An Assessment of Avian Predation on Juvenile Salmonids: A Spatially-Explicit Analysis of Smolt Losses in Relation to Survival Standards

To address concerns over the impact of avian predation on endangered salmonids in the Columbia River basin, we evaluated predation rates by birds from 10 piscivorous waterbird colonies on the survival of juvenile steelhead, yearling Chinook, and subyearling Chinook between Rock Island Dam and The Dalles Dam during 2012 and 2014. Fish were tagged as part of survival studies using the Juvenile Salmonid Acoustic Telemetry System; A network of hydrophones that provided precise, spatially-explicit survival estimates. Last known detections of tagged fish subsequently recovered on bird colonies were used to quantify where predation occurred (e.g., at dams or away from dams) and the over-all impact of birds on smolt survival in relation to regional survival standards. Spatially-explicit predation rates varied by bird species (Caspian tern, double-crested cormorant, American white pelican, California gull, ring-billed gull), colony location, river segment, and fish species/age-class, with colony-specific predation rates ranging from < 1% to > 8% of available fish. Cumulative impacts (all colonies combined) indicated that bird predation was a substantial source of smolt mortality, with upwards of 60% of all smolt mortality attributable to birds nesting in the region.