New Zealand Mudsnails

New Zealand mudsnails (NZMS) are native only to New Zealand, but spread to North America in the late 1980s. This small invader quickly spread to waters in Yellowstone National Park and is now found in many waters across the West. People spread NZMS attached to waders and fishing gear or by moving fish or bait.

  • NZMS average 1/8 inch in size but may be as small as a grain of sand. They have a gray, brown or black cone-shaped shell with 5 or 6 whorls.
  • They live in all types of waters, from silted river bottoms to clear mountain streams to estuaries.
  • Temperature tolerance is 32º – 77° F (66° F optimum).
  • They reproduce by cloning, so it only takes ONE.
  • Densities of over 500,000 per square yard have been reported in rivers in Yellowstone National Park.
  • They can survive for days out of water on moist gear.

At high densities, NZMS alter aquatic habitats and food webs by eating algae and out-competing native bottom-dwelling organisms. These ecosystem impacts can reduce trout and other recreational fisheries FOREVER. Prevent the spread of New Zealand mudsnails by taking at least one of the following actions after using gear:

New Zealand Mudsnails. Photo courtesy

  • Rinse waders and other gear with disinfectant and scrub with a stiff brush to remove mud and debris.
  • Remove snails from nooks and crannies (e.g. boot laces).
  • Use separate sets of gear for infested and non-infested waters.
  • Freeze gear for 6 to 8 hours (preferably overnight).
  • Place gear in a hot water bath of at least 130º F for a minimum of 5 minutes.
  • Dry gear completely for at least several days.
  • Clean boats and trailers after each use.



2016 Map of “Aquatic Invasive Mollusk Locations in Montana”. Graphic courtesy Montana Fish, Wildlife & Parks

Return to Chapter 6 Table of Contents