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.

Predator swamping reduces predation risk during nocturnal migration of juvenile salmon in a high‐mortality landscape

“…Avian predators also feed on smolts and often select for intermediate or even large smolts (Hostetter et al . ; Osterback et al . ).…”

“…Mortality of migrant smolts can be substantial due to a suite of piscivorous fishes and avian predators (Beamesderfer, Ward & Nigro ; Hostetter et al . ; Osterback et al . ).…”

Linking individual size and wild and hatchery ancestry to survival and predation risk of threatened steelhead (Oncorhynchus mykiss)

“…This hypothesis is tempered by other studies that have shown hatchery fish even just one generation removed from the wild can result with maladaptive antipredator responses (Jackson and Brown 2011). One important factor that may lead to higher predation risk on wild fish verses hatchery steelhead is predator satiation, where increased abundance of outmigrating juveniles may reduce the risk of any one fish to predation (Ryan et al 2003;Hostetter et al 2012). …”

“…3). Predation risk and survival of salmonids can be affected by many environmental and biological factors, including stream flow (Connor and Tiffan 2012;Hostetter et al 2012), upwelling (Scheuerell and Williams 2005), and the abundance of prey (type II or III functional response; Holling 1959). Demographic stochasticity can also contribute to variability in mortality rates and contribute to extinction risk, particularly for small populations (Lande 1998).…”

Predator-prey interactions between harbor seals and migrating steelhead trout smolts revealed by acoustic telemetry

“…The large body size of migrating steelhead smolts limits the diversity of potential predators capable of consuming large percentages of the steelhead smolt populations to a few birds (e.g. Caspian terns Hydro progne caspia, ; Western gulls Larus occidentalis, Osterback et al 2014; and cormorants Phalacrocorax auritus, Hostetter et al 2012) and marine mammals. Some work in both large and small estuaries infers predation by harbor seals based on the 'behavior' of tags from active telemetry tracking (Melnychuk et al 2013) and associations between mortality hotspots and harbor seal haul-outs (Romer et al 2013).…”