Although numbers of Caspian terns nesting in the Columbia River estuary have remained nearly stable over the last 7 years, the numbers of double-crested cormorants (Phalacrocorax auritus) nesting on East Sand Island have doubled, from ca. 5,500 breeding pairs in 1997 to 10,600 pairs in 2003. Although juvenile salmonids represented only 9% of the diet of cormorants nesting on East Sand Island in 2003, estimated smolt consumption by the cormorant colony is now as high or higher than that of the East Sand Island tern colony. The double-crested cormorant colony on East Sand Island experienced high nesting success in 2003, and is not currently limited by density-dependent feedback on colony size or reproductive success. Caspian terns and double-crested cormorants nesting in the Columbia River estuary in 2003 together consumed an estimated 10 million smolts. To reduce consumption of juvenile salmonids by piscivorous waterbirds in the estuary will likely require relocation of a portion of the terns and cormorants nesting on East Sand Island to alternative sites outside the estuary. Restorat> ion, enhancement, or establishment of colony sites outside the Columbia River estuary would likely benefit both Columbia Basin salmonid stocks and Pacific coast tern and cormorant populations, because both of these protected bird species have declined at colonies outside the estuary.
For the Caspian tern colony on Crescent Island, just below the confluence of the Snake and Columbia rivers, juvenile salmonids represent about 67% of the diet. But the Crescent Island colony is more than an order of magnitude smaller than the tern colony on East Sand Island, and annual consumption rates of salmonid smolts are about an order of magnitude less. Nevertheless, predation rates on Snake River steelhead by this tern colony have been as high as 12.4% of in-river migrants. Impacts of Crescent Island tern predation on Snake River steelhead and yearling chinook ESUs were slight, however, after accounting for the high proportion of smolts collected for transportation above Crescent Island. Nevertheless, survival of steelhead smolts from the upper Columbia River, which are not transported past Crescent Island, may be significantly affected by Crescent Island tern predation, particularly in low flow years.