Invasive New Zealand mudsnails found at Lake Sonoma fish hatchery

A tiny aquatic snail that has invaded waterways around California and much of the country has been detected at the Lake Sonoma fish hatchery for the first time, raising impediments to the release of young coho salmon into the Russian River watershed later this year and, likely, in the future.

Invasive New Zealand mudsnails were discovered during routine surveillance at an intake pipe that draws water from the reservoir for use in the U.S. Army Corps’ coho and steelhead trout broodstock program, according to the California Department of Fish and Wildlife, a partner in the program.

Mudsnails also were detected in a connected aeration pond, where extra oxygen is added to the water before it is piped to the hatchery.

Their existence in the system means juvenile fish reared in the hatchery can only be released where the gastropod already is known to exist — which includes the lower Russian River, Dry Creek and Lake Mendocino, an Army Corps official said.

But at least 12 of the 15 tributaries where endangered coho salmon fingerlings and smolts otherwise would be set free are off-limits so far: New Zealand mudsnails haven’t been detected there and, so, are presumptively absent, according to Nick Malasavage, chief of operations and readiness for the Corps’ San Francisco District.

Under California Department of Fish and Wildlife policy, “we can’t knowingly take fish from a (mudsnail) positive environment and introduce it somewhere the snail hasn’t been found,” Malasavage said.

Unlike invasive quagga and zebra mussels, which pose a threat to pipes, machinery and other human-made infrastructure when present, due to their prolific reproduction and tendency to clog up available space, the primary risk of having mudsnails in a waterway is one of competition with other invertebrate species, experts say.

As they proliferate, the invasive snails can consume up to half of the food resources in a waterway, reducing populations of mayflies, caddisflies and other more nutritious aquatic insects trout and salmon need to survive, according to the state Department of Fish and Wildlife.

At only a few millimeters in size, the mudsnails are extremely easy to transport unknowingly on boats and other watercraft, boots and waders, recreational and angling equipment, and even through the waste of fish and birds, which can excrete them alive.

“I actually thought they were just little pieces of coarse sand” upon seeing some in a jar, Malasavage said. “That’s how small they were.”

Quagga and zebra mussels pose a particular threat to lakes Sonoma and Mendocino, which provide critical water supply sources. The reservoirs both suffered near-misses — Lake Sonoma in 2020 and Lake Mendocino in 2018 — when invasive mussels were detected on boats headed for launch by specially trained sniffer dogs used for inspections since 2012.

The Sonoma County Water Agency just announced almost $400,000 in state funding to extend the program for the next two years at Lake Sonoma. It received a similar grant from the California State Parks Division of Boating and Waterways last year for Lake Mendocino.

New Zealand mudsnails, meanwhile, reproduce asexually, producing up to 230 young a year that are born live, with developing embryos already inside, Fish and Wildlife said.

One individual can produce more than 2.7 billion descendants in four years, the agency said.

“There are a lot of vectors that, because they’re so small they can attach to fish. They can be consumed by fish, if in their diets, as well as birds,” Malasavage said.

A U.S. Geological Survey “animated map” shows the first report in 1987 of the mudsnail in Idaho’s Snake River and follows its increasingly frequent detection throughout the western states and the Great Lakes region over the ensuing decades.

It was first found in California in 2000 in the Upper Owens River, west of Bishop, in the eastern Sierra Nevada, according to USGS.

The gastropod has since been observed at well over 100 locations around the state, including the Russian River watershed, where it was first detected in 2008 at several sites near Hopland, Ukiah and nearby Talmage, in Mendocino County.

In the lower reaches and Sonoma County, the mudsnail was first reported in 2009 near Wohler Bridge in Forestville, where it was found on water quality monitoring equipment at Sonoma Water’s diversion structure, where river water is filtered and diverted to aqueducts feeding regional contractors and consumers, according to USGS.

An established colony of mudsnails also was found in Dutch Bill Creek, south of Monte Rio, in July 2022, and at Salmon Creek State Beach on the Sonoma Coast two months later.

The snail also has been reported in Dry Creek, which drains Lake Sonoma, the East Fork Russian River, the Napa River and Putah Creek on the southeast side of Lake Berryessa, as well as near the mouth of the Garcia River on the Mendocino Coast.

State Fish and Widlife announced last month that an infestation had been discovered in south Lake Tahoe.

State Fish and Wildlife conducts surveillance at the 40-plus-year-old hatchery roughly quarterly, using small boxes at various sites to see if mudsnails settle there, Malasavage said.

They were found at the hatchery June 16, prompting a surge of inspections around the site and all of Lake Sonoma.

“Our first effort was to look in Lake Sonoma because we wanted to see if that was the source of the introduction to the hatchery system,” said David Hines, senior environmental scientist supervisor withe state Fish and Wildlife.

“We did kind of a blitz survey. We had well over a dozen people — maybe 15 or 20 people — out there the better part of the day, looking at different locations around the lake and the area, and we never did see them,” Hines said.

But particularly since the shoreline has risen and changed so drastically over the past year, after three years of drought shrunk the lake to its lowest level ever, it remains possible the snails are there, he said.

Teams also inspected all the sites where coho salmon are typically released — usually as advanced fingerlings between October and late December, as pre-smolts during January or February and as smolts between March and May — for signs of New Zealand mudsnails, Hines said.

Between 100,000 and 200,000 coho are raised at the hatchery each year, and about 400,000 steelhead, half each distributed to lakes Mendocino and Sonoma, Malasavage said.

The partner agencies involved in the program now have to reconfigure their release plans, which are likely to evolve as time goes on, Hines said.

But for the moment, it appears a delay is likely in the release of young coho, so those that were to be released this fall will have to wait until spring.

You can reach Staff Writer Mary Callahan (she/her) at 707-521-5249 or mary.callahan@pressdemocrat.com. On X @MaryCallahanB.

Editor’s note: This story has been updated to correct information about funding for quagga and zebra mussel inspections at lakes Sonoma and Mendocino.