Susceptibility of Juvenile Steelhead to Avian Predation: the Influence of Individual Fish Characteristics and River Conditions

Abstract

Identification of the factors that influence susceptibility to predation can aid in developing management strategies to recover fish populations of conservation concern. Predator–prey relationships can be influenced by numerous factors, including prey condition, prey size, and environmental conditions. We investigated these factors by using juvenile steelhead Oncorhynchus mykiss from the Snake River (Pacific Northwest, USA), a distinct population segment that is listed as threatened under the U.S. Endangered Species Act. During 2007–2009, steelhead smolts (n = 25,909) were captured, examined for external condition characteristics (e.g., body injuries, descaling, external signs of disease, fin damage, and ectoparasite infestations), marked with passive integrated transponder (PIT) tags, and released to continue their out-migration. Recoveries of PIT tags on a downstream colony of Caspian terns Hydroprogne caspia (n = 913 tags) indicated that steelhead susceptibility to Caspian tern predation increased significantly with decreases in steelhead external condition, decreased water discharge, and decreased water clarity. Susceptibility to Caspian tern predation also increased with increasing steelhead fork length up to 202 mm but then decreased for longer steelhead. Recoveries of PIT tags on a downstream colony of double-crested cormorants Phalacrocorax auritus (n = 493 tags) indicated that steelhead susceptibility to double-crested cormorant predation increased significantly with declining external condition of steelhead, and that steelhead of hatchery origin were more susceptible than their wild counterparts. Results indicate that steelhead susceptibility to avian predation is dependent on fish condition and length and is influenced by river conditions and rearing environment.

Received February 23, 2012; accepted July 18, 2012

ACKNOWLEDGMENTS

The U.S. Army Corps of Engineers (USACE) Walla Walla District (Scott Dunmire and David Trachtenbarg, Project Managers) provided funding for this research, access to juvenile fish collection facilities, and permission to conduct work on Crescent and Foundation islands. M. Carper, B. Cramer, J. Tennyson, and numerous seasonal technicians provided invaluable assistance in the field. We thank M. Plummer and B. Spurgeon (USACE) for providing access and logistical support at the juvenile fish facilities and S. Sebring (Pacific States Marine Fisheries Commission) and Richard Ledgerwood (National Oceanic and Atmospheric Administration Fisheries) for collaborating on PIT tag recovery. J. Adkins, D. Battaglia, K. Fone, S. Lind, P. Loschl, and M. Price provided services, logistical support, or both, for which we are grateful. The use of trade or product names does not constitute endorsement by the U.S. Government. The study was performed under the auspices of Oregon State University's Institutional Animal Care and Use Committee (Protocol Number 3722). Technical advice and feedback on earlier drafts of the manuscript were provided by D. Noakes and C. Periera.