Large Shifts in Commercial Landings of Estuarine and Bay Bivalve Mollusks in Northeastern United States after 1980 with Assessment of Causes

Abstract

Between 1980 and 2010, documented commercial landings of the four most commercially-important bivalve mollusks have declined sharply in U.S. estuaries and bays from Maine to North Carolina. The numbers of shellfishermen have declined accordingly. Landings of the following species have declined: eastern oysters, Crassostrea virginica, by 93%; northern quahogs, Mercenaria mercenaria, by 62%; softshell clams, Mya arenaria, by 66%; northern bay scallops, Argopecten irradians irradians, by 93%; and southern bay scallops, A. i. concentricus, by 91%. Their combined landings fell by an estimated 85%. In the previous three decades, 1950–80, annual landings of the same species did not decline. The declines in landings of the bivalve mollusks were a consequence of declines in their abundances which was due to falling juvenile recruitments. The declines took place after the molluscan environments changed adversely for the bivalves as the North Atlantic Oscillation index switched its phase from negative to positive in about 1982 and usually remained there until 2003. Winters then became substantially warmer while the other seasons were also warmer, but less so. The temperature increases in winter and spring probably forced weight losses in the adult mollusks, and, consequently, they spawned fewer eggs. The increased temperature also allowed heavy feeding by pelagic copepods on phytoplankton, which left too little food available for the adult shellfish and their larvae to produce good seed recruitments. Also, predation on the juvenile recruits may have declined by 85% (Table 1). The numbers of fishermen who harvested the bivalves declined similarly (Table 2). These downturns have deprived fishermen of their historical livelihoods, and many waterfronts that had once been dedicated to commercial fishing are now dominated by private and tourist housing, restaurants, and sporting vessels. Once common seasonal foods for coastal people, these bivalves have become far less available in consumer markets (authors’ observations). 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. However, the landings of northern quahogs in Connecticut have risen sharply. The landings of American lobsters, Homarus americanus, have also increased sharply in Maine, but have fallen precipitously (from 40% to 98%) from southern Massaincreased, which precipitated the declines. From Long Island Sound, N.Y., through Chesapeake Bay, Md., warmer waters allowed diseases to kill many adult oysters. In some waters, higher temperatures increased the negative effects of eutrophication, and in other waters the loss of eelgrass, Zostera marina, probably contributed to degradation of habitats. This paper also discusses the role that crustaceans, i.e., pelagic copepods, shrimps, and crabs, play in the productivity of bivalve mollusk stocks. Predation by penaeid shrimp on tiny bivalve recruits is suggested as an important factor that controls bivalve abundances. The common perception that bivalve landings declined due to overfishing is reconsidered, because separate environmental factors have had a much larger effect upon their productivity.