Susceptibility of juvenile steelhead to avian predation: Are birds only taking the sick and injured?

Identification of individual fish characteristics and environmental conditions associated with increased susceptibility to predation can aid in recovery efforts for threatened populations. To investigate these associations, juvenile steelhead (Oncorhynchus mykiss) were tagged (passive integrated transponder), externally examined, and released back into the lower Snake River to continue out-migration in 2007, 2008, and 2009 (n=25,909). Recoveries of fish tags on bird colonies indicated that at least 4.9% and 2.6% of released steelhead were consumed by Caspian terns (Hydroprogne caspia) and double-crested cormorants (Phalacrocorax auritus) nesting at nearby colonies, respectively. Results indicated that steelhead susceptibility to avian predation increased with declining physical condition of fish, whereby fish exhibiting external damage or disease were more likely to be consumed by avian predators compared to relatively undamaged fish. In addition to condition-dependent susceptibility, differences based on smolt rear-type (hatchery or wild), fork length, run-timing, and river conditions (turbidity and flow) were also observed. The higher susceptibility of unhealthy steelhead suggests that a proportion of smolt mortality attributed to avian predation is compensatory. Consideration of the results from this study may prove to be important in the development of future management plans for this federally-listed population.