The semi-annual rite of making repairs and undertaking routine maintenance at the lower Columbia River’s Bonneville Dam’s Powerhouse 2 fish ladders got a little more burdensome in late January with the need to remove some 1,700 white sturgeon – ranging from about a foot long to 10 feet long -- that had, apparently, settled in for the winter.
A crew of 12 people spent four days, Jan. 25-28, wading in knee-deep water to corral the fish and hand-loading them into a water-filled tank. The tank was then repeatedly lifted with a crane over the channel wall so that its load of fish could be released in the tailrace below the dam.
The fish collection channel stretches the entire width of the powerhouse. It has a number of entrances that are intended to draw in spawning salmon and steelhead and steer them toward the facility’s Washington shore fish ladder.
Sturgeon have in recent years been found congregated at various sites below the dam, apparently just keeping each other company as they idle through the winter. And they have been found in the channel before as well, but never in such large numbers as this winter season. About 1,000 white sturgeon were removed from the channel in December.
“This is new territory for us. We’re not used to handling so many fish,” said Ben Hausmann, U.S. Army Corps of Engineers fish biologist at the dam. He said that in most winters since he’s been on the job (2004) the number of sturgeon found in the channel has numbered in the single digits.
The Corps, which operates the dam, routinely removes fish – steelhead and other species as well as sturgeon -- when the fish ladders are dewatered for maintenance and repairs during the December-February work window when there are few spawners passing the dam.
“It’s a pretty new situation,” Hausmann said. “Why they’re in the ladder I don’t know.”
The Corps might not have had to move the big fish but for the need to fix equipment near the bottom of the channel where the sturgeon lurked. Making the repairs required that channel be nearly emptied of water.
The operation went smoothly with no recorded mortalities during the process. A few may have found their way into the mouths of Steller sea lions downstream. The big marine mammals have their own congregations downstream of the dam each winter and spring in recent years, primarily to feed on the ready supply of white sturgeon clustered below the dam.
Observers along the top of the powerhouse saw Steller sea lions take 44 sturgeon during a four-hour stretch on the afternoon of Jan. 26, which was the highest rate so far this year, though there have been spurts of 10 to13 white sturgeon taken per hour at other locations at times this year, according to the Feb. 4 “Status Report – Pinniped Predation and Deterrent Activities at Bonneville Dam, 2011.” Corps research has been ongoing at the dam each winter and spring to evaluate sea lion impacts on salmon and steelhead and other fish species.
Of the 44 white sturgeon taken during that four-hour period, only nine were preyed on in Zone 3, which is nearest the Washington shore, where the fish were being released. The tailrace below the 1,100-foot-wide powerhouse is broken up into seven zones, three (south, middle, north sides) near the dam (from the face of the dam downstream about 100 meters), three in the mid-tailrace, and one area farther downstream.
“So in general, it did seem like a couple of Stellers found and keyed in on sturgeon being released in that area, likely taking some of those that were slow to recover,” the weekly research report says. “However, this release was also going on during the afternoon of January 25, all day including morning of 26th, and the 27th with no unusual predation observed.
“The highest single day of sturgeon predation at the project was on January 11 with 122 sturgeon observed taken. January 26 was 96, the second highest this year. We are averaging about 50 per day so far this year, with several days in the 80's also,” the report says.