Sunday, January 29th, 2023
Sunday, January 29th, 2023

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From New Zealand to Wisconsin, with a vengeance

Cross Plains, Wis. — A few of these tiny critters attached to a rock or a felt-soled boot would hardly warrant a passing glance. At the size of a grain of sand, who would notice, anyway?

Five hundred thousand per square meter – well, that’s another matter.

The tiny New Zealand mud snail (potamopyrgus antipodarum), a prolific invasive species, has thrust its way into Wisconsin by way of Black Earth Creek in Dane County, one of the finest trout-fishing venues in the state. Wisconsin DNR staffers discovered the mud snail during routine stream monitoring.

A stakeholder’s meeting held Dec. 12 in Cross Plains drew a broad cross-section of conservation-minded citizens eager to learn more about the threat and determine what they could do to help. The event, hosted by the DNR, the River Alliance of Wisconsin, and the UW-Extension, sought to enlist volunteers to spread out across southwestern Wisconsin to check for other infestations of the micro-invertebrate.

“The mud snail can survive in a broad range of conditions,” said Susan Graham, DNR water resource specialist. “It lives in streams, lakes, and ditches.”

To make matters worse, it has no known predators, Graham said.

The mud snail has the potential to displace food sources, depriving fish, including trout, of their main sources of food, Graham said. The speed with which this transformation takes place is striking: More than 3 million snails can result from just one snail, according to some sources. No need for both a male and a female – most New Zealand mud snails in the United States are females that reproduce asexually.

The mud snail has an “operculum,” or trapdoor, perhaps its most significant survival adaptation. The ability to close the opening at the mouth allows it to survive when subjected to chemicals, predators, or other environmental threats. The operculum can help it survive a trip through the alimentary canal of a fish or other potential predator. It also can survive outside of its natural water habitat for up to 25 days, Graham said.

How these snails arrived in southern Wisconsin is puzzling. While the invasive has been prevalent in several western states and within the Great Lakes basin, there has been no evidence of the mud snail in Midwestern inland waters. Adding to the mystery is the genetic makeup of the mud snail found in Black Earth Creek does not match the Great Lakes clone, but rather that which is found out West, Colorado being its easternmost penetration.

Enlisting the support of conservation groups and the general public was an important objective of the stakeholder meeting. Steve Wald, president of the Southern Wisconsin Chapter of Trout Unlimited, said his group is eager to help.

“I’m encouraged by the way that the DNR and the UW-Extension have reached out with the River Alliance to educate and engage SWTU and other volunteer organizations around the problem,” Wald said. “Our chapter is committed to partnering with others to do whatever we can to advance solutions to the threat.”

Efforts to deal with the threat point toward control rather than eradication. For starters, Wisconsin’s strong invasive species law prohibits the transport of aquatic plants, live animals, and water from a water body. Officials acknowledge, however, that the effectiveness of the law depends on anglers and other users of water resources who must buy into the need to prevent the spread of invasive species, whether the mud snail or other threats such as viral hemorrhagic septicemia.

While officials consider this discovery a legitimate threat to the environment, it is no time to panic, said David Rowe, DNR fisheries team supervisor in the South-Central District.

“These snails may effect some change to the food web, and may displace some of the native insects and invertebrates. (However), they are not a predator or a disease that will kill off our desired fishery,” Rowe said.

Here are recommendations to help prevent the spread of the New Zealand mud snail:

• Inspect and remove, with a stiff brush, all mud and debris that might harbor snails from boots, waders, boats, and other gear.
• Freeze gear for six to eight hours, wash with 212-degree water, or dry for five days.
• Drain water from boat, motor, bilge, decoys, and other water-containing devices before leaving water access.
• Consider having a second pair of waders or boots if it isn’t practical to clean them when moving from one body of water to another.
• Soak boots and waders in a solution containing 2 percent Virkon for 20 minutes.

Rowe said the agency’s response to the threat includes additional monitoring in high-priority sites to understand the distribution of the organism.

“This will occur in January and February,” Rowe said. “These sites will be sampled with traditional kick nets, and then the contents of the net will be sorted to look for (mud snails).”

Citizens interested in joining the fight against the spread of the New Zealand mud snail are encouraged to take the River Alliance’s online survey at or contact Rowe at (608) 275-3282.

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