[6] Sea Lions Eat More, Enjoy It Less
A new report from the Corps of Engineers has estimated that sea lions around Bonneville Dam consumed nearly 5,000 spring chinook this year, or about 2.7 percent of the spring chinook run.
Charged with investigating the eating habits of the sea lions and reducing their predation on adult salmon at the same time, the Corps has used steel screens, loud noises, firecrackers, boat harassment, night vision goggles, monoculars, scopes. thermal imaging, and high-powered spotlights to both chase them away and see how much carnage they are responsible for.
After a court battle with the Humane Society, a federal judge approved the lethal removal of some of the biggest, hungriest sea lions. But that drastic action hasn't really helped much, either.
Readily admitting to using many assumptions in its analysis, the report estimated that the 25 sea lions removed over the past two years has allowed a thousand or more extra spring chinook to pass the dam. That's only about .3 percent of the 297,000 spring chinook adults counted passing the dam in 2008 and 2009.
Twenty sea lions were captured this year, six were branded or tagged and released, four were removed to aquariums and 10 were euthanized after it was determined they carried diseases that made them undesirable for relocation.
More Stellar sea lions showed up near the dam this year, which made up for a reduction in California sea lions, but the report says they caught more salmon per individual. In April, 82 different sea lions had been observed, less than last year's 101, but close to the numbers seen between 2005 and 2007.
According to the report, "The removal of 25 sea lions between 2008 and 2009 failed to reduce the overall salmonid consumption estimate. However, those same 25 California sea lions account for only about 7 percent (25 of 355) of the sea lions identified over the years."
Since these removed animals had been at the dam longer and ate more than other sea lions, the report says consumption estimates would have been higher if they had not been removed.
Evidently, this year's 2.7 percent predation by marine mammals at the dam is down slightly from last year's 3.2 percent. In 2007, researchers pegged their consumption at 4.7 percent and 3.1 percent in 2006 (around 3,000 fish).
Stellar sea lions ate an estimated .3 percent of the spring chinook run. They prefer to dine on white sturgeon earlier in the season. Researchers say they ate around 1,700 or so.
The report also says the sea lions ate a few lamprey, with estimated consumption around 100, but it said that number may be significantly underestimated.
One well-known sea lion even hitched a ride on the back of a towboat going through the lock at Bonneville Dam last May, and was later seen often in the dam's forebay or further upriver, as far as The Dalles Dam spillway. He was last seen in mid October.
However, salmon predation by marine mammals in the 146-mile stretch of the river below the dam is a big unknown. What is known is that 1,500 or so sea lions hang out down there. NOAA Fisheries scientist John Ferguson, speaking at a Power Council-sponsored confab on Columbia estuary science and policy, said those 1,500 sea lions may be eating 20,000 spring chinook every year
A 1999 NMFS report said that predation by marine mammals on salmon was difficult to quantify, but was undoubtedly much less in the open ocean, rather than estuaries where salmon congregate during seasonal migrations. A 1993 study had estimated that harbor seals alone, about 3,000 individuals, consumed nearly 23,000 adult spring chinook in the Columbia River, with about one-quarter of them assumed to be heading for the Snake.
Biologists say sea lions can eat about six salmon a day, if the fish are available.
In 2007, one sea lion, known as C 265, who had been in the neighborhood for the past five years, was trapped near Astoria, and weighed in at 559 pounds. After being radio-tagged and tracked -- the pinniped was down near Newport, Oregon in late March, but spent most of his time in April and May feeding on salmon at Bonneville -- he was trapped once again at Astoria on May 21 and tipped the scales at 1,043 pounds, a gain of 484 pounds in two-and-a-half months.
After he showed up again this year, C 265 was the first to be euthanized.
Meanwhile, back at Bonneville, the report recommends reducing areas where the marine mammals can haul out, like along PowerHouse II's corner collector, to make it easier to trap them and reduce their ability to hang out near the dam. It also said that the devices which created loud noises at fishway entrances to deter sea lions didn't really work, although their use on a more random basis might be more effective. -B. R.
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