Reported Fish Kill A Mystery

The cause of a fish kill in Lake Washington this week may remain a mystery, with no solid links to the start of a seismic survey using high-powered airguns.

After getting reports of hundreds of dead and dying juvenile fish on Tuesday, scientists scoured the lake Wednesday but found no fish carcasses.

Without bodies to examine, it’s impossible to determine what killed the fish, said Frank Shipley, director of the U.S. Geological Survey’s Western Fisheries Research Center in Seattle.

“At this point, we just don’t have anything to go on to try to investigate it further,” he said.

Witnesses reported seeing 200 to 300 dead and dying fish in two spots about 3-1/2 miles apart: between Mercer Island and Medina on the east side of the lake, and near the Evergreen Point floating bridge.

The first sightings were reported shortly after the start of a multi-agency earthquake-research project, in which a ship fired blasts of compressed air into the water to probe rock formations and geologic faults.

The powerful, low-frequency sound waves penetrate the ground. Seismometers are used to track the vibrations and the way they move, allowing geologists to construct a three-dimensional picture of underground structures.

Researchers had developed safety zones for marine mammals, such as seals and killer whales. They didn’t expect the tests to cause widespread harm to fish.

Lead scientist Mike Fisher said the air blasts could kill or stun fish within 10 feet of the vessel, but he said he had never seen dead fish float to the surface in the ship’s wake.

Kevin Amos of the state Department of Fish and Wildlife said it was “ludicrous” not to expect fish deaths.

“If there is something in the vicinity, it’s going to be stunned,” he said. “Those blasts are pretty violent.”

One dead fish was collected near Mercer Island, and a state expert was conducting a necropsy to find out what killed it, Amos said. But it was so far from the ship there’s very little chance the blasts caused its death.

Lake temperatures were normal, and there was no evidence pollution or toxic chemicals killed the fish, or that oxygen levels in the lake caused its demise.

Most of the dead fish were probably a species called peamouth chub, which resemble squawfish. Other species in the lake this time of year include young chinook, sockeye and coho salmon and adult steelhead, Amos said.