Hundreds of juvenile fish died simultaneously in Vancouver Lake near the beginning of the month, according to the state Department of Fish and Wildlife.
It’s not clear how many fish died in the event, but WDFW district fish biologist Matt Gardener said between 200 and 1,000 small juvenile shad are presumed dead.
Shad, a nonnative species that inhabits the Columbia River, were the only species believed to have died.
Gardener said a few of the fish are being studied for clues as to what happened, but the most likely cause is that the fish asphyxiated in low-oxygen water. Officials believe the deaths are not related to an algae bloom.
“What we think is likely the culprit is a turnover event in the lake because of the weather,” he said.
Turnover happens when colder, denser water sinks to the bottom of the water column and warmer water rises to the top. That can lead to low-oxygen conditions. In this case, Gardener said, native fish were probably better adapted to those conditions.
The department was notified about the fish Oct. 3. A member of the public reported seeing hundreds of dead fish washed up on shore.
Gardener said the fish were found primarily along a stretch of the lake close to the flushing channel, which allows fresh water from the Columbia River to flow into the lake. He said he assumes the fish were migrating from the Columbia River to the ocean when they got caught up in the current of the flushing channel and were unable to swim out.
The department plans to leave the fish at the lake, where they’ll be eaten by scavengers and dissolved back into the earth.
Fish die-offs are not an uncommon event and can occur for a variety of reasons, from human-induced pollution to natural conditions.
Gardener said it was the first die-off at Vancouver Lake in recent memory.
The lake has a lengthy history of poor water quality due to algae blooms and high-nutrient loading, among other issues.