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AbstractWe initiated a field study in 1997 to assess the impact of predation by Caspian terns (Sterna caspia) and other piscivorous colonial waterbirds on the survival of juvenile salmonids (Oncorhynchus spp.) in the lower Columbia River. Using bioenergetics models, we estimated that Caspian terns nesting on Rice Island, a dredged material disposal island at river mile 21, consumed ca. 12.4 million juvenile salmonids, or ca. 13% of the out-migrating smolts that reached the estuary during the 1998 migration year. The magnitude of predation on smolts by Rice Island terns led to management action in 1999 to relocate the colony to East Sand Island (river mile 5), where it was hoped terns would consume fewer salmonids. By 2001, all Caspian terns nesting in the Columbia River estuary were using East Sand Island. During the 2003 breeding season, the diet of East Sand Island terns averaged 24% salmonids, compared to the former diet of Rice Island terns, which consisted of up to 90% salmonids The relocation of the tern colony from Rice Island to East Sand Island resulted in a major decline in losses of salmonid smolts to tern predation in the Columbia River estuary. Consumption of juvenile salmonids in 2003 was approximately 4.2 million smolts, a ca. 8.2 million reduction in annual smolt consumption compared to 1998.
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.
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