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Old 05-08-2005, 08:55 AM
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Default New Zealand Mud Snail Found in the South Platte

[IMG2="left"][/IMG2]New Zealand Mud Snail Found in the South Platte

I don't feel that I should wait to share this info. Many of you will be going fishing on the South Platte system soon and you need to know this info. It is important.

The New Zealand Mud Snail has been found in the South Platte in 11-Mile Canyon below the dam

This is very bad news, according to the DOW and other organizations.

I have more info, but will share it in more detail as I become aquainted with it.

[IMG2="right"][/IMG2] At this time, please beware of your waders, nets and boots - anything that is in the water and might attract a wayward mud snail. If the critters attach, they will stay alive for several days or longer even though they are not in the water. Next time you wade or net a fish, they'll let go and we'll have them in Spinney, Spinney Mnt. Reservoir, Boulder Creek, Clear Creek, the Arkansas, the Big Thompson, the Poudre, the... You get the idea.

It only takes one mud snail to build an extended family.

Please: when you are done fishing, be certain to completely clean and dry your waders, boots, nets and anything else that hits the water.

Mud snails are very small, and they present a significant problem for our fisheries.


I hope you are well.

Happy Mother's Day, mommy.

Dale Darling

St. Vrain Angler

The New Zealand Mud Snail

Destroying a stream near you


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 adult 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.

It has been suggested that 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. Regardless, it's here. 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 700,000 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 65ºF and 70ºF. 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. 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. The invasion rate is amazing when put in the context that these mudsnails have been in the U.S. for less than twenty years.

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. 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 below) 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. The best source of up-to-date information is on the web at


4/22/2005. The following is an official statement from CDF&G:

"Preliminary results from laboratory tests and field trails demonstrate that solutions of 1,000 PPM copper sulfate, 2,000 PPM benzethonium chloride, or 50% Formula-409® kill New Zealand mud snails on fishing gear within 5 minutes. Fishing gear was completely submersed or put in a dry-sack with the solutions and shaken. The gear was then rinsed in fresh water. The laboratory tests demonstrated that these solutions had minimal effects on the integrity of the fishing gear. The study was funded by CALFED and completed by the California Department of Fish and Game with guidance from several angler organizations and the U.S. Fish and Wildlife Service. Study results are currently undergoing scientific and legal review."

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Old 05-09-2005, 08:23 AM
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Default Re: New Zeland Mud Snail Found in the South Platte

I pledge to do everything I can to prevent the spread of those little critters from where they already are to snail-free waters. I ask everyone here to do the same. (Especially don't spread them into the "dream-stream" section.)

Maybe the Colorado dept of fish and game should require everyone getting licenses to watch a 5-minute educational video about those things.

I'd rather hunt fish than bait deer any day.
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Old 05-09-2005, 12:20 PM
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Default Re: New Zeland Mud Snail Found in the South Platte

Because I fished recently on a California stream contaminated with these snails (and was unable to fish a very nice stream because it has been closed to prevent contamination) I am able to pass along one possible prevention technique. The freezer. According to the sign posted on the stream, waders, nets, boots etc can be put into a freezer for several days and this will kill the snails. This is inconvenient, to say the least unless you have a chest freezer dedicated to fishing equipment, but it might be more satisfactory than the chemical poison alternatives listed above.
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