CMC's Denver Fly Fishing Section
New Zealand Mud Snail Update
By Charlie Meyers
Article published 5/3/05 in the Denver Post.

Just when you thought it was safe to go back into the water, we give you ... New Zealand mud snails.

Although not exactly man-eaters, these pernicious little freeloaders appear poised to gnaw their way down many Colorado streams, perhaps to the detriment of trout fishing.

Biology can be a cruel business. For Colorado Division of Wildlife aquatic managers finally drawing a fresh breath after more than a decade of constant battle against whirling disease, now comes another rogue organism to upset the natural balance

First revealed in Boulder Creek east of the city a few months ago, tiny snails barely a quarter-inch long have turned up in one of the state's most valued trout habitats. Confirming an earlier report by anglers, DOW biologist Jeff Spohn last Thursday found large numbers of snails in the South Platte River where it spills from Elevenmile Reservoir in the canyon of the same name. The density suggests the creatures probably have been present for a couple of years.

Spohn also discovered the snails had attached themselves to the boots of anglers who said they typically visit such widespread locations as the Colorado, Arkansas and other parts of the South Platte rivers.

Eric Hughes, DOW's aquatics chief, believes it's only a matter of time before detections are made elsewhere.

"The potential for spreading is profound," Hughes said.

Balancing these concerns comes anecdotal evidence that, while extremely worrisome, mud snails may not necessarily portend disaster for Colorado trout. Coincidentally, scientists from other states who attended a workshop in Denver several days ago reported there hasn't been a substantial impact on their fisheries.

"Maybe it's not the sensational thing we thought," Hughes said hopefully. Nevertheless, DOW has launched a widespread education program that includes posting signs in Elevenmile Canyon and elsewhere urging anglers to disinfect boots and other equipment. Posters also have been placed in sporting goods stores and other outlets.

The recommended procedure is to soak or spray gear with a 50-50 solution of water and the cleaning agent Formula 409* for five to 10 minutes. Boiling for five minutes or freezing overnight also is effective.

"We're trying to do all the reasonable things," said Hughes, who urged boaters and kayakers to take similar safeguards, including power washing their craft. "While it's realistic to think this will spread, we're trying to buy some time."

Hughes immediately ordered a statewide search to discover other streams that might be infected, a pursuit that soon will be slowed by the onset of runoff. A subsequent inspection of the Dream Stream section of the Platte uncovered no snails.

The principal danger is the way this miniature creature might compete with aquatic invertebrates that serve as a primary source of trout food.

Mud snails feed on decayed organic material that also nourishes aquatic insects, a competition that could cause slowed growth rates in trout where infestation is heaviest. Fish occasionally eat the snails, but studies on brown trout indicate only about one-fourth were digested, causing a steady weight loss.

While they surely pose a detriment to the aquatic environment, mud snails thus far have been less impactful than originally feared in places where they have held a foothold for many years. Transported to Europe 120 years ago, they turned up in the western U.S. in 1987.

Studies in California, Idaho, Utah and Montana indicate snail populations expand rapidly where there is a heavy nutrient load, then collapse when the food runs out.

Concern about the danger posed to threatened populations of cutthroat trout may be ameliorated by the fact that snails seem to prefer warmer water along the transition area from mountain to plain. The rocky, high elevations where cutthroat live may be poor snail habitat.

Hughes said that while snails thrive in the same silty areas favorable to the tubifex worms that serve as host to the organism that causes whirling disease, the implication of such cohabitation isn't yet known.

*   Important Update (04/13/06)  Most Formula 409-branded products do not have the necessary disinfectant / antibacterial ingredients. Check for antibacterial labeling before using.
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