor-tagged (to monitor angler catch) before stocking. Additional hatchery trout were PIT-tagged, euthanized, and fed directly to pelicans to estimate PIT tag deposition rates at the colonies; feeding was unsuccessful for cormorants. After the juvenile pelicans and cormorants fledged in the fall, we recovered PIT tags from stocked and fed fish that were deposited at the two colonies. Deposition rates for pelican-consumed tags averaged 21% and declined exponentially as distance increased from the colonies. Pelican predation on hatchery trout averaged 18% and ranged from 0 to 48%, whereas angler catch averaged 21% and ranged from 0 to 82%. Mean angler catch was nearly four times higher when pelican predation was low (i.e., <25%) than when pelican predation was high (≥25%). Cormorant predation estimates (available for seven stocking events) were minimum estimates only (i.e., they assumed 100% of tags consumed by cormorants were recovered) and averaged 14% (range, 2–38%). Our results suggest that predation by American white pelicans and double-crested cormorants on catchable-sized hatchery Rainbow Trout stocked in southern Idaho waters often exceeds the total catch of those fish by anglers who compete directly with avian predators for use of stocked trout.

Received June 23, 2015; accepted November 8, 2015 Published online March 30, 2016

Acknowledgments

We thank numerous individuals for their invaluable help in this project, especially E. Larsen, L. Mamer, D. Daw, T. Lamansky, J. Cassinelli, J. Graham, T. Gibson, and R. Schiferl. M. Corsi and J. Messner provided earlier reviews of the manuscript. Funding for this work was largely provided by anglers and boaters through their purchase of Idaho fishing licenses, tags, and permits, and from federal excise taxes on fishing equipment and boat fuel through the Sport Fish Restoration Program.