The Newest Threat
Excerpted from: The New Zealand Mudsnail, by Ralph Cutter, California School of Flyfishing
Submitted by Bruce Dau
There is a tremendous amount of pseudo-science surrounding the New Zealand mud snail. Even "authorities" are repeating untested, untried, and simply untrue information. This article has been fact-checked by some of the world's top mud snail experts.
Description: 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 (see picture).
Natural history: A native to New Zealand, the New Zealand mudsnail (NZMS) lives in a variety of habitats ranging from estuaries and lakes to large rivers and small streams. It feeds on bottom dwelling algae and detritus. The mudsnail can reproduce sexually or through the process of parthenogenesis, which produces clones of the adult mudsnail. In its native waters the mudsnail population is primarily kept in check by trematode (small worm) parasites.
Around 1986 the mudsnail is thought to have been imported from New Zealand into a hatchery in Idaho from which it was widely disseminated through trout stocking. This 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. It 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. 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, some 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.
Habitat: 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. Diverse, natural habitats seem 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).
Distribution: The NZMS has been documented in the Columbia, Snake, Missouri, and Colorado drainages. Until the October 2003 discovery of mudsnails in Putah Creek, the California distribution was limited to the Owens River and the lower reaches of some of its tributaries. The Owens River and Putah Creek invasions were almost assuredly caused by wading anglers.
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. Birds, fish, and even cattle have been accused on an anecdotal level, but evidence strongly points towards wading anglers as the primary source of NZMS dispersion. The establishment of the Putah Creek population is an important stepping stone for the NZMS. It has finally breeched the Sierra and its obvious invasion fronts are the nearby American and Yuba River watersheds (see Editor's note below).
Control: There is little control once a NZMS population has become established. The importation of parasitic trematodes has been considered but the potential disruption to native fauna has yet to be established. Perhaps genetic engineering will provide us with a targeted weapon against the NZMS, or perhaps the mudsnail will take care of itself. Invasive exotics are well known for overshooting the carrying capacity of their new found home.
Containment: If we can't control the population of mudsnails once they become established, our first order of business is containment. Angler awareness is paramount since it is likely that the spread of the mudsnails is almost wholly dependant on wading fishermen. Thorough decontamination (see below) of wading gear is mandatory to halt the spread of NZMS. Immediate, frequent, and persistent bio assessment is needed to identify and (hopefully) contain incipient populations of NZMS.
Decontamination: There is no perfect method for decontaminating waders and wading shoes. Clorox bleach is the gold standard for killing mudsnails. A 5% solution will kill 100% of the mudsnails with a 5 minute exposure. Unfortunately many branded bleaches are only half as strong as the 6% Clorox and when opened, any bleach deteriorates rapidly. Also bleach will react quickly with organic debris (mud) attached to wading shoes and can possibly lose its efficacy before killing the mudsnails. Desiccation is very effective as long as the mudsnails are dried completely for at least four hours in the hot sun or for more then a day at room temperature. Grapefruit Seed Extract, an organic, biodegradable food product should be encouraged when decontamination temperatures are above 45°F.
The best way to decontaminate gear is to place it in a river runner's dry bag with several quarts of solution. The bag is then sealed and shaken vigorously and allowed to steep for the suggested period of time. The solution must be discarded well away from any water source.
Visit Ralph Cutter's website at: www.flyline.com/NZ_mudsnail.htm
Suggested solutions for decontaminating wading gear (minimum of 5 minute exposure):
1/2 cup Clorox/ gallon of water (any temp)
or
2 teaspoons Grapefruit Seed Extract/gal water (at least 45 degrees)
Editor's note: The 12/28/03 San Francisco Chronicle reports that NZMS have been found in the Mokelumne River east of Lodi. The article also reports that a fishing closure on Putah Creek went into effect on 12/26/03 and will last for 120 days. The Mokelumne is already closed because of spawning salmon and steelhead.