A System-Wide Evaluation of Avian Predation on Juvenile Salmonids in the Columbia River Basin Based on Recoveries of Passive Integrated Transponder (PIT) Tags

The Columbia River estuary is home to the largest known colonies of Caspian terns and double-crested cormorants in western North America. Previous research has demonstrated that terns and cormorants in the estuary consume millions of juvenile salmonids annually, including populations that are federally protected by the U.S. Endangered Species Act. Piscivorous waterbirds nesting populations are not limited to the estuary, however, as several other large avian colonies reside upstream of Bonneville Dam in the Columbia Plateau region. The impact to salmonid survival from birds nesting at these inland colonies is less understood.  

We recovered passive integrated transponder (PIT) tags on nine different avian colonies – two in the estuary and seven at inland sites – to evaluate predation on salmonid smolts during 2004-2009. PIT tags were used to calculate predation rates (percentage of available smolts consumed) based on the number of PIT-tagged smolts detected passing hydroelectric dams that were subsequently recovered on bird colonies downstream of the dam. Results indicated that terns and cormorants on East Sands Island in the estuary consumed the highest percentage of available smolts.  Predation was highest on Snake River steelhead, with combined losses to terns and cormorants estimated at 18% of available smolts.  Other bird colonies with notable predation rates included three inland bird colonies: a tern colony on Crescent Island, a tern colony on Goose Island, and a cormorant colony on Foundation Island, each with predation rates ranging from 1 to 10% of available smolts. Predation rates on smolts by two inland California and ring-billed gull colonies and one inland American white pelican colony were minor (generally < 1%) in comparison to predation rates by tern and cormorant colonies.

Per capita predation rates (i.e., predation rates per bird) were substantially higher at inland avian colonies compared to estuary colonies. Variation in per capita predation rates is related to differences in diet composition of avian predators among colonies. Differences between actual predation rates and per capita predation rates indicate that current management efforts to increase smolt survival through reductions of avian predation in the estuary would be offset if birds nesting in the estuary relocated to inland sites, as increases in bird abundance at inland sites could have a dramatic and negative impact on salmonid survival, particularly on Snake and Upper Columbia steelhead stocks.