CDFW: Unclean gear helps spread invasive New Zealand Mudsnails

Aquatic species has been detected across the state, but can be kept out of local waterways with basic maintenance, officials say.

STATEWIDE—The opening of steelhead trout fishing season in California has opened the door for the state’s Department of Fish and Wildlife to issue a public service announcement: anglers should clean their gear after every fishing trip in order to prevent the spread of New Zealand Mudsnails in local waterways.

The New Zealand Mudsail, or NZMS, is a tiny aquatic snail, measuring 3/16th of an inch.

“The resiliency of NZMS has enabled them to spread to aquatic environments across the globe, including Europe, Australia, North America and Asia,” CDFW staff stated. “To prevent further spread, anglers and others who wade or work in California waters should thoroughly clean all gear before using it in another waterbody.”

New Zealand Mudsnails were first found in Owens River in 2000; Owens River runs through Inyo and Mono Counties. The aquatic snail was again found in 2003, at Putah Creek in Yolo County and Mokelumne River. State officials have since identified the New Zealand Mudsnail in 27 other counties, including Los Angeles, Orange and Ventura.

“NZMS can survive in a variety of environments including rivers, streams, reservoirs and estuaries. They are parthenogenic live-bearers, meaning they self-reproduce. This enables a new population to begin from only one snail,” CDFW staff stated.

State officials added the New Zealand Mudsnail population is impossible to control once it reaches a certain critical mass. A large population of New Zealand Mudsnail would not allow steelhead trout to survive, state officials continued.

“NZMS have demonstrated the potential to reach high densities of up to nearly one million snails per square meter and comprising up to 95 percent of the invertebrate biomass of a river. When they reach high densities, NZMS outcompete native insects and invertebrates for space and algae, which reduces the forage available to steelhead trout. As a result, steelhead populations can decline,” CDFW staff said in a released statement.

“Once NZMS are established in a body of water, it may not be possible to eradicate them,” CDFW staff continued.

There are a few steps anglers can take to prevent to spread of New Zealand Mudsnails. Anglers are encouraged to inspect all of their fishing gear – including boots, float tubes and wades – immediately after leaving a body of water. All visible debris should be removed with stiff brush and rinse on site.

Waders and other gear should be frozen overnight – and for at least six hours – or completely dried between use.

“Use additional waders and boots in infested waters and store them separately,” CDFW staff added. “Never transport live fish or other aquatic animals or plants from one waterbody to another.”

CDFW Senior Environmental Scientist George Neillands said the New Zealand Mudsnail does not infect several watersheds and counties. The angling public, however, should take steps to ensure local waterways remain free of the aquatic snail, Neillands added.

More information about the New Zealand Mudsnail is available online at

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