What Matt Hahn thought when he heard invasive New Zealand mud snails had been found in the North Platte River isn’t fit for print.

And for good reason. The regional fisheries supervisor and lifelong angler understood the stakes. New Zealand mud snails can, in the right circumstances, choke out nearly all native macroinvertebrates in a river system and decimate local fisheries.

Now, almost one year after they were discovered, Game and Fish biologists and aquatic invasive species specialists are sampling the Platte’s river bottom to understand what native macroinvertebrates exist to prepare for the worst.

“If we see trout numbers beginning to decrease over time, at least if we have the macroinvertebrate data we will know that’s what’s causing it,” Hahn said.


Wyoming has been battling the arrival of aquatic invasive species for more than a decade. The tiny hitchhikers – zebra and quagga mussels, New Zealand mud snails, Asian clams – are slowly spreading across the US, carried in cracks and crevices on boat motors or on soles of boots and folds of waders. Once they arrive, there’s nothing fisheries managers can do.

Eric Hansen, an aquatic invasive species specialist at Game and Fish in Casper first found the snails in August 2018 near the boat ramp in the Gray Reef section of the North Platte River.

It’s his job to sample waters specifically for aquatic invasives. That means he spends much of his time squatting on the banks peering into a white, flat tray filled with mud, moss, gravel and countless living organisms.

He sampled Gray Reef in part because of its popularity among anglers from all over the country.

“I wasn’t have much luck, and thought they wouldn’t be there, then sure enough I found one, and then five more,” he said.

That triggered a need to sample other parts of the river from Pathfinder to Alcova reservoirs. He found more at the Cardwell fishing access near Pathfinder.

“I knew it was essentially a can of worms opening, but it was also good to know that if we have a population here, at least we will be able to monitor it and track what kind of progress it’s having,” he said. “If it’s becoming a bigger issue or staying where it’s at.”

Simply finding a New Zealand mud snail in a river the size of the Platte is a feat. The light brown invertebrate can be as small as the tip of a pencil to as big as the top of a matchstick.

That’s why they’re so easily overlooked by anglers and boaters. At first glance, the snails resemble a piece of sand. The creatures cling to surfaces and can survive for long periods outside of water, said Hahn. In the right circumstances – such as inside a rolled-up pair of damp waders – they can live for a month or more.

They’re also asexual, meaning one snail can turn into 100,000, and then 100 million, Hahn said.

And they don’t provide food for trout or other fish. New Zealand mud snails can survive in a trout’s digestive system for up to 48 hours. They retreat into their shell and wait until they’re expelled, outcompeting local macroinvertebrates for food and space but not providing food for anything else.


What eventual impact the snails will have is a matter of waiting and watching.

But biologists know what the worst case scenario looks like, and it’s snails covering every rock in a stream, said Sam Hochhalter, the regional fisheries supervisor for Game and Fish in Cody.

Researchers found mud snails for the first time in the lower Shoshone River outside of Cody in 2016. They don’t, at least so far, have evidence of a major shift in ecological diversity in the water, he said, but they’ll continue monitoring potential spread.

For Hahn and Hansen on the Platte, it’s important to figure out early what species already exist in the blue ribbon fishery to determine what changes could be underway.

If the snails turn into an outbreak, “we won’t lose the fishery completely, but it would be a fraction of what it is now,” Hahn said.

That’s Blake Jackson’s biggest concern. The co-owner of the Ugly Bug Fly Shop and Crazy Rainbow Fly Fishing in Casper depends on the North Platte for his business as do dozens of other outfitters and guides. The Platte, and the fish in the Platte, support a significant economy in Casper from filling hotels and buying gas to eating in restaurants and going shopping.

Nothing can be done once aquatic invasive species arrive, so Jackson, Hahn and others stress the importance of checking waders, boots and boats before moving from one water to another, even within the same state. Lift the gravel guards on waders, for example, and clean anywhere a tiny snail could hide.

Hahn is stepping up boat inspections at the river. New Zealand mud snails die with 140-degree heat, so boats, motors and anchors are flushed with hot water. Biologists are also trying to increase public awareness.

“If everybody paid attention and did due diligence in policing themselves, you could stop the spread of all invasives,” he said.

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