Sunday, February 5th, 2023
Sunday, February 5th, 2023

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Latest invader found in Superior

By Tim
Associate Editor

Duluth, Minn. — State and federal officials say they’ve
confirmed that a new foreign species has been discovered in the
Duluth-Superior Harbor of Lake Superior. The Minnesota DNR reported
this week that members of the U.S. Environmental Protection Agency
found New Zealand mudsnails during a survey last fall that was part
of a project aimed at exotic invaders in Great Lakes harbors.

The New Zealand mudsnail is a relative newcomer to the Lake
Superior, officials say, but is more prevalent to rivers of the
western United States. Its effects on ecosystems aren’t fully

“One thing we’ve seen is that it can reach very high population
densities,” said Gary Montz, DNR research scientist. “They’ve
gotten so numerous they’ve pushed out other aquatic life.”

In Western streams and rivers, mudsnails have decimated some
insect populations, and fish have in turn begun eating the snails,
which “pass quickly through the fish,” limiting nutritional value,
Montz said.

“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,” added Doug Jensen,
aquatic invasive species program coordinator for Minnesota Sea
Grant, in a DNR press statement. “More than 700,000 snails per
square meter cover the bottoms of some rivers. That’s like having
585,000 snails in your bathtub.”

One snail and its offspring can form hundreds of thousands of
clones per year, the release states.

The New Zealand mudsnails, which reach about a quarter-inch in
length when mature, first were discovered in Idaho’s Snake River in
1987. It’s believed they were introduced with stocked trout,
according to the DNR. Researchers believe they reached the Great
Lakes though ships’ ballast water.

In Lake Superior, they’ve been found in the Duluth-Superior
Harbor, and the St. Louis River Estuary.

Does that mean the snail prefers river systems – out West as
well as in the Superior area? That’s hard to say for sure.

“They are usually tied in with rivers and trout habitat,” Montz
said. “If they prefer that habitat, or if that’s just were they
were introduced first, we don’t know.”

Water temps in Lake Superior might limit New Zealand mudsnail
numbers in the main portion of the lake, Montz said, but chilly
waters likely won’t affect the species in the harbor and river

The mudsnail isn’t a filter feeder, like the exotic zebra
mussel. In fact, it’s a grazer, usually eating plant life on lake
and river rocks. According to information from Montana State
University, the New Zealand mudsnail is “very fast,” with an
estimated cruising speed of about 1 meter per hour.

“In five years (the snail) went from an undetectable level to
abundant in most sections of the Snake River in Hells Canyon,”
according to MSU.

For now, Montz said the DNR’s action regarding the snails will
be to educate boaters and anglers about the species in order that
it not be spread to other waters.

“Right now, it’s a situation like we see with a lot of exotics:
We don’t know its distribution in the harbor and river estuaries,
and not knowing where it’s at, it’s hard to know what to do,” Montz
said. Since it’s a closed-shell species, chemicals would be needed
to control the New Zealand mudsnail. Such chemical treatments can
be toxic for fish species, he said.

Further, “once they get in, it’s really difficult to achieve
control,” Montz said.

One of the scientists who discovered the mudsnail – Igor
Grigorovich, of Wilson Environmental Laboratories, Inc. – also
discovered the species in Lake Superior near Thunder Bay, Ont., in
2001. Grigorovich said the mudsnails found in the Duluth-Superior
Harbor differed in appearance from those found near Thunder

“They (the Duluth mudsnails) possess a thicker and more opaque
shell,” he said. “The Thunder Bay snails are semi-transparent,
probably as a result of low calcium content in Lake Superior

People who suspect they’ve found a New Zealand mudsnail should
preserve the specimen in rubbing (isopropyl) alcohol and report the
sighting. In Minnesota, call either Minnesota Sea Grant, (218)
726-8712 or the Minnesota DNR Invasive Species Program in St. Paul,
(651) 259-5100 or toll free 1-888-MINNDNR (646-6367). In Wisconsin,
report sightings to either the Wisconsin DNR, (608) 266-9270 or
Wisconsin Sea Grant, (920) 683-4697.

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