New Zealand mud snails are small — only slightly larger than a tiny pebble. But they are a nuisance.

Now, they're even more troublesome. The tiny invertebrates have been discovered in the Loa State Fish Hatchery.

How they were able to infect the hatchery "is anyone's guess," said Walt Donaldson, aquatic section chief for the Utah Division of Wildlife Resources.

"We know of one transport vehicle that could have brought them in. Then again, they could have come in on the sole of a wader or in mud on someone's shoe. They're easily spread but difficult to remove."

DWR officials are not sure at this point what action to take. For now, the hatchery is under temporary quarantine. No fish will be brought in, and no fish will leave.

There are no plans at this point to kill the fish, but Donaldson said it is still an option.

The snail is rapidly spreading in waters throughout the United States. In Utah it has been found in a number of waters, including the Green, Logan, Provo, Bear and Weber rivers.

The problem is the snails provide a convenient meal for fish but return very little nutritional value. Fish fed a diet of mud snails do, in fact, lose weight and eventually die.

Another problem is the snails in Utah are female and asexual, which means it takes only one snail to start a large family of snails. A single snail can also produce up to six generations per year.

In sections of the Madison River in Yellowstone National Park, the observed density of the tiny invertebrates a few years ago was greater than 100,000 per square meter, which made up more than 95 percent of the available invertebrate biomass in some sections.

"We have formed an action team and are checking with Idaho and Montana, which are dealing with the snails on some of their larger rivers, to see what they are doing," Donaldson noted.

In a study conducted by Mark Vinson of Utah State University, of 914 snails collected after passing through trout gastrointestinal tracts, 43 percent were still alive.

This means hatchery fish could be moved to clean water and left until the snails pass through the digestive systems.

"What we fear," said Donaldson, "is if we don't do this right, other fish will pick up the live snails, and then they could spread the snails to other waters if we move fish too soon."

One possible treatment is to place filter screens on the bottom of the raceways to trap the snails, and another is to constantly flush the raceways with clean water. The DWR is also looking into the possibility of using some type of chemical solution.

"Until we figure out what we need to do to make sure we don't spread the snails, we'll simply keep the hatchery closed down," Donaldson added.