Use of Radiotelemetry and Direct Observations to Evaluate Sea Lion Predation on Adult Pacific Salmonids at Bonneville Dam

Abstract

Management of protected species becomes increasingly complex when one protected population negatively affects another. This occurs along coastlines and in rivers and estuaries of the U.S. Pacific Northwest, where protected marine mammals prey on threatened and endangered Pacific salmonids Oncorhynchus spp. Over 9 years, we observed a growing aggregation of California sea lions Zalophus californianus and Steller sea lions Eumetopias jubatus preying upon adult Chinook salmon O. tshawytscha and steelhead O. mykiss at Bonneville Dam on the Columbia River. Both before and concurrent with the observation study, we monitored radio-tagged salmon at Bonneville Dam and during their upriver spawning migrations. Springtime sea lion abundance steadily increased from 2002 to 2010 and the aggregation formed earlier each winter. The principal prey species in winter were resident white sturgeon Acipenser transmontanus and migratory steelhead and then shifted to predominantly Chinook salmon when the spring run arrived. Observation-based estimates of salmonid consumption from January to May varied 12-fold among years (0.4–4.9%, mean = 2.6% of adult salmonids counted at the dam), and radiotelemetry results corroborated these estimates. The highest proportional impact was in winter and early spring. As salmonid abundance increased, per capita consumption by sea lions increased (Type II functional response) but individual salmonid risk decreased (due to prey swamping). Population-specific risk analyses indicated predation was substantially higher for early-timed than for late-timed salmon populations. The most at-risk group included Snake River and upper Columbia River Chinook salmon listed as threatened under the U.S. Endangered Species Act. These predation indices should help managers simultaneously tasked with salmon recovery and marine mammal management.

Received December 19, 2011; accepted April 17, 2012

ACKNOWLEDGMENTS

We thank all the observers in the pinniped observation program. They spent many hours in extreme weather conditions to obtain this important information for regional managers. We also thank the U.S. Department of Agriculture for supplying agents to employ harassment activities to keep sea lions away from fishways, and the Oregon Department of Fish and Wildlife, Washington Department of Fish and Wildlife, and Columbia River Inter-Tribal Fish Commission for supplying boats for harassment activities. Special thanks to the U.S. Army Corps of Engineers for designing and deploying sea lion exclusion devices, which have kept sea lions out of the Bonneville Dam fishways since their installation in 2006. We are similarly indebted to individuals at many organizations for gathering and compiling the Chinook salmon data used in this study. We especially appreciate the personnel at the many cooperating hatcheries as well as anglers and tribal fishers for returning radio transmitters. We also thank T. Bjornn, B. Burke, M. Moser, L. Stuehrenberg, A. Matter, K. Frick, and T. Bohn (National Marine Fisheries Service) for their collaboration on the telemetry project. Funding was provided by the U.S. Army Corps of Engineers; we thank D. Clugston, M. Shutters, R. Dach, M. Langeslay, and T. Mackey for their support. At the University of Idaho, T. Bjornn, R. Ringe, K. Tolotti, M. Jepson, S. Lee, C. Boggs, T. Reischel, G. Naughton, W. Daigle, M. Morasch, T. Dick, D. Joosten, and C. Morat helped with telemetry project oversight, field operations, and collection and processing of telemetry data. We also thank the two anonymous reviewers and the journal's associate editor whose constructive comments greatly improved the manuscript.