Foraging patterns of Caspian terns and double-crested cormorants in the Columbia River estuary

Northwest Science
By: , and 



We examined spatial and temporal foraging patterns of Caspian terns and double-crested cormorants nesting in the Columbia River estuary, to potentially identify circumstances where juvenile salmonids listed under the U.S. Endangered Species Act might be more vulnerable to predation by these avian piscivores. Data were collected during the 1998 and 1999 breeding seasons, using point count surveys of foraging birds at 40 sites along the river's banks, and using aerial strip transect counts throughout the estuary for terns. In 1998, terns selected tidal flats and sites with roosting beaches nearby for foraging, making greater use of the marine/mixing zone of the estuary later in the season, particularly areas near the ocean jetties. In 1999, cormorants selected foraging sites in freshwater along the main channel with pile dikes present, particularly early in the season. Foraging trends in the other year for each species were generally similar to the above but usually not significant. During aerial surveys we observed 50% of foraging and commuting terns within 8 km of the Rice Island colony, and 5% of activity occurred 27 km from this colony in both years. Disproportionately greater cormorant foraging activity at pile dikes may indicate greater vulnerability of salmonids to predation at those features. Colony relocations to sites at sufficient distance from areas of relatively high salmonid abundance may be a straightforward means of reducing impacts of avian predation on salmonids than habitat alterations within the Columbia River estuary, at least for terns. 

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Publication type Article
Publication Subtype Journal Article
Title Foraging patterns of Caspian terns and double-crested cormorants in the Columbia River estuary
Series title Northwest Science
DOI 10.3955/0029-344X-81.2.91
Volume 81
Issue 2
Year Published 2007
Language English
Publisher BioOne Complete
Contributing office(s) Oregon Cooperative Fish and Wildlife Research Unit
Description 13 p.
First page 91
Last page 103
Country United States
State Oregon, Washington
Other Geospatial Columbia River estuary
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