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...