Scott Swanson
A new fish weir installed a year ago in Foster Dam is working – mostly, a U.S. Army Corps of Engineers biologist said.
Fenton Khan said the numbers of Chinook salmon and steelhead that have passed through the weir have increased dramatically, and survival rates are up, but so is the number of fish that are injured in the fall down the chute on the river side of the dam.
The $400,000 fish weir, installed in March of 2018, was designed to make it easier for juvenile salmon and adult steelhead to get over the dam on their way to the sea, after research showed the existing one was inefficient at moving fish safely.
Over the first nine months since the new weir was installed, Khan said, researchers ran tests when the lake was at both low pool and full pool, monitoring fish for injury and survival after passage over the dam, and using sensor fish to figure out how fish were being injured in the trip over the dam.
Also, he said, radio-tagged fish were released in the reservoir, which helped researchers track them as they went over the dam and down the river.
The numbers, he said, show “big improvement” in Chinook salmon moving over the dam.
Fifteen to 30 percent of Chinook passed through the old weir. With the new weir, 60 to 90 percent are passing through. The new weir, he said, was designed specifically to help Chinook, which tend to stay deeper in the lake, make the move. The weir is shaped to create water movement that will encourage the salmon to cross the dam.
Improvements for steelhead, which tend to stay closer to the surface, are not as dramatic, but have improved from 50 to 98 percent passing through the old weir to 99 to 100 percent through the new.
“We’re passing more fish, which is a good thing,” Khan said.
The downside, he said, is that more fish appear to be injured by the time they reach the river.
“The fish are hitting the concrete and there’s not enough water flow,” Khan said. “Even though we designed the weir with a deeper, more flow coming over, this is something we didn’t expect. Fish are hitting the concrete (on the face of the dam), rolling down and having a lot of bruises. So even though survival looks fine, we don’t want these fish injured or bruised because, long-term they may die.
“If you get run over by a car and don’t go to the hospital, a week later…”
He said researchers used sensor fish to determine what was happening. Both salmon and steelhead are being injured in the fall he said.
“They’re kind of bouncing down. Lots of bruises, scrapes, head bruises.
Khan said he is working with engineers to fix the problem, possibly by creating a deeper channel for water in the spillway chute. He said any actual construction would take place later in the year because right now is the height of the fish passage season.
The weir is located in the spillway on the south side of the dam and so the center spillway, which is used to generate electricity, is being opened to fish at night.
“Most of these fish, about 98 percent, pass at night,” he said. “During the day they close the spillway to generate power. At night they don’t generate power because we don’t want fish going through the turbines.
“If we’re injuring the fish, we don’t want to do that. We’re using our regular spillway to spill at night when we’re not generating electricity.”