Invasion of the mud snail

GREEN RIVER — It may not have been particularly appetizing, sifting through the half-digested meals taken from the stomachs of trout, which included everything from bugs to other fish and large mice.

But Mark Vinson sought what was not being digested: tiny snails only slightly larger than the tip of a pen.

Of particular concern on the Green River, and now other waters along the Wasatch Front, including the Provo, Ogden, Weber and Logan rivers, is the infestation of the New Zealand mud snail.

The New Zealand mud snail, proper name Potamopyrgus antipodarum, is an exotic aquatic snail that is spreading into waters in the Western United States like a prairie fire and is pushing out native invertebrates. And, as Vinson's newest findings show, the tiny snail is providing a convenient meal but delivering little or no nutritional value to fish.

In sections of the Madison River in Yellowstone National Park, the observed density of the tiny invertebrates is greater than 100,000 per square meter and makes up more than 95 percent of the available invertebrate biomass in some sections.

Fears are that the introduction of the snail could have a negative impact on fishing.

"There's no reason to be alarmed at this point," said Vinson. "but the snail is a definite concern. It is not good for the fishery."

The snail was first noticed in the Green River in September 2001.

Since that time, through May 2004, Vinson noted, 477 locations were sampled in Utah and the snails were found in 28 locations within 16 stream basins. In 2001, the snails were found in only three basins and in 2002 eight.

Among those rivers found to hold snails are the Green, Bear, Weber, Odgen, Provo and Logan.

In one experiment, trout that were fed an exclusive diet of New Zealand snails lost 2 percent of their body weight per day and eventually died.

"The snails offer very little nutritional value for fish. There is a little nutrition, but not enough for a fish to survive," said Vinson. "Typically, these fish should gain between 1 and 2 percent of their body weight per day when they feed on native invertebrates."

Vinson, leader and research assistant professor, Utah State University Department of Aquatic, tagged along on a night fish survey by the Utah Division of Wildlife Resources on the Green River below the Flaming Gorge Dam.