New Zealand mud snail invades the Eastern Sierra


Mar 11, 2001
Reaction score
New Zealand mud snail invades the Eastern Sierra

Outdoor News Service

No one wants to be a prophet of fishery doom on the eve of one of the most hallowed events in trout fishing -- this weekend's opening of the trout fishing season throughout the Eastern Sierra Nevada. But there is a dark cloud looming on the horizon.

The cloud is in the form of a tiny, dark snail called the New Zealand mud snail. It is invading North American trout fisheries and is rapidly choking out the food chain.

The fishery biologists are looking at the data and gasping, and then whispering among themselves when they envision the scenarios. The mud snail could dramatically change the face of fisheries in North America.

"It really scares me. We're not going to be able to contain this," said Dawne Becker, a fishery biologist with the California Department of Fish and Game in Bishop.

Becker said the small species was discovered in the Owens River in 1999, and its spread is rapid and significant. It has been in the Western United States since at least 1987, when it was first found in the middle sections of the Snake River in Idaho. It had found its way to the Madison River by 1989, and by 1994, it was throughout the greater Yellowstone ecosystem. By 1997, it had spread to over 300 miles of the Snake River, moving both upstream and down.

The small snail, which grows no bigger than 12mm, or about 1/2-inch, with most being 3.5mm to 5mm, can reproduce prolifically. Most importantly, it rapidly displaces other organisms. In the upper Madison River, it has already been found at densities of 300,000 to 1,000,000 snails in a square meter of stream bottom, and it comprises up to 95 percent of the total invertebrates at some sites.

    Anglers who love flyfishing should probably reread that paragraph. The invertebrates the mud snail is displacing are the mayflies, caddisflies and stoneflies that constitute the main forage for wild trout. The mud snail could spell doom for the prolific hatches that exist now on waters like Hot Creek, the upper Owens River, and other wonderful fisheries in the West like the Madison and -- God forbid -- the Henry's Fork of the Snake River. Not only might the hatches be ravaged, but the densities of the fish will fall dramatically because the mud snail has almost no nutritional value to the fish. In fact, in species like trout, the snail can close its operculum, the door to its snail shell, and survive passage through the fish's gut.

In the Owens River, surveys at three sites in 1999 found that it comprised a mere one percent of the invertebrate biomass at one site, four percent at another, and was absent at a third. Just one year later, the numbers were 29 percent, 35 percent, and one percent at the same three sites.

"It's had to displace something. We need to research this thing and see its impacts. But it may already be too late," said Becker.

Fishery agencies could be in a situation where they are merely going to be able to document the decline and loss of aquatic diversity and fisheries.

The New Zealand mud snail has already hitch-hiked all over the world, arriving in England's Thames River in about 1883 and by the 1940s it was found in much of Europe.
Its impacts there?

Unfortunately, little is really known because so little baseline data was collected prior to the snail's arrival. There is only anecdotal evidence from the memories and writings of anglers that fisheries and invertebrate populations have crashed. Descriptions of blizzard-like hatches of insects that brought fish to the surface by the thousands were considered exaggerations, literary license abused by fishing writers of early England. But maybe they weren't exaggerating.

We will know here soon enough here. Americans have made the study of their favorite trout waters a religion. Aquatic insects have been cataloged, size and duration of hatches charted, and biomass of the trout fishery measured. It doesn't take a rocket scientist to figure out that if Hot Creek has a conversion to mud snails then its currently prolific hatches of mayflies, caddisflies, and midges will decline.The small stream will lose its appeal to anglers without those major hatches and if its rich trout resource dwindles.

In the short term, fisheries managers are trying to stop the spread of the snail by asking anglers to make sure they clean and thoroughly dry their waders and other gear that might carry a snail from water to the next. Anglers will be asked not to move trout from one water to the next, even dead ones, or clean fish in places other than where they were caught.

In the long term, it may prove that the New Zealand mud snail only will dominate under certain conditions and that -- as with whirling disease -- the snail will not cause more than localized problems for trout fisheries.

Still, as a fly-fisherman, this is a plague I find myself fearing because it could severely impact the insect populations that cause such delight in me and the trout. There is something magical about watching a major hatch of mayflies or caddisflies exploding through a stream's surface.

That magic may be in jeopardy.

Contact Information: Jim Matthews
Editor/General Manager
Outdoor News Service
P.O. Box 9007
San Bernardino, CA 92427-0007
(909) 887-3444 office
(909) 887-8180 fax


Active member
Aug 17, 2002
Reaction score
I saw that story doesn't seem like there is much we can do.. Any ideas ?? escargo maybe...

Latest Posts

Top Bottom