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The mortality rate was 12 percent for the minuscule snails gathered at 18 sampling stations near the Fifth Avenue Dam and Marathon Dam, said Allen Pleus, aquatic nuisance species coordinator for the state Department of Fish and Wildlife.
A team of scientists trying to control or eradicate the snails before their explosive population smothers the bottom of the lake and disrupts the aquatic food chain had hoped for a 50 percent die-off.
“In the short term, back-flushing the lake doesn’t look like a viable option,” Pleus said.
The lake was drained more than two weeks ago, then backfilled with marine water containing more than 20 parts per thousand of salt.
The snails, no larger than a grain of salt, can tolerate brackish water up to 15 ppt but start to die off after at least 24 hours of exposure to 24 ppt or more of salt.
The Deschutes River flows quickly diluted the salt concentrations, making it hard to achieve lethal doses, Pleus said.
“We need 48 hours of high salinity to do the job,” Pleus said. “When you back-flush the lake in the wintertime, freshwater returns too quickly.”
Filling the drained lake with saltwater might still be a useful tool to control snail populations in a dry summer when river flows are lowest, he said.
The study also showed that snails that survived the back-flush and were then subjected to salty water in the laboratory were more tolerant of salt, he said.
Mortality rates rose only slightly, to 15 percent, in test plots treated with rock salt during the back-flush, Pleus said.
City, state and federal scientists will meet next week to start crafting a long-term plan aimed at controlling the snails, which are found in concentrations too high to count and heaviest in the north basin.
“We know freezing works, and we’re not real interested in using chemicals,” said Nathaniel Jones, a planner for the state Department of General Administration, a state agency that manages the 260-acre man-made lake as part of the Capitol Campus.
Jones said the back-flush experiment suggests the snails could survive, regardless of whether the long-range decision is to manage the water body as a lake or estuary.
The snails were discovered late last fall but probably had been there for about two years. It is the only known infestation in the Puget Sound region.
They are asexual, have no known predators and can live outside the water long enough to be transported somewhere else on anything that enters the water, such as a boat or a dog’s paw.
John Dodge: 360-754-5444
jdodge@theolympian.com
