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article imageResearchers identify causes of decline in shellfish harvests

By Karen Graham     Nov 1, 2018 in Science
NOAA researchers studying the 85 percent decline between 1980 and 2010 of the four most commercially-important bivalve mollusks -- eastern oysters, northern quahogs, softshell clams, and northern bay scallops -- have identified the causes.
Along with the sharp decline in commercially important bivalves, there has been a corresponding decline in the numbers of fishermen (89 percent) who harvested the bivalves, said researchers with NOAA’s National Marine Fisheries Service.
The bivalve declines are in contrast to the previous three decades (1950–80) when the combined landings of the same bivalves were much higher and the trend in each of their annual landings was nearly level, decade by decade.
The only exceptions to the declines were seen in the harvest of northern quahogs in Connecticut and American lobsters in Maine. However, the numbers of American lobster landings have fallen precipitously - as much as 98 percent - from southern Massachusetts to New Jersey.
The researchers also found during the course of the study that a number of groups of marine and land animals have also experienced large shifts in abundance since the early 1980’s.
Declines linked to environmental factors
Warming ocean temperatures and a shift in the North Atlantic Oscillation (NAO), along with environmental degradation which has led to habitat loss and increased predation are the primary factors in explaining the decline in commercial shellfish from estuaries and bays from Maine to North Carolina.
Warming ocean temperatures have been linked to the decline in the Atlantic blue crab industry in Maryland and Virginia. In a study done in the spring of 2015, researchers concluded that ocean temperatures were running 1.3°C higher than the previous decade's average. Higher water temperatures, along with ecological damage, over-exploitation and now, migration of the species was cause for alarm.
Employees of the Hollywood Oyster company sort fresh oysters on a conveyor belt at the company farm ...
Employees of the Hollywood Oyster company sort fresh oysters on a conveyor belt at the company farm in the waters of Chesapeake Bay near Hollywood, MD on March 20, 2014
Mladen Antonov, AFP
Another study, done in October 2015, put the reason for declining catches of Atlantic cod in New England squarely on warming ocean waters. "Here is an explanation for why the Gulf of Maine's cod fishery has not recovered despite significantly reduced fishing," said Mike Sieracki, program director in the Division of Ocean Sciences at the National Science Foundation (NSF). "Management plans will need to incorporate climate change factors to be effective."
The North Atlantic Oscillation also an as issue
The North Atlantic Oscillation (NAO) is a weather phenomenon in the North Atlantic Ocean that affects both weather and climate. Shifts in the NAO affect the timing of species' reproduction, growth, and availability of phytoplankton for food, and predator-prey relationships, all of which contribute to species abundance.
Through the 1970s, the NAO had a positive ecological effect on conditions in the North-Atlantic region, keeping the water cooler than they are now. The NAO+ warming of the North Sea reduces survival of cod larvae which are at the upper limits of their temperature tolerance, as does the cooling in the Labrador Sea, where the cod larvae are at their lower temperature limits.
Algae and debris on the surface of the Susquehanna River that flows into the Chesapeake Bay.
Algae and debris on the surface of the Susquehanna River that flows into the Chesapeake Bay.
EPA
On the East Coast of the United States, an NAO+ causes warmer temperatures and increased rainfall, and thus warmer, less saline surface water. This prevents nutrient-rich upwelling which has reduced productivity. Georges Bank and the Gulf of Maine are affected by this reduced cod catch.
“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,” said Clyde Mackenzie, a shellfish researcher at NOAA Fisheries’ James J. Howard Marine Sciences Laboratory in Sandy Hook, NJ and lead author of the study.
After extensive studies, the researchers concluded that it wasn't over-fishing that caused the declines, but environmental factors causing the changes in the bivalves’ body weights, their nutrition, the timing of spawning, in addition to increased mortalities from predation, and all this was sufficient to force the harvest declines seen over the last three decades.
The study, “Large Shifts in Commercial Landings of Estuarine and Bay Bivalve Mollusks in Northeastern United States after 1980 with Assessment of Causes” has been published in the journal Marine Fisheries Review.
More about commercial shellfish, Northeastern us, warmer ocean, North Atlantic Oscillation, Environmental degradation