The coastal states of western North America and Mexico are home to a population of Caspian Terns (Sterna caspia) that are geographically disjunct and probably genetically isolated from other Caspian Terns in North America. About 74% of these birds nest at a single colony, East Sand Island, near the mouth of the Columbia River between Washington and Oregon. It is estimated by others that when the terns nested in 1998 on Rice Island, 26 km further east in the Columbia River estuary, they consumed Steelhead smolts (Oncorhynchus mykiss), Coho smolts (O. kisutch), and spring/summer Chinook smolts (O. tshawytscha) that reached the Columbia River estuary, thus involving species listed under the United States Endangered Species Act. In addition, because such a large percentage of the west coast population breeds at a single colony, there is concern that a natural or anthropogenic event at this colony may adversely impact the entire population. As a result, many federal, state, tribal and private natural resource managers are trying to develop a management plan to disperse a fraction of these terns from East Sand Island to other historic and/or newly created nesting sites throughout western North America. Among factors to be considered, managers must assess the potential impact that terns would have on fisheries, especially salmonids, at potential relocation sites before attempts are made to relocate the birds. This study documents the diet of terns in one prospective relocation area, Commencement Bay, Washington. Relative abundance of prey species, including salmonids, delivered to the colony by adults differed significantly from month to month during the breeding season, but overall terns brought an average of 52% juvenile salmonids back to the colony, a higher percentage of salmonids than are consumed by terns on East Sand Island. Therefore, we conclude that Commencement Bay may not be an appropriate relocation site for Caspian Terns.
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Vol. 25 • No. 1