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New Zealand Mudsnail Closures - Year Round

New Zealand Mudsnails by keys for size comparison, photo by Dan L. Gustafson

Stop the Spread


Help City of Boulder Open Space and Mountain Parks Stop the Spread of New Zealand Mudsnails

New Zealand Mudsnails have recently been discovered in South Boulder Creek downstream of South Boulder Road. This invasive aquatic species negatively impacts local fish populations and disrupts ecological food webs by displacing native macroinvertebrates.

What Are New Zealand Mudsnails (NZMS)?

  • Invasive aquatic snails from New Zealand
  • Accidently introduced to US in 1987, now in streams and ponds across the western US and Great Lakes region, including Colorado
  • First discovery in Colorado was in Boulder Creek in 2004, now found in 10 counties throughout the state
  • Since 2004, the snails have spread around Boulder and are now found in Boulder Creek, Dry Creek, 4-mile Creek, South Boulder Creek, Wonderland Creek, Goose Creek, and Elmer’s 2-mile Creek
  • To help prevent further spread of NZMS, OSMP has enacted year-round closures on portions of Boulder Creek and Dry Creek, and a temporary closure of South Boulder Creek from South Boulder Road to Marshall Road
  • During the South Boulder Creek temporary closure, OSMP will assess its current management of the creek and may implement additional measures and creek access restrictions

Size of NZMS compared to a dime

Photo courtesy: Colorado Parks & Wildlife

What’s the Problem?

  • They’re tiny!
    • Adults are about the size of a grain of rice and immature snails are even smaller
  • They’re clingy!
    • Because they’re so small, they can sneak aboard almost anything including waders, boots, floaties, and even between the pads of a dog’s foot. If it can carry a sand grain, it can carry a mudsnail.
  • They’re hardy!
    • NZMS can survive out of water for days
  • It only takes one!
    • NZMS reproduce asexually (i.e. cloning) so a single mudsnail can result in a colony of 40 million snails in just one year.
  • They’re tenacious!
    • Once they establish in a creek, it’s practically impossible to get rid of them.
  • They’re bad news!
    • NZMS can achieve densities of over 70,000 snails per square foot. They displace native aquatic invertebrates (which fish eat) and pass through fish digestive systems without being digested. Ultimately this can result in reduced growth rates and lower populations of fish. 

What can you do?

  • Stay out of closure areas.
  • If possible, avoid entering infested streams in areas not closed to public access.
  • If you do enter an infested area:
    • Before you leave the field:
      • Clean your boots, clothes, and gear of mud/vegetation
      • If dogs enter creeks, guardians should carefully brush their paws and bellies on dry land
    • Back at home, do one of the following:
      • Freeze boots/waders for a minimum of 6 hours (overnight)
      • Soak gear in hot water (at least 140°) for at least 10 minutes
      • Submerge gear in 1:1 solution of water and Formula 409® for at least 10 minutes (Note: simply spraying gear with disinfectant does not work)
      • Thoroughly dry your gear for at least 48 hours, preferably in direct sunlight

For more information, please see Colorado Parks and Wildlife’s New Zealand Mudsnail page.