Aquatic Nuisance Project
Fact Sheet

Species: New Zealand Mudsnail (Potamopyrgus antipodarum)

Status & Synopsis Economic Value of Potentially Affected Fisheries Geographic Distribution (USA) PSMFC Funded Projects
Publications Links Educational Materials References


Image by: Amy Benson, USGS


The State of Washington classifies the New Zealand mudsnail (NZMS) as a “prohibited species.” In Oregon, NZMS’s are not specifically classified as “prohibited,” “controlled,” or “non-controlled.” As a result, live snails cannot be possessed, imported, purchased, sold, exchanged, or offered for sale, purchase or exchange without a state permit until they are classified. California classifies NZMS’s as “restricted.” Therefore, it is unlawful to import, transport, or possess live NZMS’s in the state except under permit issued by the California Department of Fish and Game. NZMS’s are not specifically regulated by the state of Idaho. However, under Idaho Administrative Code, “no person shall import, export, transport into or cause to be transported within, release or sell within the state of Idaho any living wildlife including wildlife eggs” without first obtaining a permit from the Idaho Department of Fish and Game. In Montana, NZMS’s are listed as a Priority Class 2 species. Priority Class 2 species are species that are present and established in Montana, have the potential to spread, and for which there are limited or no known management strategies for these species (Proctor 2004).


The New Zealand mudsnail is native to freshwater streams and lakes of New Zealand and adjacent small islands. It is naturalized in Australia and Europe. Populations are widespread in the Western United States, as well as Lake Ontario and Lake Superior. The NZMS is about ¼ inch in length.

It was discovered in 1987 in the middle-Snake River in south central Idaho (near Hagerman). The Western U.S. population of NZMS was likely introduced in a batch of rainbow trout eggs brought from New Zealand or Australia. Since then, it has spread into 10 western states and Canada. Unfortunately, the snail has spread to blue ribbon trout streams across the west in California, Montana, Colorado and Wyoming (See Figure 1).

The North American population is composed of self-cloning females (triploid parthenogenetic females) – meaning a single individual can start a new population. The NZMS can form colonies dense enough to carpet a stream bottom. In the Madison River drainage, including Yellowstone National Park, researchers at Montana State University reported between 750,000 (Hall 2001) and 800,000 mudsnails per square meter (Lucas, 1959 in Dorgelo, 1987). Research has shown mudsnails have a negative effect on mayfly (Baetis spp.) survival (Cada 2004). Mudsnails have been found to deplete the standing crop of aquatic algae and periphyton (Cada 2001, Hall 2001, Hall et al. 2003). Fish in North America sometimes ingest mudsnails. These species include mountain whitefish (Prosopium williamsoni), sculpin (Cottus sp.) and brown trout (Salmo trutta) (C. Cada and B. L. Kerans, unpublished data in Proctor, 2004). These fish gain little energy from the snails, however, because studies have shown that the snails are capable of passing through the digestive canal of trout alive and intact (Bondesen and Kaiser 1949, Haynes et al. 1985).

New Zealand mudsnails hitchhike around the country by lodging in waders and other fishing gear, closing their operculum (the trap door used to seal off their shell), and then traveling as far as the host carries them. The seriousness of this problem became apparent as mudsnails spread to popular trout streams hundreds of miles from the nearest known mudsnail infestation. In 2001, for example, the Owens River, a popular trout stream in California’s Eastern Sierra, became infested; likely caused by contaminated fishing gear. In 2004, Boulder Creek, a trout stream near Boulder, Colorado became infested. The impact to anglers was significant. Shortly after the infestation, the Colorado Wildlife Commission closed fishing on a two-and-one-half mile stretch of Boulder Creek to reduce the risk of accidentally moving exotic New Zealand mudsnails to other streams and lakes. The NZMS was also recently found in Oregon’s Deschutes River, another popular fishing destination.

New Zealand mudsnails are difficult to eradicate once in a stream. Research on potential biological control methods includes the use of a trematode (a fluke), which shows some promise (Emblidge and Dybdahl 2004). There is a concern that NZMS could negatively affect anadromous and resident fish resources in the western U.S. These fisheries provide substantial economic benefit (see below).

