Point Reyes Light - June 17, 1999

Green crabs a blooming problem in West Marin

Bolinas fisherman Josh Churchman warned members of the Bolinas Lagoon Technical Advisory Committee Friday that the number of European Green crabs is growing rapidly in Bolinas Lagoon, posing a threat to shellfish, crabs, and other aquatic life.

However, scientists are currently trying to develop parasites that will control the crustaceans.

Churchman reported he has seen youngsters fishing from the dock on Wharf Road catch an abundance of the inedible crabs. The fisherman said he and a friend set traps off the dock and captured nearly 1,400 Green Crabs in four days.

Nor is the local problem limited to Bolinas Lagoon. The crabs have shown up in Estero de San Antonio, Estero de Americano, and Tomales Bay.

Hog Island Oyster Company on Tomales Bay this week reported suffering a significant loss a few years ago when the company's Manila clams, which are grown in bags along the bay floor, became infested with Green crabs.

Terry Sawyer, an owner of Hog Island Oyster Company explained, "The crabs settle into the bag during the [crabs'] larval stage. We were pulling up bags that [seemingly] had fat, healthy crabs," but the clam shells were empty.

Sawyer said that the crab population in Tomales bay has dropped significanly this year and last, perhaps because La Niña's waters are cooler and the crabs aren't reproducing as rapidly. "Of course you have to consider all of the factors that come into play, such as salinity and sediment loads," he added.

"Since 1993," noted marine biologist Ted Grosholz of UC Davis' marine laboratory in Bodega Bay, "we have seen a 90 percent reduction in local populations of native clams and small shore crabs due to the European Green crab. It just eats everything up."

The European Green crab is a highly adaptable, invasive species and thrives in a wide range of temperatures and salinity. It's been found in inlets of the Mediterranean Ocean, South Africa, and the Atlantic coast from Nova Scotia to Chesapeake Bay. It reproduces quickly with a single female laying as many as 200,000 eggs a year. It doesn't do well in the open sea but makes its home in bays and estuaries.

The Green crab eats just about anything that it gets its claws on. Feeding on mussels, oysters, other crabs, small fish, and snails the Green Crab disrupts the balance of the ecosystems of bays and their bottoms.

Bay bottoms, sometimes called the mother ferment, support the smallest organisms in the food chain, such as: plankton, algae, and other micro-organisms.

Nor does the crabs' mischief stop there. They compete with shorebirds for food and prey on the young of commercially important fish.

"First discovered in the San Francisco Bay in 1989, the European Green crab has been reported in water bodies as far north as Coos Bay and parts of Washington," said biologist Grosholz. "Specimens have been found in Bolinas Lagoon, Tomales Bay, Drake's Estero, Bodega Bay, and Humboldt Bay," he added. Recently the crabs have also begun to appear as far south as Elkhorn Slough and Morro Bay in San Luis Obispo County, said Grosholz.

Biologists believe that the European Green crab, Carcinus maenas, was introduced to North America by ships discharging ballast water. In the crabs' larval stage, ocean currents may have carried them to northern regions, the scientists add.

In Australia, the Center for Research on Introduced Marine Pests (CRIMP) is working to discover parasites and pathogens that can biologically control the crabs' population explosions.

Marine farmers in Tasmania, have banned together in an effort to evict the pest with traps designed by a local shell fish farmer.

The Environmental News Network has reported that Armand Kuris, a zoology professor at UC Santa Barbara, and his colleagues would like to "introduce a parasite that would serve as a biological control to eliminate the species."

A parasitic barnacle, called rhizocephalan, is able to castrate the crabs by blocking their reproductive systems. Researchers are testing crabs here to see which, if any, are susceptible to rhizocephalan.

Point Reyes Light Cover | News | Coastal Traveler