Columbia Plateau

Recent observations indicate that the number of double-crested cormorants nesting on the Columbia Plateau has decreased slightly from 2004-2010. For example, the cormorant colony on the Potholes Reservoir has decreased from ca. 1,160 breeding pairs in 2006 to ca. 830 breeding pairs in 2010.  The size of the colony at Foundation Island on the Columbia River was estimated to be ca. 310 breeding pairs in 2010, the same as the average colony size estimated for this colony over the previous eight years. Smaller colonies have also become established at several other locations on the Columbia Plateau and do not appear to be growing in size.  Nevertheless, salmon managers and local fishers have raised concern over the impact of cormorant predation on survival of salmonid smolts from the Columbia and Snake rivers. While the proportion of salmonids in the diet of Foundation Island cormorants is a third that of Caspian terns nesting nearby on Crescent Island (ca. 66%, average 2000-2010), the estimated number of smolts consumed was similar to or exceeded the number consumed by the tern colony on Crescent Island. This is largely due to the greater food requirements of double-crested cormorants relative to Caspian terns. 

Similar to predation by Crescent Island terns, steelhead were in general more vulnerability to predation by Foundation Island cormorants in 2007-2010. Unlike terns, however, Foundation Island cormorants also keyed in on groups of Chinook salmon (both yearlings and sub-yearlings) migrating through McNary Pool, just below the confluence of the Snake and Columbia rivers. In contrast to the Foundation Island cormorant colony, there is little evidence to suggest that cormorants nesting at the larger colony on Potholes Reservoir are affecting the survival of juvenile salmonids from the Columbia or Snake rivers during the nesting season, based on the paucity of PIT tags from Columbia Basin salmonid smolts recovered at the colony in 2009 (n = 20 smolt PIT tags).

Any management of double-crested cormorants on the Columbia Plateau to reduce smolt losses will require a status assessment of this population in the context of the entire Pacific Coast population and a demonstration that cormorant predation on the Columbia Plateau negatively affects recovery of ESA-listed salmonid stocks.