Scientists have only begun documenting (and publishing) impacts in the past 5-6 years; with no studies directly linking mudsnails to significant disruption of the aquatic ecosystem and adverse impacts to fish. This may explain why funding for mudsnail management activities is minimal when compared to other invasive species issues such as zebra mussel prevention and ballast water management and research. The timeline of NZMS arrival in North America mirrors that of the zebra mussel in the 1980′s. Despite arriving about the same time, there have been scores of studies documenting zebra mussel impacts.

Economic Value of Fisheries Resources That Potentially Could Be Affected by the New Zealand Mudsnail

For the Pacific Ocean Salmon Fishery (coho, chinook) from Cape Falcon, Oregon to the Canadian Border, (Pacific Fisheries Management Council 2005):

  • Estimates of ex-vessel value for Council-adopted 2005 non-Indian commercial troll salmon fishery: $1.798 million.
  • Coastal community income from recreational ocean fishery in 2004: $7,625,000.

Chinook Salmon, State of Idaho: During the 2001 salmon season, recreational fishing for salmon was responsible for $89,880,015 in expenditures in Idaho (Reading 2001).

Geographic Distribution (USA): New Zealand Mudsnail (Potamopyrgus antipodarum)


Source: Department of Ecology, Montana State University, Bozeman


PSMFC Funded Projects


Educational Materials



Montana State University
National Management and Control Plan for the New Zealand Mudsnail (Potamopyrgus antipodarum) DRAFT



Bondesen, P. and E. W. Kaiser. 1949. Hydrobia (Potamopyrgus) jenkinsi (Smith) in Denmark illustrated by its ecology. Oikos 1:252-281.

Cada, Chelsea. 2001. Effects of New Zealand mudsnails on native invertebrates in Darlington Ditch, Montana. Minutes of the First Annual Conference on New Zealand Mudsnails in the Western USA, July 9-10, 2001. Bozeman, Montana.

Cada, Chelsea. 2004. Competitive interactions between the invasive Potamopyrgus antipodarum and baetid mayflies: temporal variation and community-level consequences An Annual Report to the Montana Water Center, US Geological Survey. Montana State University-Bozeman, Bozeman, MT. 14pp

Dorgelo, J. 1987. Density fluctuations in populations (1982-1986) and biological observations of Potamopyrgus jenkinsi in two trophically differing lakes. Hydrobiological Bulletin 21:95-110.

Emblidge, Alison and Mark Dybdahl. 2004. Third Annual Potamopyrgus antipodarum Conference, 215 Cheever Hall, Montana State University, Bozeman, MT.

Hall, R.O. 2001. Estimating New Zealand mudsnail impact based on consumption rates of algae in 2 rivers in Yellowstone National Park. Montana. Minutes of the First Annual Conference on New Zealand Mudsnails in the Western USA. Bozeman, Montana.

Hall, R.O., J.L. Tank, and M.F. Dybdahl. 2003. Exotic snails dominate nitrogen and carbon cycling in a highly productive stream. Frontiers in Ecology and the Environment 1:407-411.

Haynes, Alison, B. J. R. Taylor, and M. E. Varley. 1985. The influence of the mobility of Potamopyrgus jenkinsi (Smith, E. A.) (Prosobranchia: Hydrobiidae) on its spread. Archives of Hydrobiologie 103:497-508.

Lucas, A. 1959. Les Hydrobia (Bythnellidae) de le Ouest de la France. Journal of Conchology 99:3-14.

Proctor, Tina. 2004. Management and Control Plan for the New Zealand Mudsnail (Potamopyrgus antipodarum) DRAFT Prepared by the New Zealand Mudsnail Management and Control Plan Workgroup August 2004. USFWS, Denver Co. 57 pp.

Reading, Don. 2001. The Economic Impact of the 2001 Salmon Season in Idaho. Ben Johnson Associates. 6070 Hill Road Boise, Idaho 83703.

ANS Examples

New Zealand Mudsnail


(Potamopyrgus antipodarum)