That crane looming over Interstate 405 Tuesday morning in downtown Renton was used to place a fish trap in the Cedar River that will assist in counting salmon fry moving into Lake Washington.
Using winches, cables and brute strength, a crew with the state Department of Fish and Wildlife moved the trap across the river, where it will remain through July.
The trap is mounted on a platform made of aluminum that is anchored in place by cables attached to the pilings of the I-405 bridge over the Cedar River. They entire trap weighs about 10,000 pounds.
Previously, the trap was just downriver of Logan Avenue, but the river bottom has changed and state officials decided to move it upriver to the Cedar River Park.
For the past 10 years or so the state has used the 5-foot- diameter trap to help monitor the number offspring – fry – produced by the adult salmon, mostly sockeye, that returned the previous fall.
This year’s count is also helping to answer an important question: What impact did the near-record January flooding have on the salmon redds or nests?
The floods “didn’t help,” said Kelly Kiyohara, a fish biologist with the state Department of Fish and Wildlife who was leading the installation.
“The flood has severely impacted the production and out- migration of juvenile salmon,” she said.
Right now, there are no hard numbers to put an exact figure to the loss. However, the flood scoured out nests, put down a layer of sediment that smothered eggs and generally altered prime spawning habitat.
This spring’s out-migration of sockeye fry was expected to be small anyway – about 2 million fish – because it is the product of a small return of adult salmon last fall, about 22,000 fish.
The previous year, about 40,000 adults returned, resulting in a fry count of about 25 million fish. That reflects the variance in each of the runs. The low count this year will be felt in the return of sockeye adults in about four years, what Kiyohara calls a “vicious cycle.”
The young fish will spend time in Lake Washington before they are ready to head to the Pacific Ocean, where they will mature – and face the dangers of the open sea.
On the positive side, what she has seen in terms of monitoring is that fresh-water survival of the fish so far isn’t as bad as what she had expected. And, it’s better than in previous years with high river flows.
The fish traps, which vary in size depending on the depth of the river, are widely used on rivers in the state to track the production rates of salmon. Half the trap is underwater.
The trap basically acts as a screw, drawing the fish into a holding box, where they are counted twice a day and released. Using a system of “tagged” fish, the state is able to estimate the outmigration of young salmon.
The state will trap young fish through July. Then, starting in August or early September, the state and Seattle Public Utilities again will install a fish weir just upriver from the fry trap to count the next class of returning adults.
Safety for recreationists, including innertubers and rafters, is a big issue for the state, Kiyohara said.
Mounted on the trap are two gates that will direct errant boats or rafters away from the trap. There’s a sign hung from the pedestrian bridge under the I-405 freeway, warning recreationists to move to the left just downriver from the Carco Theater.
King County is paying for the trap on the Cedar River and one on Bear Creek in northeast King County. The Cedar is the largest spawning ground for sockeye in the Lake Washington watershed.
