Tiny stream invaders may harm Western trout
High Country News
Every two months for the past six years, ecologist Dave Richards has scouted a 60-foot-long stretch of the Snake River near Twin Falls, Idaho. He collects cobbles from the river, carefully rinses them in a bucket of water, then counts what he’s washed off the rocks: hordes of minuscule New Zealand mud snails.
Richards estimates there are 100,000 to 500,000 New Zealand mud snails per square meter on the rocks along about 60 miles of the Snake. The snails graze on algae, and in such high densities, he says, that “they gobble up everything.” That’s a troubling image, because Richards’ sampling area is the beachhead of a biological invasion ” it’s where New Zealand mud snails were first discovered in the United States, back in the mid-1980s. Since then, the invaders have spread to every Western state except New Mexico. Every few months, researchers find the snails have colonized more streams.
Richards, who works for a consulting firm, EcoAnalysts, says his monitoring effort is the longest continuous study of the invaders. He’s documented them crowding out six species of native Idaho snails that are threatened or endangered. Other researchers have found they may crowd out aquatic insects and harm trout. But their cumulative effects on aquatic ecosystems are largely unknown. “That’s the million-dollar question,” Richards says.
A Westwide effort to answer that question and devise methods to control the invader has produced some worrisome results.
In their home country and adjacent islands, New Zealand mud snails are controlled by parasites and predators that don’t exist in the United States. The snails probably traveled here in shipments of trout eggs imported by fish farmers, who raise trout in springs along the Snake River. Then the snails hitched a ride to other watersheds on the gear of anglers and boaters.
They’re a Western problem; east of Colorado, they’ve only been found in a few places. It’s not clear why they take over some stretches of water and not others. But only a few habitats repel them: high-mountain creeks that are cold year-round, warm-water Plains streams with intermittent flows, and warm springs, says Dan Gustafson, a Montana State University ecologist. They can even live in 50 percent seawater in Oregon’s coastal estuaries.
They’re aggressive colonizers. Almost all are females that reproduce by cloning; in two years, the offspring from a single female can multiply to 12 million snails, Richards says.
They occur in densities as high as 750,000 per square meter in several streams in Yellowstone National Park, according to Robert Hall, a University of Wyoming researcher. In some reaches, the tiny creatures amount to more than 90 percent of the invertebrate biomass. That degree of dominance by a single species is typically found only in the most disturbed environments, such as highly polluted streams and sewage ponds. Also, because the snails “consume a lot of algae that would go to mayflies, stoneflies and caddis flies,” they reduce the available food for insect-eating trout, Gustafson says.
Trout eat some types of snails, but they don’t do well eating the foreign snails, according to Mark Vinson, a researcher at Utah State University. Last year, he fed a dozen rainbow trout an experimental diet: three months of only New Zealand mud snails, alternating with three months on a normal diet. Trout flourished on the normal diet, but on the mud snails diet, one-third died and the rest suffered dramatic weight loss.
The snails are so hardy, many survived their passage through the trouts’ digestive systems, Vinson found, which means that fish can spread the invaders. Vinson says, “We need to worry quite a bit” about the long-term impact on trout in the wild.
Anglers report declining trout populations in some areas infested by snails, but researchers are just gearing up to verify such impacts. “The effects could be subtle,” Gustafson says. “The trout population might not plummet, but it’ll affect the growth rate and the size of trout.”
Wildlife agencies have temporarily banned fishing on a few streams in Montana, California and Colorado, to reduce the chances of spreading the snails. But most people ignore recommendations to wash their gear to remove the hard-to-see snails. A survey of anglers on the Missouri River in Montana in 2004, for instance, showed that only 38 percent cleaned their gear and boats regularly.
Researchers hope to develop biological controls ” organisms that would prey on the invaders. State and federal agencies, universities, and groups like Trout Unlimited are providing some funds for the work, but far more support is needed, researchers say. Richards has to stitch together funding for his Snake River sampling from several agencies and the Idaho Power company, which has dams on that river and is obligated under the Endangered Species Act to help ensure the survival of the native snails. A California irrigation district is underwriting Richards’ attempts to develop a strain of bacteria that would kill the invaders.
In a lab in Bozeman, Mont., he and an assistant, Tristan Arrington, put mud snails in petri dishes and expose them to different types of bacteria. “It’ll take several years” of testing to determine whether a bacteria would hit the invaders without harming other species, Richards says. For inspiration, Arrington has drawn a New Zealand mud snail on the lab’s blackboard, with a message in chalk: “Welcome NZMS, prepare to die!”
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