Tiny Snails Could Make Big Changes in Spirit Lake and Connected Waterways
Nestled in a rocky valley in the blast zone below Mount St. Helens, Spirit Lake lies isolated from most of the world. It is accessible to the public only by foot via the Harmony Trail on the east side of the lake. And yet, aquatic invaders have somehow made it across the globe into these chilly waters.
The invaders are New Zealand mud snails (Potamopyrgus antipodarum), tiny freshwater snails that were first spotted at Spirit Lake in 2016. These miniature mollusks reproduce quickly, and in some cases they can produce outsized ecological effects.
Along the southwest shore of Spirit Lake, they are so thick that “it almost looks like somebody peppered the bottom of the lake” with their shells observed Jim Gawel, a professor of environmental chemistry and engineering at the University of Washington Tacoma.
Left unchecked, the snails can outcompete other small invertebrates for food. “They kind of eat everything” Gawel explained. “What normally would be eaten by insect larvae and things like that now gets eaten by snails.”
This competition at the base of the food chain has effects throughout the local ecosystem. The snails can displace organisms that would normally be eaten by fish such as trout. Trout can and do eat the snails, but they get little nourishment from them.
“They’re almost indigestible to fish” Gawel said. “The fish eat them, and then they pass right through their system undigested. It’s like empty calories.”
“[Their shells] are like a trap door. Like some other snails, they can kind of close themselves up,” Gawel continued. “And the fish don’t have a way to crunch them up.”
The invasion of Spirit Lake is in it’s early stages, and it’s not yet clear how exactly the snails will interact with the ecosystem. To inform our response to the invasion, a team of researchers recently set out to determine how widespread the snails are in the lake and when exactly they showed up.
To determine when the snails arrived, they examined the gut contents of rainbow trout that were collected on the lake between 2000 and 2021. The earliest evidence of the snails was found in samples from 2015, the year before they were first noticed in the lake.
To check how widespread the snails are in the lake, the researchers used several methods. They took water samples from throughout the lake and tested them for New Zealand mud snail DNA. They also plucked and examined aquatic plants throughout the lake that are known to harbor the snails. Finally, they checked rainbow trout gut contents from fish caught in multiple locations.
All methods pointed to the same result: For now the snails are staying put along the southwest shore of the lake.
Moreover, the researchers found that, so far, the snails appear not to be outcompeting other native snails in the lake. Nor do they appear to be starving the fish, although the fish are indeed eating them without digesting them.
“We know that they’re there, and we know how they’re spread out. The question is, what is it, if anything, that’s keeping them in check?” Gawel said. “Are there natural things in the lake that are preventing these snails from spreading? If that’s the case, what we don’t want to do is interfere with anything that’s naturally keeping them in check.”
Such interference could come from the Spirit Lake Outflow Safety Improvement Project. To prevent potentially catastrophic flood events, Spirit Lake kept below its natural outflow point, drained through a 1.6 mile tunnel on the west side of the lake. This tunnel is scheduled for significant maintenance in 2025.
“There’s a bunch of work that’s being planned for Spirit Lake, and how you do that work matters,” Gawel said. “As they do all this work, we need to come up with a plan to not make this problem worse than it currently is.”
The snails spread rapidly when carried from place to place by unwitting humans. They can easily become attached to boats, gear or clothes, especially to the felt soles of wading boots. And it only takes a single snail to spawn an enormous, thriving population in a new waterway.
“They clone themselves,” Gawel explained. “They can sexually reproduce, but a lot of times they just clone themselves and make huge numbers.”
Nobody knows how the snails made it to Spirit Lake, but Gawel speculates that they could have hitched a ride on gear or equipment belonging to researchers who work on the lake, or on the boots of an unwitting hiker visiting from the Harmony Trail.
Now that they are in the lake, Gawel says that it is critical that maintenance workers do not accidentally transport snails to the outflow from the southwest shore. “If they get through the outlet tunnel, they can get all the way down through the whole river system down below, into the Cowlitz,” he said.
Once the snails colonize a waterway, there is no known way to remove them. At spirit lake, one option would be to draw down the lake to dry them out. However, Gawel explained that, because they can burrow into the mud and close their shells, they can survive for a long time in a dry lake bed. And if even a single snail survives, it can clone itself to repopulate the lake.
Another option would be to treat the lake with snail-killing pesticides called molluscicides, but these would also kill the native snails that live in the lake.
For now, Gawel says the best approach is to contain the snails by avoiding accidental spread. Hikers, fishers and lovers of the outdoors can avoid spreading New Zealand mud snails and other aquatic pests by properly cleaning and treating their clothes and gear between outings.
Image Credits:
“View above Mount St. Helens National Volcanic Monument in Washington” by Jeffhollet, CC BY-SA 4.0.
“New Zealand Mudsnail (Potamopyrgus antipodarum)” by Columbia River Research Laboratory (CRRL), public domain.
