| Tuesday, May 03, 2005
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
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
"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
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
The Colorado Division of Wildlife has postedsigns warning of the
spread of New Zealand mud snails.