Blue Crabs: Back From The Brink
Blue crabs are fierce predators and armored cannibals, with their claws as quick to snap up one of their own young as they are to devour a fish or worm. But Callinectes sapidus also embody the spirit of the Chesapeake Bay. They are the beautiful swimmers that have clawed their way back from the brink to continue their reign as the irascible czars of the nation’s largest estuary.
After plummeting to a near-record low in 2007, blue crab populations in the Bay have nearly tripled over the last five years because of restrictions on catching females imposed by Virginia and Maryland in 2008.
A scientific dredge survey of crabs this past winter estimated 764 million blue crabs in the Chesapeake, which was up 66 percent from the 461 million in 2010, and nearly three times the 255 million in 2007, according to the Maryland Department of Natural Resources. After these impressive numbers were announced in April, however, harvests of blue crabs in the early summer were relatively low.
Why didn’t the boom translate into brimming crab pots? CBF Director of Fisheries Bill Goldsborough explained that an unusually high proportion of the crabs about three quarters counted in the 2011-2012 winter survey were juveniles, still too small to catch by early summer.
Many of these crabs were spawned in the late summer or fall of 2011. Blue crabs normally need about 12 months of growing time before their shells are large enough to meet the 5-inch legal catch limit, Goldsborough said. The wave of juveniles was spawned by a large number of female crabs protected the previous year by Virginia and Maryland’s catch restrictions. The number of young crabs may have also been boosted by tropical storms in the fall of 2011 that swept more crab larvae into the Bay from the ocean.
This year’s spike in young crabs may lead to higher harvests next summer. But the fact that the crab population often experiences booms and busts like this points to a continuing problem, Goldsborough said.
“The crab population is truncated, meaning we catch crabs so quickly, there are very few crabs bigger than legal size,” Goldsborough said. “As a result, the fishery is very dependent on each year class that comes in. And that creates instability, which is not a good thing. We want more stability in the crab population and more older crabs so that we have ongoing high reproductive potential. This would be good for both the crabs and the crabbers.”
One way to create stability in the crab population is for Maryland and Virginia to continue the science-based restrictions on catching female crabs imposed in 2008. Although some watermen have called for relaxing the restrictions, lifting the protections now could lead to another collapse.
Another strategy to improve the long-term stability and health of the Chesapeake’s crab populations is to cut significantly the nitrogen and phosphorus pollution into the Bay. Nitrogen and phosphorus stimulate explosive growth of algae, Goldsborough said. Algal blooms darken the water, blocking light and killing underwater grasses that crabs need for shelter.
Algal blooms fed by polluted runoff quickly die and decay, sucking up oxygen and creating “dead zones.” The dead zones force crabs to find oxygen in shallow waters where they are more easily caught, Goldsborough said.
“Dead zones” also kill the food that crabs eat, destroying or preventing the growth of 75,000 metric tons of clams and worms a year in the estuary, according to a scientific study in the journal Science. That is enough food to support half the commercial crab harvest.
In an effort to shrink the Bay’s “dead zones,” the U.S. Environmental Protection Agency issued pollution targets for the Chesapeake Bay in December 2010. These targets have come under attack by industry lobbying groups, however, and CBF is fighting to defend them in court and in Congress. At the center of this fight are blue crabs and the more than 6,000 watermen and other workers who depend on the crabs for their livelihoods.
Callinectes sapidus the latin name for “beautiful savory swimmers” are strange and feisty mascots for the Chesapeake Bay. Blue crabs swim sideways through the water. They have five sets of legs, and are propelled by tiny flippers called swimmerets. Blue crabs deploy their claws to hunt everything from fish to clams, oysters, worms, insects, and a large number of their fellow crabs, including their own offspring.
Blue crabs range along the Atlantic coast from Nova Scotia to Argentina. They mate in the Chesapeake Bay in the spring through fall. Male crabs called “Jimmies” wrap themselves around mature females “sooks” to protect them from cannibals and predatory fish for several days as the females shed their shells.
After being fertilized, female crabs swim in the fall to the southern Bay in an extraordinary spectacle called the “march of the sooks” or the “sook run.” They hibernate at the bottom of the Bay. And then in the spring, the females release millions of larvae, which drift into the Atlantic Ocean before being swept back into the Bay by winds and currents.
Blue crabs can live for up to about four years and can grow up to 10 inches across the shelll from point to point. But until Virginia and Maryland imposed restrictions on catching crabs in 2008, two thirds or more of all crabs in the Bay were caught annually. This meant that most did not live much beyond one or two years or grow much larger than five inches.
“The blue crab was really affected most by overexploitation by fishing,” said Dr. Thomas Miller, Director of the Chesapeake Biological Lab. “A decade ago, they were experiencing 70 percent removal rates. And you can’t take 70 percent of the trees and still have a forest. And if you take 70 percent of the crabs, you no longer have a healthy crab stock.”
Since 2008, fisheries managers in Maryland and Virginia have been keeping total annual harvests below 46 percent of all crabs. Regulators have also now adopted a target of allowing no more than 25.5 percent of all females to be caught in any one year.
If these limits continue, and the Bay states are successful in reducing pollution, the frequent crashes in the blue crab populations should moderate. And this should allow the Chesapeake’s scrappy fighters to continue their peculiar sideways march to restored health.