Friday — February 20, 2004

Exotic invaders threaten famous fishery

Silver Creek being overrun
by New Zealand mud snails

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By GREG STAHL
Express Staff Writer

The world-famous fly fishing waters of Silver Creek are being invaded by a minuscule creature that could have a big impact.

New Zealand mud snails are small, but thick in many rivers throughout Idaho and the West. Courtesy photo

The New Zealand mud snail is one of dozens of plant and animal species that are threatening agriculture, industry, wildlife and recreation throughout Idaho, and, like other invasive species, it is spreading unchecked through the waterways of Idaho and North America.

Gov. Dirk Kempthorne announced Tuesday, Feb. 17, at a conference on invasive species in Boise that the Idaho Department of Fish and Game will team up with The Nature Conservancy-Idaho to seek ways to prevent the snail from devastating Silver Creek, near Picabo, and other Idaho streams.

"Silver Creek will become an outdoor laboratory on how to limit the spread of these species," Kempthorne said.

As its name indicates, the New Zealand mud snail is native to the freshwater lakes and streams of New Zealand. A single snail is tiny, measuring several millimeters across, but they populate streams in extremely high densities.

The snails were first detected in the U.S. in the mid-1980s in the mid-Snake River near Hagerman. Since then, they have mushroomed to the waters of Montana, Wyoming, California, Arizona, Oregon and Utah. They have infiltrated the Firehole River and other popular fishing streams in Yellowstone National Park.

The spread of the snails is caused, at least in part, by anglers or boaters who pack muddy waders or equipment from river to river.

For that reason, cleaning stations will be established along Silver Creek to slow or stop further spreading of mud snail populations.

The serenity of Silver Creek belies the turmoil under the water, where an invasive species, New Zealand mud snails, is waging a war against the stream’s natural ecosystem. Express photo by Ken Retallic

According to TNC, the extraordinary density of snails that has been discovered in some Western streams is a cause for serious concern. In some cases, biologists have counted more than 500,000 snails in a square meter of river. They can out-compete aquatic invertebrates like mayflies and caddis flies and have a relatively low nutritional value as a food source for fish, said TNC communications director Matt Miller.

In extreme cases, mud snails comprise up to 95 percent of the invertebrates in a river.

Mud snails were one of the featured species discussed at TNC’s Silver Creek Symposium held in November at Sun Valley. Because mud snails are relatively recent invaders, little is known about their long-term impacts on waterways. Also, because they reproduce asexually, even one survivor is enough to repopulate a stream, Miller said.

Invasive species, defined as any species non-native to a region that harms economic, environmental or human health, cost Americans an estimated $137 billion each year.

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