Searching for survivors in Capitol Lake

OLYMPIA - The first saltwater assault on invasive New Zealand mud snails in Capitol Lake is over, but the effectiveness of the pilot project won't be known for a few days.

The state Department of General Administration began drawing the lake water levels down through the Fifth Avenue Dam early Friday so biologists could take snail samples back to the laboratory from more than two dozen test plots on the lake bottom. The question is, Are the snails dead or alive?

“We probably won’t have an answer until later next week,” said Allen Pleus, aquatic invasive species coordinator for the state Department of Fish and Wildlife.

The goal was a 50 percent die-off for the snails exposed to the saltwater from Budd Inlet, researchers said last week when the lake was filled with saltwater.

The asexual snails, which are no larger than a grain of rice, can tolerate brackish water up to 15 parts per thousand, but they begin to perish when exposed to 24 ppt of salt for more than 24 hours.

On Tuesday, after the lake was drained and back-flushed with marine water, salt concentrations in the lake were 15 ppt to 24.6 ppt beneath the surface of the two lake basins north of Interstate 5, said David Hallock, a natural resources scientist with the state Department of Ecology.

By Thursday, the concentrations ranged from 1 ppt to 19 ppt as more freshwater from the Deschutes River and Percival Creek entered the lake, Hallock said.

The Capitol Lake infestation, first discovered in the fall, is the only known one in the Puget Sound region.

The lake is off-limits to the public and pets to keep the snail from spreading. A fence along the Heritage Park gravel beach in the lake’s north basin will start to go up Monday in a bid to keep people and dogs away from the water, General Administration spokesman Jim Erskine said.

While the lake was drawn down last weekend, it became abundantly clear how easily the snails could be moved to another water body.

For instance:

 • Pleus found 122 snails on his boots after his survey work Sunday.

 • He also counted 200 snails in a 5-inch-diameter scoop of mud he took from a dog’s paw print.

The snails can multiply rapid-ly, have no known predators and can attach themselves to shoes, fur, clothing, boats and other surfaces for easy transport.

If left unchecked, they could form a dense mat on the lake sediments and rocks, choking out tiny invertebrates at the bottom of the food chain.

The results of the back-flush experiment will help determine the next step in snail control, Pleus said.

“We’re hoping saltwater is a part of our future treatment strategy,” Pleus said. “We’re trying to avoid chemicals.”

Normal lake water levels for this time of year should be restored by Monday, state officials say.

John Dodge: 360-754-5444

jdodge@theolympian.com