BEALS — For years, Maine shellfish harvesters have been complaining that there are fewer softshell clams while arguing that the diggers who go out on the mud flats aren’t the cause of the problem.
A recent study by researchers from NOAA’s Northeast Fisheries Science Center and the Maryland Department of Natural Resources backs them up on both counts.
According to Clyde L. MacKenzie Jr. of NOAA and Mitchell Tarnowski from the Maryland Department of Natural Resources, between 1980 and 2010, documented landings of the four most commercially important inshore bivalve mollusks along the Northeast coast — eastern oysters, northern quahogs, softshell clams and northern bay scallops — dropped by 85 percent.
The principal cause, they say, was warming ocean temperatures associated with a shift in the North Atlantic Oscillation which resulted in damaged shellfish habitat and increased predation from Maine to North Carolina.
“My first response is that the article confirms what I have been seeing with soft-shell clams over at least the last decade or so,” Brian Beal, a professor of marine ecology at the University of Maine Machias and director of research at the Downeast Institute on Great Wass Island, said last week.
The North Atlantic Oscillation is a fluctuation of atmospheric pressure over the North Atlantic that affects both the weather and the climate along the East Coast, especially in winter and early spring.
According to NOAA, shifts in the oscillation can affect the timing of a species’ reproduction and growth, the availability of microscopic organisms for food and predator-prey relationships.
Over a period of several years, MacKenzie and Tarnowski interviewed shellfish wardens and harvesters along the New England coast, as well as examining landings records and other research in an effort to determine the “true causes” of the precipitous drop in shellfish landings.
Eventually, they determined that “habitat degradation from a variety of environmental factors, not overfishing, is the primary reason” for the decline.
“A major change to the bivalve habitats occurred when the North Atlantic Oscillation (NAO) index switched from negative during about 1950 to 1980, when winter temperatures were relatively cool, to positive, resulting in warmer winter temperatures from about 1982 until about 2003,” MacKenzie said.
The effects of those changes are still evident, Beal said in an email last week.
“I have observed intense soft-shell clam predation by green crabs, milky ribbon worms, and a host of other predators that have decimated juvenile populations of soft-shell clams.”
Too much shellfish harvesting isn’t the cause of reduced clam landings.
“I have been saying for some time that the decline of clams is not due to overfishing,” Beal said. He added that the problem also wasn’t related to “a variety of other red herrings that are commonly used to show that if we reduce harvesting pressure through various management techniques that clams will somehow miraculously ‘come back.’”
The real problem, Beal said is a superabundance of tiny predators — primarily juvenile green crabs — that thrive in warmer waters and devour juvenile shellfish.
“The clam industry is fighting an uphill battle against a foe that is microscopic, and there’s no way that removing a few hundred thousand pounds of crabs by fishing them is going to make any dent in their predatory activities,” Beal said, “because the focus of any fishery is on the adults when the bulk of predation is occurring at the level that is smaller than the size of your thumbnail.”
That is right in line with the NOAA researchers’ finding that “this climate shift (brought on by fluctuations in the ocean oscillation) affected the bivalves and their associated biota enough to cause the declines.”
Landings figures are not an entirely satisfactory indicator of the health of a particular species of shellfish resource.
Beal said soft-shell clam landings diminished, in part, because of changes in the market.
“What has also happened is that demand for soft-shell clams is not what it used to be because,” Beal said, “people have ‘moved on’ to other shellfish that have more consistent production either because they are cultured and/or are being imported.”
During the 1980s and 1990s, the NOAA researchers found, consumer demand for oysters fell and so did landings of oysters — primarily wild harvest shellfish from areas such as Narragansett and Chesapeake bays.
With federally imposed changes in safe shellfish handling practices imposed in the 1990s, the demand for oysters — and oyster landings — has rebounded.
Landings of Maine oysters, virtually all cultivated, have increased from less than 1 million pounds in 2013 to nearly 2.7 million pounds last year.