State wildlife officials say this week’s freezing weather could kill off half or more of an invasive New Zealand mudsnail population afflicting Capitol Lake in downtown Olympia.
With temperatures falling into the teens overnight, the theory is that the mudsnails — which are the size of a grain of rice — will simply die of cold.
To maximize the effect, the Department of Enterprise Services began draining the lake Tuesday, and by Wednesday morning a scientist from the Department of Wildlife was inspecting the newly uncovered shore areas where the tiny mollusks live.
Wildlife’s aquatic invasive species coordinator Allen Pleus did some poking around on the beach areas of the lake on Wednesday morning. He said later that the cold spell — which hit 15 degrees early Wednesday morning and is predicted to continue through the weekend — is promising.
“The situation is good. It’s dry. We are not getting rain or snow,” Pleus said, adding that rain fills up the basin more quickly with warmer water, and snow provides an insulation blanket for the snails. “At the end of this if we got 50 percent mortality, I’d be really pleased. We might get higher mortality around areas that are more exposed.”
The mudsnail is considered a potential threat to other species because it competes for food such as algae and aquatic insects that are eaten by other species in the food chain. In theory, this could affect caddisflies eaten by young salmon passing through the lake.
The areas of the lake with the worst infestation are in the north basin, but the snails are spreading inside the lake. Jim Erskine, spokesman for Enterprise Services, said a survey this year found evidence of the snails spreading to the lake’s south basin and Percival Cove but not into the Deschutes River, Percival Creek or Black Lake.
As part of its effort to contain the snail, the state has tried other methods, such as flooding the lake with salt water to kill off the snails, whose appearance in the lake is still a bit of a mystery.
The snails were discovered in the lake in 2009, and no one knows whether it was a fisherman, dog, waterfowl or something else that brought the snail to the lake from another infested area.
It was after that discovery that the lake saw its last really good stretch of freezing weather, but the drawdown didn’t start until after a 3-inch layer of ice formed, sheltering the snails. But where the ice was removed, samples showed 98 percent mortality, Pleus said.
With a drawdown beginning early this time, the hope is for better results.