Most people familiar with the blue crab industry along the Atlantic Coast have long been aware of the ongoing problems with a declining blue crab population in the Chesapeake Bay. In the 1980s, there was a drastic reduction in the oyster population, in part due to overly abundant fresh water flooding into the oyster beds in the bay.
Because limitations on oyster harvests were put in place, fishermen turned to blue crabs, extending the harvest into the fall when they would have normally turned to oyster harvesting. By the mid-1990s, the crab population had been reduced to about 300 million, half of what it should have been. Not only were the oysters depleted, but so were the crabs and the underwater grasses that provided them with food, shelter and oxygen.
Against this depressing background of dwindling blue crab populations, David Johnson of the Marine Biological Laboratory began studying the northward migration pattern of the blue crab. In the Chesapeake Bay region, the Maryland Department of Natural Resources and Virginia Institute of Marine Science had been conducting yearly surveys that were beginning to show a marked increase in the crab population in the bay, that is, until the 2012-2913 survey.
Surprisingly, the 2012-2013 survey showed the blue crab numbers had again dropped to 300 million, and officials were devastated. At the same time, Johnson was studying the northerly migration of the blue crab into the Gulf of Maine and further north into Nova Scotia. Historically, the northern limit of the blue crab is Cape Cod, Massachusetts.
But Johnson hypothesized that between 2012-20913, ocean temperatures were running 1.3°C higher than the previous decade’s average. “Climate change is lowering the thermal barriers that kept species from moving toward the poles,” he says. “Climate change presents a challenge not only for ecologists, but for fisheries managers as commercially important species shift their ranges in response to warming oceans.”
In all fairness, this is not the first time this migration northward has occurred. In the 1950s, a similar migration of the blue crab was observed. When the ocean waters returned to their average temperature, the crabs disappeared. “It’s too early to determine if the current blue crab population in the Gulf of Maine is permanent or ephemeral,” Johnson says. “However, models predict an increasing warming of the world’s oceans and recent observations of blue crabs may be a crystal ball into the future ecology of the Gulf of Maine.”
Researchers have documented other commercially important marine species, such as lobsters, hake and flounder moving northward in the Northeastern U.S. as the warming trend in the world’s oceans continues. The blue crab is actually the second crustacean Johnson has documented moving into the Gulf of Maine. In 2014, he published his findings on the migration of the fiddler crab, Uca pugnax, into the Gulf. “As the world’s oceans continue to warm, we will continue to see climate-driven range expansions,” he predicts.
Johnson’s study was recently published in the Journal of Crustacean Biology, under the title: The savory swimmer swims north: a northern range extension of the blue crab Callinectes sapidus?
