San Diego Union-Tribune (Sunday)
RESEARCHERS DIG FOR ANSWERS
Study in Oceanside, Coronado to assess Pismo clam population
Beach visitors in Oceanside or Coronado recently may have noticed a team of mostly graduate students digging long, narrow trenches in the sand at low tide.
The research team from Concordia University Irvine is finishing up a multiyear population study of the Pismo clam, with a few more clam-digging excursions planned in San Diego and Orange counties before the end of this year.
“There’s a lack of information on this historically iconic species in California that seems to be struggling,” said Sean Bignami, the university professor leading the research. “The population is low, and we are not sure if it’s recovering, declining or staying stable. But it’s still there.”
Pismo Beach on the Central California coast is named for the once-abundant bivalve. The Pismo clam was a staple food of coastal Native Americans for thousands of years.
Once common in the tidal zone from Monterrey to Baja California, the clams were harvested commercially for decades until the 1940s. Recreational clam-digging became popular in the 1970s and ’80s, until the population began to decline and many people lost interest.
“The bright spot is there are areas where the population seems to be persisting,” Bignami said. “We are finding small clams throughout the range.”
Most of the Pismo clams found in the survey so far are 4 years old or less, he said. They take seven to 10 years to grow large enough to be harvested. The legal size for harvest varies by location, but generally is 4.5 inches in diameter or larger.
The largest clam recorded in California came from Pismo Beach. It was 7.37 inches across and estimated to be 26 years old, according to the state Department of Fish and Wildlife.
Size does not always correlate with age. Several of the clams, as determined by rings on the shell, have been more than 35 years old, and one collected from Zuma Beach, near Malibu, was estimated at 53 years old and measured only 5.25 inches across.
Bignami and his team of six to eight graduate students have been surveying Southern California beaches at the lowest tides of the year using methods specified by the Fish and Wildlife Department.
They dig a long trench the width and depth of a shovel blade, perpendicular to the water line, to identify where the clam bed starts. Then they load the sand from the trench into a garden cart, like a child’s wagon, with a quarter-inch mesh screen that captures anything larger than about a centimeter.
Most of the clams they find are the more common bean clam, which only lives a few years and gets no larger than 1 inch long.
“You find them in the millions in the swash zone ... where the waves come onto the sand,” Bignami said. “Those clams are really abundant.”
Bean clams are asymmetrical and are mostly on or near the surface of the sand.
Pismo clams grow larger and are more symmetrically shaped. They have a longer siphon through which they feed, so they can go deeper in the sand, but no more than 8 or 10 inches. The color of their shells varies, and often they have stripes radiating from the hinge where the two shells are attached.
Most of the clams are in sand that’s exposed at low tide, so they are easily harvested, usually with rakes. And because they grow slowly, the fishable population is easily decimated by a few people.
Clams are legal to collect, though a current saltwater fishing license is required, and the limit is 10 clams per person.
And there could be other reasons the populations are low. Clams are a favorite food of sea otters, and the animals can eat large amounts in a short time. Sharks, bat rays and seagulls also eat them.
Also, larger populations of clams could be living in shallow water past the waves and out of reach of the university surveys. Natural mortality could be another factor.
Further study is needed, Bignami said, and he’s working on a way to track and tag clams to learn more about how clams move, travel and burrow in the sand.
“No one has studied these things for 40 years,” he said. “We’re pretty sure they stay put, but we don’t know.
“Bivalves in general perform important ecoservices,” Bignami said. “They filter the water (to collect plankton for their food). When they used to be abundant, they were probably filtering a substantial amount of water along our coast.”
More information about the species would be useful in many ways, from adjusting seasons and harvest limits to planning important projects such as beach sand replenishment.
philip.diehl@sduniontribune.com Twitter: @phildiehl