The Invasion of the Killer Mud Snails:
A New Threat To Salmon and Trout

by Dan Bacher

December 9, 2003

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California’s fisheries, already beset by invasions of mitten crabs, northern pike and other exotic species that disrupt the aquatic food chain, are now threatened by the New Zealand Mud Snail.

Faced with an infestation of these snails in Putah Creek below Lake Berryessa, the California Fish and Game Commission on December 3 unanimously recommended a 120-day emergency fishing closure of the creek below Highway 128 and Lake Solano to prevent the snail’s spread.

A special Commission hearing on the emergency closure was set for December 16.  The reason for the emergency action is because the snail is notorious for having a severe impact upon the forage of wild trout and salmon, since it replaces the aquatic invertebrates that these fish depend upon and offer almost no value to feeding trout. The mud snail could have a dramatic affect upon threatened and endangered trout and salmon if it spreads to other waters in the state.

Ken Davis, an independent aquatic biologist on contract to state agencies who testified before the commission, discovered the mud snail by accident when he was fly fishing in Putah Creek on October 30. The DFG’s Bioassesment Lab confirmed that they were indeed the invader snails on November 3.

Davis, local fly fishing groups and state and federal agencies began an assessment of the creek to find out where the snails were found. Granite Bay Flycasters and other groups began putting up posters along the creek and in local sporting good stores and fly shops alerting locals about the infestation.

“We don’t know how far down the creek the snails are yet, so we started surveying the creek below Winters on December 1,” said Davis. “It’s common sense if a patient has TB, that you don’t wait until the disease has spread throughout the population before you do something about it. Likewise, it’s prudent to isolate the creek to prevent the spread of this invasive species.”

These tiny snails can form colonies as dense as 500,000 per square meter. “They graze across habitat normally used by mayfly larva and other grazing aquatic invertebrates,” noted Davis. “Research in Yellowstone National Park has shown a severe reduction in native mayflies and stoneflies in some areas. At those sites, the NZMS now constitute 50 percent of the total invertebrate population.”

If accidentally introduced into the Upper Sacramento, Hat Creek, McCloud River or other Central Valley streams, these snails “have the potential to severely alter the aquatic and riparian food web forever,” added Davis.

If the snail is found to have already spread to other waters in the Central Valley, there isn’t much that can be done about it. However, if it is just Putah Creek and Lake Solano where the pest is found, then the infestation is potentially controllable.

Hal Cribbs, representing the Granite Bay Flycasters, Trout Unlimited and the Northern California Federation of Fly Fishers, explained how anglers are a key vector in the spread of the snail, since the small 4 to 5 mm snails are easily transported on waders and boots. He urged that the Commission send a letter to Governor Arnold Schwarzenegger asking for an exemption to his Executive Order putting an 180-day hold on all administrative regulations enacted since 1999 and subjecting them to a 90 day review.

Gary Flanagan, president of the Granite Bay Flycasters, emphasized that Davis and fly fishing groups had already done an intensive outreach to the fishing public, urging them to treat their waders after fishing and putting up posters on the creek about how to clean their waders to kill the snail.

The snails, which can survive for many days tucked away in wading shoes, can be killed by soaking equipment in 10 percent household bleach for an hour. Freezing, dessication and soaking equipment in water heated to at least 130 degrees F will also kill the snail.

However, Flanagan noted that voluntary compliance wasn’t enough, since studies in Montana show that that there is less than a 1 percent compliance rate by anglers on posted streams.

“An emergency closure for the creek is warranted, since it’s a unique, special case,” said Ed Pert of the California Department of Fish and Game. “In this case, we may have a chance to eradicate the snail before it spreads.”

Commission Member Robert Hattoy questioned Pert as to whether the problem could be solved in 120 days. Pert responded that this period can allow the Department and other agencies enough time to figure out a plan for controlling the infestation. The survey of Putah Creek to see how far the snail has been spread will be accompanied by surveys for the snail in other waters west of the Sierra.

Mike Flores, president of the Commission, quipped before the Commission’s vote, “What we need is a stone fly that eats the mud snails.”

Unfortunately, there is no such predator. For the time being, we can only hope that the invasion of the mud snail hasn’t already spread to other waters. Right now, Putah Creek is the only watershed in California, other than Owens River and Hot Creek in the Eastern Sierra Nevada, where the snail has been found.

For more information on the New Zealand Mud Snail, visit the Granite Bay Flycasters website at www.gbflycasters.org.

Daniel Bacher is an outdoor writer/alternative journalist/satirical songwriter from Sacramento California. He is also a long-time peace, social justice and environmental activist. Email: danielbacher@hotmail.com


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