Corps: Sea Lion Gates
by Kathie Durbin, Columbian staff writer
A two-day experiment that removed sea lion barriers from a fish ladder at Bonneville Dam has prompted the U.S. Army Corps of Engineers to conclude that the barriers are not to blame for record-low counts of returning spring chinook salmon at the dam.
The number of salmon passing the dam jumped dramatically during the 48-hour test, from 60 on Sunday to 444 on Tuesday. But the increased numbers were evenly divided between salmon that passed through the Branford Island fishway on the Oregon side, where all the sea lion exclusion devices, or SLEDs, remained in place, and the Powerhouse 2 fishway near the Washington shore, where two of the 12 devices had been removed.
"Based on the increasing fish counts, we feel pretty confident that the spring migration has begun," said Roger Willis, environmental branch manager for the Corps. "There was almost no difference (Tuesday) between the number of fish moving through Bradford Island and the Washington shore fishways."
Willis noted that the number of fish passing the dam began to increase even before the barriers were removed.
Although the long-awaited migration appears to have begun, Willis said, the number of salmon passing the dam remains low. "It's still a very late run and the numbers are very small."
As of last Sunday, only 488 spring chinook had passed through the dam. At the beginning of the season, biologists predicted 88,400 salmon would enter the mouth of the Columbia River during the spring run.
Stuart Ellis, a fish biologist with the Columbia River Inter-Tribal Fish Commission, said he concurs with the Corps' conclusion that the steel grates were not to blame for the record-low numbers of fish passing the dam.
"Some other factor has caused these fish to start moving," he said. "It's about as strong a result as we could have expected. It seems like it's a pretty good indication that the SLEDs were not adversely impacting the chinook passage."
Ellis, who chairs a group of fish managers that coordinates run size and harvest data, said the increased numbers don't necessarily mean the size of the spring run will meet early projections.
"Because the run timing is so late, and the count has been so low for so long, it's really out of the range of the typical run timing data that we use to help predict these runs," he said. "We don't have any great tools to apply to this year's situation that would allow us to make a prediction one way or another."
In addition to counting fish as they pass the dam, the Corps has begun trapping and radio-tagging small numbers of salmon in the fish ladders, then releasing them downstream near Beacon Rock to learn how quickly the fish travel back up to the dam. New radio antennas located near the fish ladder entrances will allow biologists to monitor fish behavior near the steel grates.
Corps spokeswoman Diana Fredlund said biologists hope to tag 600 returning chinook salmon by the end of the spring run.
The Corps installed the exclusion devices after other tactics, including hazing with firecrackers, failed to drive hungry California sea lions away from the fish ladder entrances.
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