Columbia Plateau

Recent observations indicate that the double-crested cormorant population on the Columbia Plateau is slowly increasing in both the number of breeding pairs and number of colonies. For example, the colony at Foundation Island on the Columbia River has grown from approximately 100 breeding pairs in 1998 to ca. 310 pairs in 2009. The cormorant colony on the Potholes Reservoir has increased in the last decade and now consists of ca. 810 breeding pairs. Smaller colonies have also become established, and nesting attempts have been noted at new locations in recent years. Furthermore, there appears to be a substantial and increasing number of non-breeding cormorants in the region. These trends apparently reflect a general post-DDT era recovery of the Pacific Coast population. 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 less than half that of Caspian terns nesting nearby on Crescent Island (ca. 66%, average 2000-2009), the numbers of smolt PIT tags recovered from the cormorant colony was similar or exceeded the number recovered from the tern colony in 2007-2009. In fact, of all the piscivorous waterbird colonies studied on the Columbia River in 2007, the Foundation Island cormorant colony had the highest per capita consumption rate of PIT-tagged juvenile salmonids (ca. 11.3 PIT-tagged fish per breeding adult). These results suggest that consumption of salmonid smolts by Foundation Island cormorants is similar to, if not higher than, Caspian terns nesting 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 2007-2009. 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.