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New Zealand Mudsnails Found in Duluth-Superior Harbor

New Zealand Mudsnail.

New Zealand Mudsnail. Photo: Daniel L. Gustafson

The New Zealand mudsnail (Potamopyrgus antipodarum) is another invasive species now living in the Duluth-Superior Harbor and St. Louis River Estuary. The announcement of the discovery by the U.S. Environmental Protection Agency (EPA) has natural resource officials from Minnesota and Wisconsin urging anglers and other river users to take steps to prevent accidentally spreading mudsnails.

Over 100 snails were collected last fall by a research team from the EPA’s Mid-Continent Ecology Division in Duluth. The team was conducting a species survey as part of a project designed to look for new invaders in Great Lakes harbors. This is the first finding of the tiny snail in Minnesota and Wisconsin waters.

Igor Grigorovich, a contract scientist who was part of the team, identified the New Zealand mudsnail among the samples. Following confirmation of the mudsnail’s identity, the EPA disclosed preliminary results of the survey in May.

“I kind of expected to find them,” said Grigorovich of Wilson Environmental Laboratories, Inc. “The St. Louis River Estuary is a more suitable habitat for this snail than Lake Superior because it’s not as cold.” Grigorovich was also first to find the species in Lake Superior near Thunder Bay, Ontario, in 2001.

Grigorovich and the team discovered the infestation while analyzing bottom
sediment samples. Grigorovich said the New Zealand mudsnail varies in appearance and that the snails he found in the harbor look different than the ones he found in Thunder Bay.

“They possess a thicker and more opaque shell,” Grigorovich said. “The Thunder Bay snails are semi-transparent, probably as a result of low calcium content in Lake Superior water.”

New Zealand mudsnails cause concern because their sheer numbers can disrupt the ecosystem.

“They have adapted so well in Western rivers that they have pushed out almost all of the native insects, snails, and other invertebrates that are important food for fish,” said Doug Jensen, aquatic invasive species program coordinator for Minnesota Sea Grant. “More than 700,000 snails per square meter cover the bottoms of some rivers. That’s like having 585,000 snails lining your bathtub!”

Another concern is that they can spread easily on aquatic plants, waders, and other gear used in infested waters. They are able to close their shells, allowing them to survive out of water for days. Also, just one snail can start a new infestation because they reproduce without mating, essentially cloning themselves. Native fish and wildfowl eat the snails, but because they are so prolific, nothing seems to control infestations in North America.

New Zealand mudsnails are tiny mollusks (about the size of a peppercorn) native to New Zealand. Their spiral-shaped shells are usually dark gray or dark brown to light brown with a right-handed coiling pattern and 5-6 whorls. Some native snails look similar to the mudsnail, which makes identification difficult.

They were first found in the U.S. in Idaho’s Snake River (1987). It is believed they were accidentally introduced with stocked imported rainbow trout. The snails have impacted Rocky Mountain trout streams, apparently spread by anglers. Researchers suspect they arrived in the Great Lakes via ship ballast water.

Avoid Spreading Mudsnails

Anglers and others who may use gear in infested waters are encouraged to:

  • Inspect and remove visible aquatic plants, animals, and mud from waders, hip boots, and field gear before transporting.
  • Rinse waders, hip boots, and gear with hot water (120 degrees F or 45 degrees C), or
  • Dry gear for five days before reuse.

If you suspect you've found a New Zealand mudsnail, please preserve the specimen in rubbing (isopropyl) alcohol and report your sighting. In Minnesota, call either Minnesota Sea Grant, (218) 726-8712 or the Minnesota Department of Natural Resources (DNR) Invasive Species Program in St. Paul, 1-888-MINNDNR or (651) 259-5100. In Wisconsin, report sightings to either the Wisconsin DNR, (608) 266-9270 or Wisconsin Sea Grant, (920) 683-4697.

New Zealand mudsnail identification cards are now available from Minnesota and Wisconsin Sea Grant programs and DNRs. (Order item X 109 from the MN Sea Grant products page.)

The DNRs in both Wisconsin and Minnesota are working to designate the New Zealand mudsnail as a prohibited invasive species. The designation means it will be illegal to import, transport, possess, and place mudsnails into other waters in the state, as it is with other prohibited species such as zebra mussels. An exception is if a person is bringing the mudsnail to the DNR for identification or reporting purposes.

The Minnesota DNR is working to designate Lake Superior and the St. Louis River below the Fond du Lac Dam as mudsnail-infested waters.

By Marie Zhuikov
June 2006

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