The embattled Carmel River steelhead face a new obstacle to recovery – invasive snails.

The Carmel River steelhead, a federally threatened species, can’t seem to catch a break.

Despite the removal of the San Clemente Dam in 2015, the species’ recovery has been impeded by invasive striped bass, which can prey on steelhead.

Now, there is a newer invader – New Zealand mudsnails, tiny invertebrates about the size of a large grain of sand. Biologists with the Monterey Peninsula Water Management District first discovered the snails in the river system from samples taken in November 2016, which were identified in a lab in February 2017.

That’s bad news for the steelhead: The tiny snails multiply rapidly, and have since established themselves in the river from Highway 1 to the western edge of Garland Ranch Regional Park. In at least one place in the river, the snails already account for up to 90 percent of the invertebrate biomass.

And while steelhead feed on invertebrates (among other things), the problem with the invasive snails is that the steelhead cannot digest them, but they eat them and later pass them, alive. That means their continued spread – as steelhead swimming upstream pass them through their digestive tract – seems all but assured.

They are also easily spread by people – on boots, clothing, etc. – which is how they arrived in the first place.

John Olson, a professor at CSU Monterey Bay, has been spreading the word about the invasive snails, and met with the local chapter of nonprofit Trout Unlimited in Salinas on May 8 to discuss what, if anything, can be done.

“They are very difficult to control,” Olson says, adding that they reproduce asexually, so that a single transported snail can soon lead to thousands. “Currently, we have no way of eliminating them from the system.”

Potentially, Olson says, a parasite that keeps the snails in check in their native New Zealand could be introduced, but much study would have to be done before embarking on that path.

Tom Dudley, a researcher at UC Santa Barbara, did some of that research in recent years – the results were promising, he says – but dropped it after the snails’ populations later dropped in Santa Barbara County streams. He and other scientists aren’t sure why, but he suspects it was some kind of native parasite.

“They probably acquire naturally occurring parasites,” Dudley says. “It just takes a while to build up a new parasite load in a new environment.”

Regardless of whether that happens, Beverly Chaney, a biologist with the Water Management District, is trying to ensure the district limits the snails’ spread further upstream, or into tributaries.

That can be done, she says, by ensuring any clothing or objects submerged in the river are allowed to dry out for at least four to five days – the amount of time it takes the snails to dry out and perish.