Green crab predation identified as cause of Maine clam decline

phys.org | 5/16/2018 | Staff
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Juvenile soft-shell "steamer" clams are not surviving to adulthood due to high levels of predation, according to Brian Beal, a professor of marine ecology at the University of Maine at Machias and director of research at the nonprofit Downeast Institute (DEI), who partnered with the Maine Clammers Association to conduct the research.

The team discovered that post-settlement mortality of soft-shell clams, Mya arenaria, is as high as 99 percent in southern Maine, putting the state's iconic soft-shell clam fishery in jeopardy unless major changes in resource management occur.

Statewide - Clam - Landings - Years - Landings

Statewide, clam landings have plummeted in the past 30 years, with 2017 landings the lowest since 1930. During the same time period, seawater temperatures in the Gulf of Maine have risen substantially and winters in the region have become milder. During this period of warming ocean temperatures, populations of invasive green crabs, Carcinus maenas, have increased rapidly in Maine, especially along the southern Maine coast.

DEI's experiments were designed to disentangle the drivers of the clam decline. Six large-scale field experiments, which excluded predators such as green crabs, were deployed in three tidal estuaries in southern Maine over the course of two years. Two of the experiments, conducted in Freeport, were part of the largest intertidal research project in state history.

Studies - Predation - Clams - Beal - Predators

"These studies reveal just how severe predation on soft-shell clams is," Beal says. "The major predators are invasive green crabs, especially small crabs that feast on juvenile clams. But even native species such as milky ribbon worms are having a significant impact on clam populations.

"Results suggest that current predation rates are so severe that less than 0.01 percent of juvenile clams survive beyond their first year," says Beal. This means that most clams are being killed by predators before they can reach commercial sizes."

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