Invasive mud snails tiny, but pose risk for local streams

The effects of New Zealand mud snails can be seen on the leaves above. - Courtesy Photo, City of Bellevue
The effects of New Zealand mud snails can be seen on the leaves above.
— image credit: Courtesy Photo, City of Bellevue

Kit Paulsen plunges her hand into a stream of cold water. She pulls up a yellow leaf and draws it closer to the camera, pointing to several brownish-black specks.

The shells, shaped like ice cream cones, range in size from a grain of sand, to one-fifth an inch. They’re New Zealand mud snails, says Paulsen, a Watershed Planning Supervisor of Bellevue Utilities.

“Once mud snails are in a stream, there’s nothing we can do to get rid of them. They’re there,” explained Paulsen. “They have no natural predators that keep them in balance.”

New Zealand mud snails, an invasive species barely the size of a pin-head, were first identified in Kelsey and Valley creeks in August. Though tiny, mud snails are known to reproduce at a rapid pace, damaging a fragile ecosystem as they multiply.

The risk, says Paulsen, is twofold. Mud snails may compete with other native species for food resources and when ingested, they cause fish to lose weight. Their petite, hard shells make them difficult to digest, often passing through the fish unharmed. Paulsen explains that fish may digest high volumes of the mud snails, but gain very little nutritional value. In a study conducted on rainbow trout, 91 percent of the mud snails passed through intact, and 50 percent survived digestion.

The long-term consequences are unknown.

“It’s difficult to understand what the effects are…There’s very little money out there to do research,” said Allen Pleus of the Washington Department of Fish and Wildlife. “In many invasive species, it can take decades and sometimes up to a century [to see the harm done].”

Mud snails were first found in Olympia’s Capitol Lake in 2009, but the nature of the infestation made them much easier to control. The lake was closed down and fencing put up in some public areas. The following year it was drained and filled with saltwater in an attempt to kill off the species. Kelsey Creek poses a more complicated risk.

“When dealing with Kelsey or Thornton Creek, your ability to actively manage the snail is basically nonexistent at this point,” said Pleus. “In an urban landscape, it’s very difficult to contain those systems.”

The snails have no natural predators and can tolerate a range of climates, temperatures and water qualities, making them particularly difficult to eradicate. According to the WDFW website, snails can reproduce so quickly that after four years in an environment a population of 2.7 billion may exist. Paulsen says that the mud snails were likely spread by humans — hitchhiking on pets or clothing, or on the gear of researchers and staff involved in the restoration and scientific work conducted along both waterways.

But while it is too early to assess the effects of this invasive species, Paulsen maintains a cautionary tone. The snails have already spread at an alarming rate and they could complicate the city’s salmon recovery efforts.

“When you have a species that’s endangered or threatened, they’re already highly susceptible to minor changes in their environment,” explained Pleus.

In the meantime, both Paulsen and Pleus agree that the best approach is education and outreach with residents. The resilient snails can live for weeks on damp gear, so she suggests avoiding entering streams, when possible. Clothing should be thoroughly scrubbed down after exiting any body of water. And gear should be rinsed and allowed to dry for 48 hours or more.


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