New Zealand mud snails spotted in Bitterroot

Montana Department of Fish, Wildlife and Parks has notified Trout Unlimited and landowners in the area of Mitchell Slough, a 19-mile long spring-fed channel of the Bitterroot River, that it now hosts an identified population of New Zealand mudsnails. New Zealand mudsnails (NZMS) are a small aquatic snail native to New Zealand. They are relatively new to Montana. They were first detected in North America in Idaho’s Snake River in 1987 and were first detected in Montana in 1995. Now they are found in the Madison, Jefferson, Beaverhead, Ruby, Bighorn, Missouri and Yellowstone Rivers. 

They are considered an Aquatic Invasive Species due to the negative impacts they can have on the ecology and other native species. They compete for food with native bottom-dwelling species and can crowd out native species. They can outcompete or displace native snails, mussels, and aquatic insects which native fish species depend on for food, but they are not an alternative food source to native fish since they have very low nutritional value and most often pass through a fish’s digestive track unharmed. 

The critters are also very good at reproducing and can reach extraordinary densities in some locations. Researchers at Montana State University have reported densities of up to 750,000 snails per square meter in Yellowstone National Park. Large colonies of NZMS can comprise up to 95% of the total macroinvertebrate biomass and consume up to half of the available food in a stream. 

New Zealand mudsnails are not totally new to the Bitterroot. An infestation was discovered at a fish hatchery near Hamilton in late 2019. The infestation was tracked back to a hatchery in South Dakota. According to Pat Saffel, Fisheries Manager for FWP, the hatchery itself was drained and the facility disinfected and protected from any re-entry of the snails. Some snails were identified outside the facility, but FWP was unable find any surface connection to the river from that area.

“Because of that, “said Saffel, “our hunch is that this infestation is not connected to the problems at the hatchery.” 

Saffel said that the snails do not expand much on their own. He said they seem to have a preferred habitat that tends to keep them confined to cold, stable, spring-like waters. Though they are not that good in moving themselves around, it seems, they can be easily transported in a number of ways by people, birds and other animals.

The snail’s small size makes them very easy to overlook and accidentally transport to new locations. They can cling to fishermen’s waders and fishing gear. They can cling to boats and trailers. 

They can be spread locally on the fur or feathers of terrestrial wildlife and pets or when consumed and dispersed in the excrement of local fish species. 

According to Saffel, the agency received an anonymous report of the snails being seen in Mitchell Slough and went to take a look this summer. Snails were identified at Victor Crossing and Bell Crossing. He said there was no sign as yet of any kind of ecological impact but they will be watching and educating people about how the impacts can be minimized by being conscious of the possibilities and cleaning up your fishing gear and waders, and cleaning, draining and drying your boat before leaving the area.

“It’s frustrating to get another invasive species into a new area,” said Saffel. “But hopefully the impacts in this case will not be too harmful. But you never know.”

The state has a very aggressive program to identify AIS such as mudsnails, zebra mussels and quagga mussels and has placed Boat Inspection Stations around the state. Snail observations at the inspection stations are increasing.