Why were so many Pismo clams lying on sand at Oceano Dunes? Here’s what scientists think
A video posted to YouTube in August showed a car driving over and crushing dozens of Pismo clams that surfaced on the beach near Oceano Dunes State Vehicular Area in southern San Luis Obispo County.
It was a shocking sight that had researchers scratching their heads: Why were so many of the iconic clams lying out on the sand at Pismo State Beach?
Pismo clams, which are found along the Central and Southern California coasts, typically remain beneath the surface of the sand — firmly rooted by muscles known as “feet” that they also use to bury themselves. The clams sometimes show up on top of the sand, but rarely in the huge numbers seen in the video taken in early August.
“We’re not sure what caused this,” said Sonke Mastrup, the California Department of Fish and Wildlife’s (CDFW) marine region invertebrate program manager. “We are trying to look at it.”
Cal Poly assistant professor Ben Ruttenberg, who has studied the clams for the past several years alongside graduate students at the San Luis Obispo university, agreed with Mastrup.
“We don’t really know why this happened,” said Ruttenberg, director of Cal Poly’s Center for Coastal Marine Sciences. “Definitely, in the time I’ve studied them, we’ve never seen this many of them on the surface like that.”
Originally, CDFW biologists speculated that a red tide in August may have caused the mass surfacing of clams.
Red tides, which are caused caused by a so-called “bloom” of phytoplankton, create waves that appear maroon, brown or bright red, and can even glow with bioluminescence at night.
The red tides occur every few years in California, usually on the coast between San Diego and Santa Barbara, according to UC San Diego’s California Sea Grant.
Some algal blooms or red tides can produce bacteria that are known to weaken or cause paralysis in clams, Mastrup said. That could have been a reason the Pismo clams surfaced on the sand near the Oceano Dunes, he said.
CDFW sent some clams from Pismo State Beach to a pathology laboratory in early September to be tested for the harmful bacteria.
However, Mastrup said, “We haven’t been able to find really any conclusive evidence of that yet.”
“All the clams that were tested all looked fine,” he said.
Mastrup said the CDFW sent another batch of clams to the lab two weeks ago for additional testing. The results for paralytic shellfish poisoning came back negative, and the lab is now running tests for domoic acid, a toxin found in shellfish that is caused by certain types of algae, Mastrup said.
The inconclusive test results leave researchers again searching for answers as to what caused the clams to surface in such large numbers.
According to the CDFW, additional theories as to why the clams surfaced include rough tide events that could have caused the sand to shift considerably and unbury the clams, and potential overcrowding of the clams.
Is Pismo clam population recovering?
If overcrowding is the issue, Ruttenberg said, at least it’s a relatively good problem to have.
The Pismo clam population has struggled to return to the numbers it reached in the early 1900s.
The last legally harvestable clam — measuring at least 4.5 inches — was found on the Central Coast in 1993, according to CDFW.
But the clams have seen a resurgence in the past two years.
“We’ve never seen a population pulse this big,” Ruttenberg said. “It’s exciting. We’re seeing them return.”
The clams that are on the beach are still too small to harvest. Ruttenberg and Mastrup said it may still be several years before people can start bringing Pismo clams home for dinner.
Poaching is a “forever” problem, Mastrup said, but the agency has seen a sharp increase in the past two years in the number of clams illegally harvested and caught by CDFW enforcement rangers. That’s likely due to the fact that there are simply more clams to take.
In 2015, the agency issued zero citations for poaching Pismo clams. Three years later, 175 citations were issued for a total of 4,133 poached clams, according to CDFW data.
In 2020, the agency issued 226 poaching citations for a total of 25,165 clams.
This year, through Aug. 28, the CDFW has issued 131 citations for a total of 11,012 poached clams, according to the agency’s data.
“Clearly, taking undersized clams from the beach, or driving over the clams is not good,” Ruttenberg said. “But we don’t actually know how big of an impact that has on the population.”
If 11,000 Pismo clams are poached, does that translate into 1% or 50% of the population?
That’s one of the questions Ruttenberg said he and grad students at Cal Poly are trying to answer.
Ruttenberg and his team are studying how or if the Pismo clam population is recovering and what environmental and human factors may be impacting the population since about 2014.
See a clam surfaced on the beach? Here’s how to rebury it
To help the Pismo clam population continue to grow and mature, the CDFW encourages anyone who finds surfaced Pismo clams on the beach to rebury them.
California State Parks interpreter Danielle Bronson explained to The Tribune how to properly rebury Pismo clams to ensure they will survive.
According to Bronson, the first thing you do when you find a surfaced clam is to look for the clam’s hinge side, which is opposite the curved opening. You’ll see a ligament, she said, that looks like a small, fleshy bump, on one side of the clam’s hinge.
Dig a small hole about two inches deep into the wet sand, as close to the ocean as you can get. If you’re comfortable standing in the surf to rebury the clam, that’s even better, Bronson said.
Place the clam in the hole you’ve dug with the hinge side facing toward the ocean and the ligament side up, she said. Then, cover it with sand.
This story was originally published September 16, 2021 at 5:00 AM.
