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Sea Lions Avoid Traps In Favor Of Dam's Concrete Pad; Stellers Taking More Salmon
Columbia Basin Bulletin May 8, 2009
There have been 11 California sea lions trapped and removed from the area below Bonneville Dam on the lower Columbia River so far this spring but in recent weeks state biologists' trapping efficiency has dropped to zero.
The Oregon-Washington effort began March 10 and produced success in three of the first four weeks by capturing specific animals that were targeted for removal. But lately none of the big pinnipeds has lingered aboard any of the three floating cages long enough for the trap doors to be triggered.
The last animals captured, on April 16, were two California sea lions that were not eligible for removal. They were branded and released.
The California sea lions, and Steller sea lions, had often used the traps early in the season to "haul out" and rest between forays to prey on salmon and other fish below the dam. But nowadays they seldom frequent the cage platforms, and are instead plopping down on a concrete pad along the dam's corner collector or "rafting" -- forming their own multi-mammal flotation device -- nearby.
"I think it's a combination of things," the Oregon Department of Fish and Wildlife's Charlie Corrarino said of the sea lions' sudden lack of interest in the traps, and the lack of trapping success.
"We've got the trap-happy animals -- the ones that said 'hey, that's a cool place,'" Corrarino said. In all 22 California sea lions have been removed from the area since the states received authority in March 2008 to lethally remove individually identifiable animals preying on salmon and steelhead stocks listed under the Endangered Species Act.
Last year seven sea lions were trapped intentionally and six were shipped off to zoos and aquariums. One died while under anesthesia during a post-trapping medical examination. Four others died of heat exhaustion when trap doors inadvertently, and inexplicably, were tripped and the animals were caged together overnight.
The 22 animals represent roughly 20 percent of the California sea lions that are known to have made repeat visits to the dam in the late winter and spring.
Corrarino said that consistent cold, blustery weather this spring may be causing the animals to seek shelter in the water instead of exposing themselves to the elements aboard the traps, which float along the north shore of Cascades Island. It juts straight downriver from the dam. The corner collector is a passage channel for juvenile fish migrating toward the ocean. Its long flume runs down the island.
Some of the animals also may have exited the area because the salmon run has been very late to arrive. The sea lion population's peak at the dam coincides generally with the peak of chinook salmon passage. The pinnipeds prey on salmon that mill below the dam in search of its fish ladders.
Or they could have figured out boarding the rafts is not a wise decision. The sea lions are clever creatures, "but I think that's probably giving them too much credit," Corrarino said.
Despite the poor recent success, state crews will continue the trapping, probably until about the end of May. By then most of the animals will have begun their journey to the Channel Island breeding grounds off the coast of Southern California. The crews have been manning the traps two to three days per week.
Little can be done this year, but the U.S. Army Corps of Engineers is mulling options for preventing sea lion access to the concrete pads next year.
The dynamics of the visiting sea lion populations are a bit different than in years past.
"California sea lions numbers have been relatively steady, and much lower than previous years, likely due to the removal of 11 animals this year and 11 last year, while Steller sea lion numbers are higher than previous years," according to the May 1 weekly update from Corps researchers at the dam. The research, ongoing since 2002, aims to evaluate the impact the sea lions are having on salmon and steelhead.
In years past California sea lions have greatly outnumbered Steller sea lions. The record California count was 106 individually identifiable sea lions in 2003. That number has ranged to as low as 69. This year only 47 California sea lions have been spotted.
On the other hand, 10 or fewer Stellers visited the dam each year until that number jumped to 17 last year. This year that total is 26.
In the past the California sea lions have focused almost exclusively on salmon and other smaller fishes while the Stellers concentrated on white sturgeon. The trend is generally the same this year although the Steller sea lions have for the past month or so been taking more salmon, according to Robert Stansell, who leads the Corps research.
Last year Stellers were seen taking 162 salmon over the entire season. Observers atop the dam tally, if possible, prey species being taken by the marine mammals. This year already 175 chinook have been taken.
A shift that sees more Stellers taking more salmon is "somewhat alarming," Stansell said.
"So far this year, salmonids have made up about 16.1 percent of the diet for Steller sea lions (excluding all the additional ones they steal from California sea lions), compared to 3.8 percent last year and lower for previous years," the weekly update says.
Another trend is unchanged, and that could be a good sign, Stansell said Thursday.
"Up to 13 of the California sea lions appear to be new visitors to Bonneville Dam, with the remainder repeats from previous years," according to the May 1 update. That means that roughly one-third of the sea lions seen this year are making their first visit. That ratio is similar to that of past years, meaning fewer new and repeat visitors made the 146-mile trip from the ocean to the dam this year.
The take of chinook salmon is also down this year. The sea lions have been observed taking 1,875 through the end of April this year; last year the total through April 28 was 2,416.
Page Updated: Sunday May 10, 2009 03:14 AM Pacific
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