Caspian tern predation on juvenile salmonids in the mid-Columbia River

We used a bioenergetics approach to determine the magnitude of predation by Caspian terns Sterna caspia on juvenile salmonids Oncorhynchus spp. in the mid-Columbia River during 2000 and 2001. Caspian terns nesting on Crescent Island, Washington, located below the confluence of the Snake and Columbia rivers, consumed several hundred thousand juvenile salmonids each year of the study. Tern consumption of smolts was higher in 2001 (679,000 smolts; 95% confidence interval [CI]: 533,000-825,000 smolts) than in 2000 (465,000 smolts; 95% CI: 382,000-547,000 smolts) as a result of an increase in tern breeding pairs, fledging success, and percentage of salmonids in the diet. On-colony detection rates of passive integrated transponder tags from in-river migrating smolts were also higher in 2001 (0.90-12.40%) than in 2000 (0.03-1.60%); the higher predation rates in 2001 were probably caused by extreme drought conditions that resulted in reduced spill from hydroelectric dams, lower river flows, and increased travel times for in-river migrating smolts. Tern predation rates on juvenile steelhead O. mykiss were higher than those on yearling Chinook salmon O. tshawytscha in both years. The impacts of tern predation on steelhead smolts and yearling Chinook salmon from the Snake River were slight after accounting for the high proportion of smolts collected for transportation above Crescent Island. Survival of steelhead smolts from the upper Columbia River that are not transported above Crescent Island may be significantly affected by tern predation, particularly in low-flow years. Appreciably higher predation rates on salmonids by Crescent Island terns than those observed in 2001 are unlikely considering the constraints on tern colony expansion, limited capacity for increased per capita smolt consumption by terns, and current high transportation rates for Snake River smolts.
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