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The New Zealand Mud Snail: Destroying a stream near you



Get the full report of California Department of Fish and Wildlife and US Fish and Wildlife NZMS decontamination testing.

Get the report from Colorado division of Fish and Wildlife comparing Formula 409 to Sparquat 256. Please note that the formulation of Formula 409 used in the study is no longer available.

Sparquat 256 is a highly effective molluscicide that is also proven effective against whirling disease. It is considered the gold standard for disinfecting wading and aquaculture equipment by many agencies, but is not readily available to the average consumer. Sparquat 256 can be sourced here.


2) 3/4 tsp (3.8 grams) of 99% Copper Sulfate Pentahydrate (available on-line or from farm supply) in 1 gal.of water

Gear can either be put in a bucket to soak, or placed in a plastic bag with solution, shaken and allowed to steep for 5 minutes. The copper solution can also be sprayed on gear (409 doesn't work sprayed). To remove any snails that may not have been immediately killed, thoroughly rinse gear with clean water before transporting to a new location. Both chemicals cause snails to withdraw into their shells making them easier to remove than using water alone. Dump used solution into municipal sewage facilities if possible or onto bare earth well away from water. There is heated debate within the California Department of Fish and Game whether copper sulfate (a highly toxic substance) is suitable for general public use.


3) Freeze gear solid . This might take 6-12 hours or more.


4) Completely dry gear.

This isn't as easy as it sounds, a slight damp spot under the insole can harbor live snails for over a month. Heat will assist in killing snails during drying.

5. Consider dedicating a set of wading gear to waters known to harbor NZMS.


A brown to black very small (maximum size of 5 mm) mudsnail with a sharply conical shell with five (rarely 6) spirals. The small size and 5 spirals are distinguishing characteristics for layman’s field identification.

Natural history

A native to New Zealand, the New Zealand mudsnail (Potamopyrgus antipodarum) (NZMS) lives in a variety of habitats ranging from estuaries and lakes to large rivers and small streams.  It feeds on bottom dwelling algae (periphyton) and detritus. The mudsnail can reproduce sexually or through the process of parthenogenesis, which produced clones of the female mudsnail. In its native waters the mudsnail population is primarily kept in check by trematode (small worm) parasites that sterilize the snail or change mudsnail behavior making it more likely that mudsnails are eaten by foraging waterfowl.

Probably around 1986 the mudsnail was imported from New Zealand into a trout hatchery in Idaho from which it was widely disseminated through trout stocking. Another theory suggests it made its way into the States via Australia. In September 2005 a second morph, possibly a second species, of NZMS was discovered in the Snake River. The Western American strain is clonal and apparently did not bring the normally associated trematode parasites with it. Without its natural enemies, the mudsnail has spread uncontrolled through some of the most productive waters in North America.

The mudsnail has a tremendous propensity to rapidly populate its environment, and upwards of 750,000 mudsnails per square meter have been found in some waters. The mudsnail does not appear to be self-limiting from density dependent effects. Their sheer numbers dominate the base of the food web and they can consume over 80% of a river’s productivity. Their presence exerts a negative impact on mayflies, stoneflies, caddisflies and some midges. In one Montana stream infested with NZMS only 3% of the macroinvertebrate biomass now consists of indigenous species. Conversely scuds and some midges thrive on the mudsnail’s nitrogen rich feces.

Even when deprived of other food, trout and sculpins avoid feeding on the NZMS and one study suggests that even when they are consumed, mudsnails can pass through the digestive system of trout unscathed. Whitefish and some crayfish do feed on the mudsnail, but studies suggest that mudsnails are a poor source of nutrients compared to other aquatic invertebrates. Though quantitative analysis is not yet published, it appears quite likely that the presence of large numbers of NZMS can have a profoundly negative impact on a trout or salmon fishery.


In lab studies the mudsnail selects gravel as its preferred substrate; in the wild they can inhabit just about any stable substrate but seem to do best in areas where streambeds and riparian zones have been disturbed or altered. Spring fed creeks and tailwaters produce the highest densities of snails. Areas of high water velocities produce the least suitable habitat. Diverse, natural habitats seem fairly resistant to NZMS colonization, but this may be a result of the habitat preference of particular clones that have been introduced. So far, high-gradient first order streams and lakes have not been colonized in the West (Lake Ontario supports a NZMS from a European clone. This appears to be a mudsnail specifically predisposed to dwelling in lakes).

