California Department of Fish and Wildlife
New Zealand mudsnails around a U.S. dime

New Zealand mudsnails around a dime

New Zealand Mudsnails FAQ

What is a New Zealand mudsnail?

A very small aquatic snail native to fresh waters of New Zealand.

When were they discovered in the U.S.?

They were first discovered in North America in the late 1980's in the Snake River, Idaho and Madison River, Montana.

When were mudsnails first discovered in California?

They were first discovered in 2000 in the Owens River. A review of past samples shows that there were low densities in 1999, but the snails were misidentified.

What is their current distribution in California?

There are occurrences of New Zealand mudsnail scattered throughout California. These animals spread very easily, so if they are found in one spot, it is possible that they are present upstream or downstream from that location, even though it may have not been reported from other sites in that watershed yet.  To see a map of infestations in California,  go back one step to the CDFW mudsnail page and follow the directions under “Map of CA Infested Areas”.

What are the potential impacts to fish and wildlife resources?

To date, there has been little research on the potential impacts of New Zealand mudsnails on other aquatic resources. Impacts could be significant if nothing is done to control its spread. If the snails become very dense and comprise a large percentage of the macroinvertebrate biomass, impacts can be substantial. They can reduce food resources and populations of other macroinvertebrates, particularly mayflies, caddisflies and chironomids. They can also reduce whole-stream algal production. There is very little information New Zealand mudsnail as a food resource for fish, but it does not appear as though they are the preferred food of trout. There is general consensus that New Zealand mudsnail could have a significant impact on trout fisheries, including federally listed species.

How are they spread?

It is believed they are spread into new river systems primarily by humans. Recreationists such as anglers, boaters and others transporting contaminated gear can easily move mudsnails to new locations. Biological consultants and researchers are others who frequent rivers and streams and, therefore, should clean their gear carefully. It is also possible that wildlife could spread NZMS.

Can they be eradicated?

Unfortunately, the New Zealand mudsnail is here to stay in the western U.S.

How can we control the spread?

Individuals working or playing in California waters should be aware of the potential for New Zealand mudsnails to be present. Remove visible snails with a stiff brush and follow with rinsing. If possible, freeze or completely dry out any wet gear.

What should I do if I think I find New Zealand Mudsnails?

Clean your gear thoroughly as recommended above, and send an e-mail reporting the location with your contact information to the CDFW Invasives Species group: