Published February 6 at 5:31 AM Updated February 6 at 10:09 AM
The federal government killed thousands of double-crested cormorants living on a Columbia River island between 2015 and 2017 in an effort to help young salmon make it to the Pacific Ocean alive. But Oregon state biologists say the birds just moved upriver—possibly tripling the number of salmon each bird ate.
The killed 5,576 cormorants and destroyed 6,181 nests in an effort to prevent the birds from eating an estimated 12 million young salmon each year as they swim past East Sand Island, just east of the mouth of the Columbia as it flows into the Pacific.
Biologists say the mass slaughter may have caused the collapse of the birds' largest breeding colony. It also may have been for nothing.
James Lawonn, biologist in charge of avian predation for the , is preparing a new study on the effects of the birds' redistribution along the river. (His study was .) Lawonn says his agency "expects little to no gain in survival" from the corps' actions for young salmon swimming through the Columbia River estuary.
That's because cormorants are now living farther upriver—still in huge numbers. And where they live makes a difference. Cormorants that live closer to the ocean choose from an extensive menu of ocean fish that form huge schools in the Columbia estuary, like anchovies, herring and smelt. Upriver, they eat a far higher proportion of salmon and other freshwater fish.
In 2017, during the bird kill, most of the colony on East Sand Island fled, in an event the Audubon Society called a "catastrophic collapse" of the largest population of double-crested cormorants in the world. The corps says there's no direct evidence to link the two events. But the mass exodus came during the corps' third year of shooting thousands of birds out of the air, destroying their nests and setting off explosives on the island.