Though the NZMS can tolerate temperature extremes from near freezing to 90°F, they thrive between 65°F and 70°F.  Water chemistry appears to play only a minor role in growth and reproduction rates.


The NZMS has been documented in the Columbia, Snake, Missouri, and Colorado drainages. In 2000 snails made landfall in California in the Lower Owens River near Bishop. Since then it has moved throughout the Owens drainage including Hot Creek, Rush Creek and Lone Pine Creek. In October 2003 mudsnails were discovered in Putah Creek and two months later in the Mokelumne. In January 2004 a well established population was discovered in an eleven mile reach of the Calaveras River. A population was found in the Napa River drainage later that summer that was possibly spread by construction equipment that had been used on the Mokelumne. NZMS were later documented in Piru Creek and by early 2006 NZMS populations were popping up in small creeks throughout Southern California. On December 29, 2006 NZMS were found in the spring that feeds the Department of Fish and Game's Hot Creek hatchery near Mammoth Lakes. Director Ryann Broddrick made the decision to continue to plant the contaminated trout into waters already infected with NZMS.

In 2007 NZMS were discovered in the Lower American River near Sunrise Bridge (home to one our largest hatcheries); in Alameda creek then Hayward Creek, the largest watershed within the San Francisco Bay region; and in Santa Cruz's San Lorenzo River - home to a nearly extinct population of coho salmon. In late 2007 snails were found in Lake Shasta - the only colony of lake dwelling snails in the US outside of Lake Ontario. It may be a new strain, or simply the original California strain that has adapted to still water. A year later they were discovered in Big Lagoon, and in the nearby tidewater narrows between Lakes Earl and Talawa, Humboldt County. Later in 2007 they appeared in Antioch Creek in the South Bay. In 2008 NZMS were found in the Russian, Smith, and Klamath Rivers. As of September 2009 additional infestations were discovered in Baxter Creek in the East Bay and in the Stanislaus River. In May of 2013 NZMS were confirmed in the Truckee River near Reno, Nevada.

The invasion rate is amazing when put in the context that these mudsnails have been in Current distribution of NZMS in California.

Within a river system downstream drift and spread is facilitated by the mudsnail's propensity to cling to leaves and "raft" with the current. Upstream spread is poorly understood. The invasion is rarely a smooth march outward, but hop scotches like spotting before a forest fire which suggests spreading by vector. Birds, fish, and even cattle have been implicated on an anecdotal level, but evidence strongly points towards wading anglers as the primary source of NZMS dispersion. In a recent survey of 50 wading anglers at Putah creek, 100% of the fishermen had mudsnails on their waders and/or in their boots (an average of 33 snails per angler!). The possibility has been raised that the Mokelumne may have been inoculated from the Calaveras by salmon monitoring activities.


With current technologies, once established NZMS are unlikely to be controlled. The importation of parasitic trematodes has been considered but the potential disruption to native fauna has yet to be established. Since the mudsnail is clonal it has very little genetic resistance to a clone specific disease process and perhaps genetic engineering will provide us with a targeted weapon against the NZMS. Perhaps the mudsnail will take care of itself. Invasive exotics are well known for overshooting the carrying capacity of their new found home. NZMS populations in Lake Zurich rose dramatically then crashed. The populations in Denmark have all but disappeared. Scientists still do not know why.


Since NZMS populations can't be controlled once they become established, the first order of defense is containment. Angler awareness is paramount since it is likely that the spread of the mudsnails is strongly associated with wading. Thorough decontamination (see top of page) of wading gear is mandatory to halt the spread of NZMS. Though inspection of wading gear has been suggested as a measure of control, the fact that snails may be a small as a fleck of pepper make this route highly unreliable. Immediate, frequent, and persistent bio assessment is needed to identify and (hopefully) contain incipient populations of NZMS.


Further information

There is almost as much urban legend as fact regarding the NZMS. Even some “facts” are questionable and information is changing by the week. Excelent up-to-date information is available at and