Mussels make moves in Mountain Lake
After 7 years’ effort, there’s new evidence that native freshwater California floater mussels, reintroduced to Mountain Lake, are thriving. The mussels are a keystone species, vital to maintaining stable, healthy water conditions. Until now, the biologists responsible for restoring Mountain Lake’s habitat could only guess about how the mussels were doing. That is, until they put their heads underwater to get a more complete answer.
While we work the mussel, we meditate the hustle
Just about a year ago, I was on my hands and knees in a creek bed outside Sacramento. Alongside other naturalists (Jon Young, Presidio Trust Wildlife Ecologist, David Harelson, Presidio Trust Biological Science Technician, and Liam O’Brien, local lepidopterist legend), I was groping in the dark, cold water to gather freshwater mussels. The other mussel gathering trips in 2019 were a bust, and this November’s effort would be the last chance to use the 20 mussel quota allowed by California Fish and Wildlife.
The spot we’d chosen was deep enough to harbor mussels — here, the creek had water year round. But the bottom wasn’t sandy. It was all rocks. Rounded river rocks, each about fist-sized. So, everything I felt underwater might be a mussel, until I picked it up and checked its heft. But before long, everyone was pulling up mussels. Big, mature, adult mussels. We managed to fill the quota in a half hour, which never happens. We’d struck mussel gold.
These mussels were bound for San Francisco. They were the latest recruits in an effort to reestablish California Floater mussels in Mountain Lake, one of a handful of natural freshwater bodies in the city.
As part of the habitat restoration of Mountain Lake, mussels were reintroduced to the lake in 2013. Freshwater mussels were suspended in cages, designed to keep them out of reach of predators. But these mussels didn’t survive. In 2016, they tried something new by putting mussels in a new enclosure, more like their natural habitat. Big trays, the size of a card table, were installed a few feet above the lake floor. They were filled with sand, and mussels. It gave the mussels room to move around, and settle where they wanted. These mussels survived. But the few dozen mussels in their sandboxes are just the beginning. The goal is to get the mussels to reproduce, and populate the lake.
Push out the jive, bring in the love
Freshwater mussels are a key species in aquatic environments. They’re called “ecosystem engineers,” because, like beavers, they modify their habitat to suit their needs. Mussels constantly filter water, taking in organic particles, like bits of algae, bacteria, and plankton. What the mussels don’t digest, they return to the water. This process alters the water for both mussels and their neighbors. Plants can pull more nutrients from the water, and can collect more sunlight. Sediment clumps and sinks to the bottom, instead of clouding the water.
But it takes a lot of mussels to get this process up and running. And that’s where the newest batch of 20 mussels comes in.
Cheeky mussels
Mussels, like oak trees, aren’t especially mobile. So, like oak trees, mussels rely on other critters to help their offspring spread. Oak trees let squirrels carry their acorns away, some of which will sprout at some distance from the parent tree. Mussels recruit fish to carry their young. Simply put, female mussels squirt their larvae into the water when they detect fish are nearby. The larvae have minuscule fangs that they use to latch on, to fins or gills, and ride along with the fish until they mature a bit. Then the mussels let go, and drop to the lake bottom.
Three spined stickleback fish were successfully introduced to the lake, and the idea was, mussels would rely on the sticklebacks to move baby mussels around. To jump start this mussel-fish relationship, the biologists acted as matchmakers. They stocked aquariums with fertile mussels. Then they added stickleback fish from Mountain Lake, who spent a few weeks in the aquariums with the mussels. Once it was apparent that the fish were carrying mussel larvae, the fish were re-released in the lake. This began in 2017. The 20 mussels we brought back from Sacramento joined this program as well, seeding sticklebacks with larvae.
Still, there was no way to tell if any of this was working. The mussels in the trays were doing well, but they were all introduced as adults. The signal for success is young mussels across the lake bottom. From a dinghy atop the lake, Jon and David could only guess.
All that changed a few weeks ago. October is the end of the dry season in San Francisco, so the lake was as shallow as it ever gets. And the clarity of the water has continued to improve. Jon and David were wading in the lake, peering below the surface, and they saw something new. They spotted a mussel. And then another, and another. Young mussels, mostly buried in the sand, everywhere they looked. This was the first evidence that mussels were reproducing and establishing themselves across the lake.
There was one way to get a better look at what was happening. Out came the wetsuits and snorkels. An informal underwater survey revealed more surprises than just mussels. While they got some answers, this survey surfaced new questions about how, and where, this lake restoration effort is going.
Clams got legs
My prior experience with mussels didn’t prepare me for the video Jon and David made of their snorkel survey. Picking up mussels from the creek bed, looking at photos and illustrations, mussels look the same: two symmetrical shells, buttoned up tight. The video Jon and David made revealed how they look in the wild — from above, snuggled deep in the sand, with their mouth, and, uh, back holes wide open and pumping water.
California floater mussels don’t stay put. They have a foot, and they use it. “Foot” describes what this loose muscle does. But it looks and acts more like a meaty, white tongue. Mussels waggle and push and shimmy with their single tongue-foot, digging deeper into the sand, or heading out for a new spot. Unlike oysters and many other sorts of mussels, California floaters roam (albeit very slowly).
And it turns out the mussels are everywhere. Wherever Jon and David checked the lake bottom, they found mussels. The first evidence the naturalists have seen that they successfully created conditions were mussels could thrive. Next, a mollusk specialist will survey the lake more formally, generating data and checking for really young mussels.
We have met the enemy, and he is us
David solved one long-standing dispute about Mountain Lake. It’s never been clear if the lake was spring-fed, or only filled by runoff. In the lake, David found cold water streaming up from the bottom: a spring. They saw, or didn’t see, a few other things of note.
They saw bass. Invasive bass. Dozens of bass that shouldn’t be in the lake, full stop. Like the bullfrogs I wrote about recently, again, some asshat released bass into the lake (the lake is part of a National Park, so, federal offense. Just saying). It may or may not be related to the bass, but the lake used to crawl with crayfish. David and Jon didn’t see one crayfish during their dive. Sticklebacks should be abundant, but the only fish they saw were bass.
Are the bass going to be a big problem? It’s not clear. The mussels may be using the bass, as well as sticklebacks, to truck their young around the lake. And crayfish are a threat to baby mussels and aquatic plants, so maybe there’s an upside to the bass. Are the bass eating all the sticklebacks? It’s not clear.
And that’s where these updates from Mountain Lake often end. Some good news, some bad news, and a fresh pile of new questions. And I hope that whoever slipped the illicit fish into the lake, snuck in for some illegal angling, and panfried a big bass is reading this article now. I talked about it with a fellow volunteer/community scientist, who said, “I wouldn’t eat anything out of that lake. People shit in it.”
(apologies to Wu Tang Clan, The Simpsons, B.C. comic strip, Pogo, and the UK)
(all images not credited are courtesy of Jon Young and David Harrelson)
More resources
Video
California Floater Mussels Take Fish For an Epic Joyride | Deep Look
Reading
Importance of Mussels, Minnesota Dept. of Natural Resources
Stickleback Help with Mountain Lake Mussel Reintroduction, National Park Service
Mountain Lake and Presidio articles by this author
Bullfrogs? You’ll need snakes too
What’s up my turtles?
Measure of Success
El Jefe Has a Posse
Return of the Checkerspot
The Frogs of Mountain Lake
The Mountain Lake Turtles
