Ducks top salmon predator, study says
July 1, 2004
"They are the unknown predator that you've had in your system the whole time," Parrish told commissioners with the Chelan County Public Utility District on Monday. "Yet there is no management of the merganser in the Columbia."
The utility hired Parrish in 2002 to study bird predation on salmon. The $3 million study is the first comprehensive look at salmon-eating birds on the Columbia, and the results likely will be used by dam operators throughout the river system to manage the predators.
The work was expected to focus primarily on California gulls, ring-billed gulls, Caspian terns and double-crested cormorants.
Parrish said that when she started the study, she expected to verify a commonly held belief that gulls were the greediest salmon-eaters among all the birds.
Also surprising was that the feed was occurring in the calm reservoirs behind the dams, rather than the tailraces, where fish often lie dazed after their journey through the turbines or bypass system.
Eighty percent of all the fish eaten by birds are caught in the reservoirs behind the dams, not below them, the study found.
The utility's wildlife experts were surprised by the findings.
"For years, we've thought that all the mortality occurred right below the dams, and that it was gulls and terns doing all the eating," Todd West, the utility's fish and wildlife supervisor, told The Wenatchee World for a story in yesterday's edition.
For the study, Parrish observed birds along 62 miles of river between
Rock Island and Wells dams, killing about 1,500 birds to examine their
stomach contents and fat tissue.
Among all birds, gulls also eat 75 percent of northern pikeminnow, a fish that preys on salmon, so Parrish also cautioned officials that salmon could suffer in other ways if they try to reduce gull populations.
Parrish also exposed the misconception that the larger cormorants and terns were eating volumes of endangered young salmon and steelhead.
She said that while the migratory birds have voracious appetites, they don't show up in the mid-Columbia until later in the summer, after the small salmon and steelhead have passed through the reservoirs.
Parrish advised the utility to use a habitat approach to control mergansers and gulls to push them away from the areas where they traditionally feast on fish.
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