In the News

New Zealand Mudsnails Threaten Tahoe and Truckee River Waters
date posted: December 1 2005

Lake Tahoe Report #140
Oct. 24, 2005
By John Cobourn and Heather Segale

A tiny snail from New Zealand is infesting streams throughout the western United States and wreaking havoc on the aquatic food web.

The New Zealand mudsnail (Potamopyrgus antipodarum) is so small that it is difficult to see. An adult snail is about the size of a pencil lead. However, when these snails infest a stream, they can greatly reduce the food supply for trout.

The New Zealand mudsnail lives in a variety of habitats, ranging from estuaries to large rivers and small streams. It can reproduce sexually or through the process of parthenogenesis, which produces clones of the adult mudsnail. In its native waters, the mudsnail population is primarily kept in check by trematode (small worm) parasites that sterilize the snail or change mudsnail behavior, making it more likely that they will be eaten by foraging waterfowl. However, there are no natural predators of the mudsnail in the United States.

The mudsnails can rapidly populate an environment. More than 700,000 mudsnails per square meter have been found in some waters. They feed on bottom-dwelling algae (periphyton) and detritus (dead leaves, etc.). Because they are so numerous, they dominate the base of the food web, with an ability to consume over 80 percent of a river’s productivity. This leaves little algae and decaying organic matter to feed native aquatic organisms. The mudsnails decrease the supply of mayflies, stoneflies, caddis flies and some midges. These are very important food for trout.

The mudsnails have been documented in the watersheds of the Columbia, Snake, Missouri, and Colorado Rivers. In 2000, they made landfall in California in the Lower Owens River near Bishop. Since then, they have moved throughout the Owens drainage, including Hot Creek. In October 2003, mudsnails were also discovered in Putah Creek near Davis. Two months later, they were found in the Mokelumne River in the Sierra. In January 2004, a well-established population was discovered in an 11-mile reach of the Calaveras River.

Evidence strongly points toward wading anglers as the primary source for the spread of mudsnails. In a recent survey of 50 wading anglers at Putah Creek, all of the fishermen had mudsnails on their waders and/or in their boots (an average of 33 snails per angler!). The majority of the snails were less than 1mm and nearly invisible, yet fully capable of cloning themselves in a remote site and infecting an entire watershed.

Since mudsnail populations can't be controlled once they become established, the first order of defense is containment. Angler awareness is paramount. Thorough decontamination of wading gear is mandatory to halt the spread of these mudsnails.

To decontaminate, anglers should mix equal parts of Formula 409 degreaser/disinfectant and water, and then soak their gear in the solution for five minutes. Then, they need to dunk their gear in a bucket or put it into a river runner’s “dry” bag, shake it, and let it steep for five minutes.

Testing to develop this decontamination of fishing gear was performed by the California Department of Fish and Game and the U.S. Fish and Wildlife Service, in conjunction with the Federation of Fly Fishers, California Trout and private anglers. Cal Fed provided funding. Orvis, Patagonia, and Simms generously donated wading equipment.

Ralph Cutter, of Truckee’s California School of Fly-fishing, is helping these agencies spread the word about how to prevent the spread of the New Zealand mudsnail. According to Cutter, “The Truckee, Carson and Walker River watersheds are excellent mudsnail habitat. The potential environmental and economic harm to the region is enormous. Our angling and tourist-based communities need to wake up and work proactively to keep the snail at bay. Once it arrives, there will be nothing we can do but watch and wait to see how bad the impact will be.”

Watch for the “Lake Tahoe Report” in the Tahoe Daily Tribune each Wednesday, and tune in to KOLO-TV News Channel 8 Tuesdays at 5 p.m. to learn more. Coauthor John Cobourn is a water resource specialist with University of Nevada Cooperative Extension. Coauthor Heather Segale is the education and outreach coordinator for the University of California, Davis. For more information, contact Heather Segale, (530) 583-3279, or logon to www.lteec.org, www.tahoe.unr.edu, or terc.ucdavis.edu.