ESPN Outdoors

New Zealand snails invade the Owens River

A small snail native to New Zealand may endanger native fish stocks

By Bill Becher
Special to ESPNOutdoors.com

On a dime: New Zealand mud snails from the Lower Owens River in California.

BISHOP, Calif. — Alien invaders that clone themselves and can't be killed by ordinary means? Is this a summer blockbuster movie starring Will Smith? Nope, it's a tiny snail from New Zealand that has been found in the Lower Owens River near Bishop, California.

Biologists from the California Department of Fish and Game are concerned about possible impacts of the snail on this popular sport fishery in the Eastern Sierras.

While the snails do not directly harm fish, they don't provide much food value to trout and other fish species. The snails can pass through a trout's digestive system unharmed, according to Fish and Game biologist Dawne Becker.

The snails can potentially out compete other invertebrates, which can reduce fish forage that support trout and native fish species.

Don't expect to see New Zealand Mud Snails Avec Garlic Butter featured at your favorite French restaurant. They're tiny, only about a quarter of an inch long. But they multiply quickly, and seem to have no natural predators in California waters.

	“	This is definitely cause for concern based on the impact we've seen in other areas. ”
		— Darrel Wong, Environmental Specialist Supervisor, DFG

Potamopyrgus antipodarum (the scientific name for the snails) were first discovered in the United States in the Snake River in Idaho in 1987. Since that time they have been found in waters in and near Yellowstone National Park.

Exactly how they got to the United States is unclear, though one theory is that they were in a shipment of trout eggs from New Zealand. Becker has sent the Owens River snails to a lab in Florida for DNA testing to see if they are from the same strain as the Yellowstone snails.

According to the National Park Service, "These small mollusks have the potential to 'cover the stream bottom', similar to impacts observed with the zebra mussel in the Midwestern U.S."

In other trout water, such as the Madison River near Yellowstone in Montana, as many as 300,000 snails have been found in one square meter of river bottom.

I accompanied Becker when she sampled a section of the Lower Owens River near Bishop. She found caddis larvae and stonefly nymphs in her net, prime trout food, but no snails, though this was not a complete survey.

Becker, who is tracking the snail's spread, says "We know that they are in the Upper Owens River, from Crowley Reservoir, upstream about five miles. They are also in the lower section of Hot Creek. They have just been found in the Bishop Creek Canal, in the Owens Valley."

Fish and Game biologist Dawne Becker examines her net for signs of New Zealand mud snails.

According to Becker, "If waders or other equipment that entered New Zealand mud snail habitat were transported without completely drying out, upon entering a new water, any snails attached to the equipment would be released. If a snail was kept moist and not exposed to excessive heat, its operculum, a hard structure that allows it to close its soft body in its shell, would enable it to survive out of water for up to 25 days. The New Zealand mud snail is parthenogenic, which means it only takes one snail to start a new population."

Like my time in high school, the mud snail has no sex life. Each snail carries up to two hundred or so snail eggs in it's reproductive system, and doesn't need to mate to produce offspring, which are in essence clones of the original snail.

The snails are hard to kill. Pat Dwyer of the U.S. Fish and Wildlife Service tested different ways to remove the snails from equipment.

He tried immersion in strong bleach and in copper sulfate, common disinfectants. But the snails survived. Only submerging them in water heated to 133 degrees (about the temperature of hot tap water) for over a minute killed the snails.

When I checked the Owens River above Crowley Lake, where the snails were first discovered last fall, they were easily visible, like BB's scattered on the river's bottom. It is this remarkable ability to reproduce that has scientists worried.

According to Becker, high densities of mud snails are correlated with lower densities of some mayflies and stoneflies, prime trout food. But establishing exactly what the impact on fisheries may be is difficult, as the snails have not been found in areas where fisheries biologists have long-term, base line fishery population and condition information.

Slow the spread

According to Dawne Becker, biologist with the California Department of Fish and Game, "Traveling from one water body to another can transport the New Zealand mud snail, as well as Chytrid fungus (we think), and Whirling Disease and who knows what else!

Allowing waders to dry completely prior to going to a different water will help slow the spread of the snails.

If that is not feasible, if an angler plans to fish in a known New Zealand mud snail site, it would be preferable to fish other sites prior to the contaminated site.

In areas of high snail densities anglers may be able to see the substrate peppered with little black snails. If this is the case, please clean waders well prior to visiting another water."

Becker says, "It will obviously affect the ecosystem balance and will compete directly and indirectly with aquatic invertebrates, many of which provide a forage base for our recreational fisheries."

According to Darrel Wong, Environmental Specialist Supervisor for the Department of Fish and Game, "This is definitely cause for concern based on the impact we've seen in other areas."

He adds that the DFG has good baseline data for the Owens River, and hopes to encourage researchers to undertake long-term studies of the impacts of the mud snails. The current infestation is relatively close to the Hot Creek and Alpers fish hatcheries, but what, if any impacts, the invasion of mud snails into hatcheries would cause is unknown at this time according to Wong.

The Department of Fish and Game's Statewide Invasive Species coordinator, Susan Ellis, is working on putting together a State Invasive Species Management Plan, which will standardize how the DFG deals with new and existing invasions, as well as make the state eligible to receive Federal funds.

While the DFG and others are studying ways to control the New Zealand mud snail, introducing poisons into the Owens River, which is part of the Los Angeles water supply, is not an option. "We may have to learn to live with the snails," says Becker.

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