As dusk falls on a popular trout stream, an angler in thigh-deep current takes one final cast before wading to the bank. He'd stay until dark, but he has an out-of-state fishing trip planned for the next day and needs to pack his gear. He doesn't know it, but he'll be taking more than his fishing tackle on the trip.
A few tiny snails have settled into the laces of his waders, and a few more are caught in the soles. They can survive for weeks out of water, and they reproduce asexually. When the unwitting sportsman wades into his next river, chances are good that he will introduce these hitchhikers to a new watershed.
Biologists believe this scene has been played out repeatedly over the past decade. Some of the most famous and productive trout fishing streams in the West have become infested with billions of tiny, potentially destructive snails, and fisheries managers are hoping Alaskans can help keep the invaders out of the state.
The New Zealand mud snail has been introduced to waters in 10 Western states, most extensively in Idaho and Montana. Biologists suspect the freshwater snails first arrived in the United States along with young fish or eggs shipped to a Pacific Northwest fish hatchery from New Zealand in the mid-1980s. They were discovered in the late 1980s after they had become established in the Snake River system in Idaho.
By the early 1990s they had spread to the Madison, Firehole and Yellowstone rivers in the Yellowstone region, three legendary "Blue Ribbon" trout streams of the American West. The snails have been found at two locations in Lake Ontario, but have largely spread in the West and are now found in streams and rivers in Washington, Oregon, California, Wyoming, Arizona and a few spots in Nevada and Colorado.
"It looks like one of the main ways this is spreading is on the gear, particularly the wading boots of sport anglers," said Denny Lassuy, the invasive species coordinator for the U.S. Fish and Wildlife Service in Anchorage. Last year Lassuy, working with the Flyfishers of America and the Alaska Department of Fish and Game, prepared a flyer detailing how anglers can help keep this invasive out of Alaska, and distributed it to about 1,500 registered Alaska sport fishing guides.
Lassuy and other invasive species biologists hope to avoid the situation developing in rivers such as the Yellowstone, where densities of a half-million snails per square meter have been found. The National Park Service is closely monitoring the situation in Yellowstone and reports that in some places, the snails comprise 95 percent of the invertebrate biomass - at the expense of the native invertebrates like mayflies, stoneflies, and caddis flies.
The tiny snails consume the algae that important native insects rely upon for food. The aquatic insect larva, which salmon fry and trout eat, can be starved out, and potentially, fish can suffer. The mud snails can pass through a fish's gut unharmed and thus don't appear to be a food source for fish.
The snails can survive in clear, brackish or silty waters ranging from zero to 80 degrees. The snails are BB-size when grown; immature snails are as small as the period at the end of this sentence. Although they are very small individually, given time they can carpet a stream bottom.
A tiny "door" called an operculum closes to seal the opening of their shell, allowing them to survive out of water - up to two months if they are in a damp environment. Because they can reproduce asexually, even a single snail can colonize a new area.
At this time it's not clear what effect the snails may have on fish. Trout populations in rivers where the snails have become established have not crashed, but biologists say it's too early to tell what the impact will be. The potential for the snails to disrupt the food web to the detriment of fish has scientists concerned.
"We're still dealing with unknown potential impacts," said George Nandor, a biologist with the Oregon Department of Fish and Wildlife.
Bob Piorkowski, the invasive species coordinator with the Alaska Department of Fish and Game, is concerned that freshwater anglers visiting Alaska could bring the snails north with their fishing gear. He's put Alaska fishing lodges and guides on the alert, and hopes that Alaskans who host freshwater-fishing visitors this summer can spread the word.
Piorkowski recommends that visiting anglers clean and dry their waders and other tackle that could harbor the snails before coming up. Piorkowski suggests another option, instead of bringing fishing gear, Alaskans could loan fishing tackle to their guests. Alaskans who fish outside the state should remember to clean and dry their fishing gear before returning to Alaska.
Several treatments have been recommended to kill the snails. Tap water at 130 degrees or hotter will kill the snails, and waders should be rinsed and then allowed to soak in the hot water until the water cools. Soaking boots for five minutes in a bleach solution (12 cup chlorine bleach in a gallon of water) is another option. Gear should be thoroughly dried - four hours in the hot sun is ideal. Oregon Fish and Wildlife recommends spraying gear down with 409 window cleaner, then allowing it to thoroughly dry in the sun.
Piorkowski said with cooperation from anglers, chances are good these invasive snails can be kept out of the state and Alaska's world-class trophy fishing can be protected for the enjoyment of future anglers.
Riley Woodford is a writer with the Division of Wildlife Conservation at the Alaska Department of Fish and Game. For comments or questions, he can be reached at firstname.lastname@example.org.