by Robin Meadows
The San Francisco Bay-Delta estuary has been a food desert for fish for two decades and the introduced overbite clam is a major culprit. The fish eat tiny creatures called zooplankton, which in turn eat tiny plant-like organisms called phytoplankton. But overbite clams eat so much phytoplankton that there’s hardly any zooplankton for fish.
“It’s a huge problem,” says Madison Dunlap, a University of California, Davis ecologist.
Now Dunlap and aquatic ecologist John Durand, also at UC Davis, present an out-of-the-box idea for controlling overbite clams in hopes of boosting fish food. In a June California Water Blog post, they explore dredging the clams in Suisun Marsh, which is between the Bay and the Delta.
Suisun Marsh is also a bright spot for at-risk native fish like Delta smelt, longfin smelt, and young salmon migrating toward the ocean. “It’s their last refuge,” says Durand.
CLAM INVASION
Overbite clams, so called because one shell overhangs the other, are small but mighty. These shellfish spawn year-round, starting when they’re just a couple of months old, and a single female can produce up to 220,000 eggs in her short lifetime of about two years.
Native to Siberia, China, Korea and Japan, overbite clams were likely introduced to the San Francisco estuary as larvae in the ballast water of ships. These prolific non-natives were first documented in Suisun Bay in the fall of 1986 by community college biology students.
The area was ripe for invasion. That spring, a powerful flood washed away the bottom-dwelling organisms that had been living there, leaving a clear field for the clams.
By 1987 overbite clams carpeted the bottom of Suisun Bay and by 1990 had spread to the San Francisco Bay. They now account for more than 95% by mass of bottom-dwelling organisms at many sites in these waters. Suisun Marsh is hardest hit, averaging a staggering 2,000 overbite clams per square foot.
That said, some parts of Suisun Marsh still have fish food, notably the small side channels in the marsh interior. “They don’t have overbite clams and do have phytoplankton,” Dunlap says. This may be because water exchanges so slowly in these channels that the abundant debris from tules and other marsh plants builds up, smothering the little clams. Another possibility is that fish that thrive in these phytoplankton-rich channels also eat the clams.
DEARTH OF FISH FOOD
Overbite clams are prodigious consumers. They eat by siphoning water through themselves and filtering out their food in the process. The clams can filter the entire water column―which goes from the bottom to the top―more than once a day in deep marsh channels and up to 13 times a day in shallow ones. At these rates, clams can eat phytoplankton faster than it can grow.
How can clams all the way down on the bottom of channels get phytoplankton from the water surface? The answer is that it’s delivered to them by tides and the wind. “There’s a lot of mixing,” Durand says. “That moves the phytoplankton down to the bottom where it’s sucked up by clams.”
To make matters worse, overbite clams also filter out larvae of the copepods, a type of zooplankton that is a mainstay of fish diets. This makes the clams a double whammy when it comes to depleting fish food. “The clams are a competitor for phytoplankton and a predator of zooplankton,” Dunlap says.
Indeed, the overbite clam invasion was followed by a sharp drop in fish food. Phytoplankton began dropping in 1987, within a year of the clam’s first detection in the estuary. A decade later, zooplankton―which eat phytoplankton and feed fish―were in short supply.
“Delta smelt went into an extinction spiral, and salmon and longfin smelt began to decline,” Durand says.
THE CASE FOR DREDGING
While many Suisun Marsh waterways are infested with overbite clams, a 2017 study found that breeding adults are restricted to a single large slough, fittingly called Suisun Slough. Wide and winding, this channel is up to 125 feet across and 40 feet deep and meanders 13 miles through the marsh.
The finding that overbite clams breed only in Suisun Slough raises the possibility of controlling the marsh’s population by taking them out. Dunlap and Durand suggest doing this literally by dredging the bottom of the slough to a depth of 5 inches, which is as deep as the clams go.
This may seem like a wild idea but another suggestion that once seemed far-fetched has turned out to be true. “People laughed at the idea that Suisun Marsh duck clubs could produce fish food decades ago,” Durand says. But recent research shows that when duck club ponds are flooded for wintering waterbirds, they produce a wealth of phytoplankton that is exported into marsh waterways when the ponds are drained.
In support of dredging clams out of Suisun Slough, a 2012 study found that after dredging invasive Asian clams from test plots at the bottom of Lake Tahoe, populations remained diminished well over a year later.
The odds of success might be enhanced by dredging Suisun Slough shortly before a big freshwater release to the Delta. This is because overbite clams colonize more heavily during drought years.
Based on similar dredging projects in the estuary, the team calculates that dredging Suisun Slough would cost between 2 and 13 million dollars. This may sound like a lot of money but it’s far less than the hundreds of millions the state has spent to restore habitat―an effort aimed in part at increasing fish food―in the estuary.
“The focus has been trying to mitigate the impact of the clams rather than the clams themselves, ” Dunlap says, adding that restoration is a good thing but that dredging Suisun Slough could also be a good thing.
Downsides of dredging Suisun Marsh appear to be low. The clams have taken over so completely that dredging wouldn’t disrupt any other bottom-dwellers. Rather, the worst-case scenario seems to be that the clams might just come right back. “But that’s unknown―and nothing else works,” Durand says. It’s also possible that clearing out the clams could make room for other, less harmful bottom-dwellers to reinstate themselves.
“Why not try something new?” Dunlap says.
