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

Abstract

AbstractIdentification 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.