Columbia River Estuary

The numbers of double-crested cormorants nesting in the Columbia River estuary have increased dramatically in the last two decades. During the period 1997-2010 the cormorant colony on East Sand Island increased 170% to ca. 13,600 breeding pairs, the largest known breeding colony for the species in western North America. Although juvenile salmonids represented only ca. 10% of the diet of cormorants nesting on East Sand Island in 1999-2010, estimated smolt consumption by the cormorant colony (19.2 million smolts; 95% c.i. = 14.6–23.8 million) is now greater than that of the Caspian tern colony on East Sand Island. The large numbers of smolts consumed by the East Sand Island cormorant colony are due to both the larger size of the cormorant colony and the greater food requirements of cormorants relative to Caspian terns. The double-crested cormorant colony on East Sand Island has experienced high nesting success in recent years (2.1 young raised/breeding pair in 2001-2010), perhaps contributing to the increase in colony size and its current and future impacts on smolt surival.

Resource management agencies decided that management of the large colony of double-crested cormorants on East Sand Island is warranted in order to reduce losses of ESA-listed juvenile salmonids in the Columbia River estuary. Elsewhere in North America, management of double-crested cormorants has consisted primarily of lethal control (i.e., shooting of adults, egg oiling, and destruction of nests in trees). Non-lethal management approaches, such as relocating a portion of the colony to alternative colony sites along the Pacific Coast, seem more appropriate in the context of the cormorant colony on East Sand Island, whose very large size has been at the expense of other colonies in the region. Also, the size of the Pacific Coast population is more than an order of magnitude less than the cormorant population that inhabits central and eastern North America. Pilot studies designed to test the feasibility of employing habitat enhancement and social attraction (i.e., decoys, audio playback systems) to relocate nesting cormorants have had mixed results; cormorants were attracted to and successfully nested on Miller Sands Spit, an island in the upper Columbia River estuary, where no successful cormorant nesting had previously occurred. But social attraction techniques failed to establish a new cormorant colony on a floating platform at Fern Ridge Reservoir and Summer Lake. Colony relocation will not succeed as a management action if suitable nesting habitat outside the Columbia River estuary is not either available or provided.

As was the case with Caspian tern management in the Columbia River estuary, any management of double-crested cormorants to reduce salmonid smolt losses will likely require additional research and NEPA analysis, including assessments of (1) the status of the Pacific Coast population of double-crested cormorants, (2) availability of suitable nesting habitat for the population in the Pacific Northwest outside the Columbia River estuary, and (3) the potential enhancement of salmonid recovery rates in the Columbia River Basin due to management of cormorants in the estuary.