Study: Decline in Nearshore Shellfish Landings Linked to Environment, Not Overfishing

The team examines the contents of a bottom drag for bay scallop recruits (this year’s seed) among adult scallops, predators of the recruits, and eelgrass in Cape Poge Pond. Mackenzie and the shellfish constables have been monitoring bay scallop beds on Martha’s Vineyard monthly during warm weather for several years. Photo © Deborah M. Gaines

HYANNIS – Researchers say they have identified the causes of the sharp decline in inshore shellfish landings since 1980.

Between 1980 and 2010 the combined landings for eastern oysters, northern quahogs, softshell clams and northern bay scallops dropped by 85 percent.

Warming ocean temperatures associated with a positive shift in the North Atlantic Oscillation have led to habitat degradation and increased predation for the species in estuaries and bays between Maine and North Carolina.

“Since the North Atlantic Oscillation went into a positive phase things have changed for the shellfish,” said lead author Clyde Mackenzie, a NOAA Fisheries shellfish researcher from the James J. Howard Marine Sciences Laboratory in Sandy Hook, NJ, who also has long connections to Martha’s Vineyard.

“For recruitment of the shellfish there is much less seed and much less recruitment than there had been and their stocks have gone way down.

The NAO is an irregular fluctuation of atmospheric pressure over the North Atlantic Ocean that impacts both weather and climate, especially in the winter and early spring in eastern North America and Europe.

The study recently published in “Marine Fisheries Review” also indicated that the NAO shifts affect the timing of the species’ reproduction, growth and availability of phytoplankton for food, and predator-prey relationships.

In the past, declines in bivalve mollusks have often been attributed to overfishing.

“We have determined what the cause is and it’s not fishing,” Mackenzie said. “Reducing fishing isn’t going to do anything. At least in this study we have identified the causes.”

Researchers also note there was a decline in the number of shellfishermen who harvest the mollusks by an average of 89 during the timeframe of the study. The landings of the shellfish between 1950 and 1980 by contrast were significantly higher.

Mackenzie said the next steps will be to think about what can be done to stop the population decline.

Exceptions to the declines have been a sharp increase in the landings of northern quahogs in Connecticut and American lobsters in Maine. The landings of lobsters between southern Massachusetts and New Jersey have drastically declined due to warmer water temperatures.

Mackenzie added that the only two places in Massachusetts that continue to produce nearshore bay scallops are Cape Poge Bay off Martha’s Vineyard and Madaket Harbor off Nantucket.

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