Relative Vulnerability of PIT-Tagged Subyearling Fall Chinook Salmon to Predation by Caspian Terns and Double-Crested Cormorants in the Columbia River Estuary

Abstract

We quantified the percentage of PIT-tagged subyearling fall Chinook Salmon Oncorhynchus tshawytscha that were consumed by Caspian terns Hydroprogne caspia and double-crested cormorants Phalacrocorax auritus nesting on East Sand Island in the Columbia River estuary by electronically recovering PIT tags that were deposited on the bird colonies. We released 23 groups of PIT-tagged subyearling fall Chinook Salmon from hatcheries in the lower Columbia River downstream of Bonneville Dam from 2002 to 2010. Vulnerability to avian predation was compared between PIT-tagged subyearlings of two Columbia River basin stocks: tule and upriver bright (URB). Recoveries of PIT tags revealed that overall predation rates were significantly different between the tule stock (22%) and URB stock (3%); for fish that were detected as entering the lower Columbia River during the same week, predation rates also differed between stocks (tule: 21%; URB: 2%). Minimum predation rates on tule subyearlings originating from hatcheries downstream of Bonneville Dam were among the highest documented for any salmonid species in the Columbia River basin to date, occasionally exceeding 35% of the available fish. The ratio of URB fish consumed by the two avian predators indicated that the percentages were nearly equal (cormorant [%]: tern [%] = 51:49), whereas the ratio for tule-stock fish consumed by the two avian species was not uniform (cormorant: tern = 81:19). Differences in predation rates between the tule stock and the URB stock may be attributable to migration behaviors exhibited in the estuary. We estimate that more than 8 million tule fall Chinook Salmon subyearlings released from hatcheries annually are consumed by double-crested cormorants and Caspian terns nesting on East Sand Island; ongoing management actions by multiple federal, state, and tribal governments, if successful, will decrease predation on fall Chinook Salmon stocks.

Received October 9, 2012; accepted May 8, 2013

ACKNOWLEDGMENTS

This research was supported with funding by the U.S. Army Corps of Engineers (USACE) Walla Walla District. We thank our USACE representatives: Scott Dunmire, Chris Pinney, and David Trachtenbarg. In addition, we are grateful to the numerous support staff from NOAA Fisheries and PSMFC as well as Ken Collis, Daniel Roby, and biologists from Real Time Research and Oregon State University. We thank hatchery personnel from the Oregon Department of Fish and Wildlife, Washington Department of Fish and Wildlife, Sea Resources, Clatsop Economic Development Council, and Warrenton High School for allowing us to PIT-tag their fish. We also thank April Cameron, Rory Cameron, Amy Cook, Matthew Morris, Michael Morrow, Steve Porter, and the Warrenton High School Fisheries Ecology students for their participation in PIT-tagging operations. Finally, we thank Edmundo Casillas, Earl Dawley, G. Curtis Roegner, Laurie Weitkamp, and Jeannette Zamon for providing comments that improved the quality of this manuscript.