Researchers Find Decline in East Coast Commercial Shellfish Landings Linked to Environmental Factors — ecoRI News

By ecoRI News staff

Researchers studying the sharp decline between 1980 and 2010 in documented landings of the four most commercially important bivalve mollusks — eastern oysters, northern quahogs, soft-shell clams, and northern bay scallops — have identified the causes.

Warming ocean temperatures associated with a positive shift in the North Atlantic Oscillation (NAO), which led to habitat degradation including increased predation, are the key reasons for the decline of these four species in estuaries and bays from Maine to North Carolina, according to the National Oceanic and Atmospheric Administration (NOAA).

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. Shifts in the NAO affect the timing of species’ reproduction and growth, the availability of phytoplankton for food, and predator-prey relationships, all of which contribute to species abundance. The findings appear in Marine Fisheries Review.

“In the past, declines in bivalve mollusks have often been attributed to overfishing,” said Clyde Mackenzie, a shellfish researcher at NOAA Fisheries’ James J. Howard Marine Sciences Laboratory in Sandy Hook, N.J., and lead author of the study. “We tried to understand the true causes of the decline, and after a lot of research and interviews with shellfishermen, shellfish constables, and others, we suggest that habitat degradation from a variety of environmental factors, not overfishing, is the primary reason.”

Mackenzie and co-author Mitchell Tarnowski, a shellfish biologist with the Maryland Department of Natural Resources, provide details on the declines of these four species. They also note the related decline by an average of 89 percent in the numbers of shellfishermen who harvested the mollusks. The landings declines between 1980 and 2010 are in contrast to much higher and consistent shellfish landings between 1950 and 1980.

Exceptions to these declines have been a sharp increase in the landings of northern quahogs in Connecticut and American lobsters in Maine. Landings of American lobsters from southern Massachusetts to New Jersey, however, have fallen sharply as water temperatures in those areas have risen.

“A major change to the bivalve habitats occurred when the North Atlantic Oscillation 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. “We suggest that this climate shift affected the bivalves and their associated biota enough to cause the declines.”