Clams and Predators

Green Crabs and Clams 
(Published by The Quoddy Tides,
August 10, 2001)

It's not easy being a clam. From the moment life begins, the odds of survival are low. And the odds got a good deal worse some years ago when green crabs made their way to Cobscook Bay.A lot of people here depend on healthy clam populations for their livelihood, so understanding the relationship between soft-shell clams and this effective predator has special importance. 

Although green crabs were found here in the 1970's, as recently as 1995 there were many clammers in the bay who had never seen one. This summer, it is difficult to not find them. Dana Dupee, a fisherman from Perry, says, "The past few years there have been more and more green crabs. When you're out digging, especially around rocks with seaweed, you can easily catch a hundred. They're turning up in lobster traps in great big numbers, as well as scallop and urchin drags."

Dupee adds, "A study done in East Bay last summer shows that 80% of the juvenile clams were lost due to green crabs. These are clams that could have been harvested in future years."

Researcher Robin Seeley, who has been monitoring the green crab populations in Cobscook Bay since 1985, has also seen their numbers rise dramatically. Seeley says, "I saw the beginning of it in 1999, but last year was the explosion year at several of my sites."

Seeley counts how many green crabs she can find in an hour.At one particular site, throughout the 1980's she found less than ten crabs.In 1999, she found 50 crabs at this same site, then last year the number jumped to 260.

Researchers have spent decades studying the soft-shell clam and the leading causes of clam mortality. Brian Beal, Associate Professor of Marine Ecology at the University of Maine at Machias, has had a particular interest in clam predators for the past ten years. "Clams get hit at all stages in their life. As larvae floating around in the water column, they are eaten by plankton feeding animals. Those that survive to settle to the bottom face new risks."

Initially, these miniature clams attach themselves to sand or rocks with tiny byssal threads secreted by a gland in their foot. Only a small percentage of these clams survive to adulthood.

"Where they end up settling and burrowing into the sand has a tremendous effect on their survival rates. Clams in the upper reaches of the flats may never reach harvestable size, but they do have a better chance at avoiding most water-borne predators," reports Beal.

Historically, the moon snail has been the most voracious predator of soft-shell clams in Cobscook Bay. The tell-tale sign of moon snail predation is a perfectly round hole that is beveled or countersunk into the empty clam shell. However, green crabs now rival moon snails as one of the top causes of clam mortality.

The green crab is a non-native invasive species that has worked its way up the coast of Maine. Because green crabs are not native to our coast, clams have not evolved any defenses against the green crab. Clam shells are thin, and although adult clams can burrow deep into the sand, juvenile clams can not, and are easy pickings for a hungry crab. 

"Green crabs take clams in with their walking legs, put their pincher claw through the shell, and then clean out the tissue, " Beal says. "Sometimes all you find is the two areas around the hinge."

Although eaten by seagulls and some other predators, green crabs have had no effective natural enemies in Maine until now. The Asian shore crab was discovered in South Portland this summer; unfortunately, this species of crab also preys on clams.

Dr. Dana Wallace of Brunswick was the Department Head of the Resource Conservation branch of the Department of Sea and Shore Fisheries (now the Department of Marine Resources) when green crab populations first exploded in the mid-1900's.

"Green crabs didn't hit their peak of abundance until the late 1940's into the 1950's," says Wallace. "Clam production went down from 500,000 to less than 100,000 bushels statewide by 1959.The number of diggers went down to a little over one thousand. The green crab explosion had a tremendous economic impact along our coast."

Cummings Beach in Jonesport was one of the early sites where he saw the impact first hand. "We brought some good size clams from Scarborough to transplant to the flats in Jonesport. A couple weeks later, we found the clams had all been dug out and eaten by green crabs."

"It took us about twenty years to develop effective fencing that kept the crabs away from the larger clams," Wallace says.To make this netting available to communities, the legislature established the Green Crab Fund from license fees received by DMR. 

"Around the same time, the winters turned colder, which caused a sudden reduction in the numbers of green crabs. Millions were seen dead on the flats and in burrows. Towns were no longer interested in the netting," reports Wallace. The recent warm winters have meant fewer green crabs are dying from the cold.

"Heavy intensive trapping has had limited success in reducing green crabs - you can't say you can trap them out," states Wallace. "Very few of the people around today are old enough to have seen the effects of the green crab. They don't realize what happened, the extent of the predation on clams."

The potential impact is more serious as the value of clams has increased. Wallace says, "Clams are more sought after today. With an average of $70 a bushel, the economic impact is much greater." With green crab numbers on the rise again, the government and towns have to plan ahead to minimize the damage this predator can do. 

The Department of Marine Resources stopped monitoring green crab populations in 1985 .And now to the surprise of many, DMR has proposed new regulations to manage the green crab as a sustainable fishery.

What happens to clam populations in Cobscook Bayhas a direct impact on the local economy and could affect the local ecology in ways we can't yet guess. More research on clam ecology and the impact of green crabs would be helpful in maintaining healthy clam populations in the future.

This column was prepared by Cheryl Daigle. Cobscook Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information.

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