Colonial Waterbird Predation on Lost River and Shortnose Suckers in the Upper Klamath Basin

Abstract

We evaluated predation on Lost River Suckers Deltistes luxatus and Shortnose Suckers Chasmistes brevirostris by American white pelicans Pelecanus erythrorhynchos and double-crested cormorants Phalacrocorax auritus nesting at mixed-species colonies in the Upper Klamath Basin of Oregon and California during 2009–2014. Predation was evaluated by recovering (detecting) PIT tags from tagged fish on bird colonies and calculating minimum predation rates, as the percentage of available suckers consumed, adjusted for PIT tag detection probabilities but not deposition probabilities (i.e., probability an egested tag was deposited on- or off-colony). Results indicate that impacts of avian predation varied by sucker species, age-class (adult, juvenile), bird colony location, and year, demonstrating dynamic predator–prey interactions. Tagged suckers ranging in size from 72 to 730 mm were susceptible to cormorant or pelican predation; all but the largest Lost River Suckers were susceptible to bird predation. Minimum predation rate estimates ranged annually from <0.1% to 4.6% of the available PIT-tagged Lost River Suckers and from <0.1% to 4.2% of the available Shortnose Suckers, and predation rates were consistently higher on suckers in Clear Lake Reservoir, California, than on suckers in Upper Klamath Lake, Oregon. There was evidence that bird predation on juvenile suckers (species unknown) in Upper Klamath Lake was higher than on adult suckers in Upper Klamath Lake, where minimum predation rates ranged annually from 5.7% to 8.4% of available juveniles. Results suggest that avian predation is a factor limiting the recovery of populations of Lost River and Shortnose suckers, particularly juvenile suckers in Upper Klamath Lake and adult suckers in Clear Lake Reservoir. Additional research is needed to measure predator-specific PIT tag deposition probabilities (which, based on other published studies, could increase predation rates presented herein by a factor of roughly 2.0) and to better understand biotic and abiotic factors that regulate sucker susceptibility to bird predation.

Received January 19, 2016; accepted June 25, 2016 Published online October 7, 2016

ACKNOWLEDGMENTS

This project was funded by the Bureau of Reclamation (BOR), with support from the U.S. Fish and Wildlife Service (USFWS) and the Klamath Sucker Recovery Implementation Team. We especially thank Kristen Hiatt (BOR) and Josh Rasmussen (USFWS) for their assistance and support. We thank John Beckstrand (USFWS) for his assistance in sowing PIT tags to measure detection efficiency at the Clear Lake NWR colonies and for granting us access and providing transportation to islands in Upper Klamath NWR following the nesting seasons. We thank Tim Lawes of Oregon State University and Aaron Turecek of Real Time Research, Inc., for providing assistance with aerial imagery and colony counts. We thank Alta Harris, Amari Dolan-Caret, and Summer Burdick (U.S. Geological Survey, Klamath Falls Field Station) for invaluable assistance regarding the availability of PIT-tagged suckers. We thank Paul Schmidt of the U.S. Army Corps of Engineers for assistance and support on a related but an independent study of Caspian terns in the region. Finally, this work would not have been possible without the hard work and dedication of numerous field researchers, for which we are grateful. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.