Invading NZ snail shuts US hatchery

A tiny New Zealand snail has put a California trout farm off-limits after fish were found infested with the gastropod.

California state biologists this month found the mudsnail in the headwaters of the Upper Owens River, above a famous commercial trout hatchery and "fishing ranch".

The Department of Fish and Game has quarantined the hatchery in the Sierra Mountains, the San Diego Union-Tribune newspaper reported.

The tiny (5mm) aquatic snail was first found in the United States in the Snake River, Idaho, apparently after being imported among trout eggs from New Zealand in 1985. A single snail can clone itself without sexual partners.

The species has now spread through the western US.

The snails displace virtually all other bottom-dwelling species.

"Native invertebrates can decline from 50 to 90 per cent, with similar impacts on fish," Sacramento biologist Ken Davis told the newspaper. "They're tremendously harmful - a real cause for worry."

The mudsnail has reached densities as high as 750,000 per square metre in sections of rivers in Yellowstone National Park.

They eat the algae that would otherwise go to the larvae of caddisflies and stoneflies, important food for fish such as rainbow trout, native steelhead and salmon.

In New Zealand, the snails are kept in check by small worm parasites known as trematodes.

- NZPA