From North Carolina State University
Scientists study life of economically important blue crab
Jan. 26, 2001 -- When the chill of winter moves down from the north, female blue crabs of the Chesapeake Bay head for the salty waters near the mouth of the estuary to spawn the next generation of their species. But exactly when and where they go – and how they get there – is a mystery.
Such questions are no small matter, because blue crabs are one of the most economically and ecologically important fishery species in the Chesapeake Bay, as they are in the coastal waters of North Carolina. State and national policymakers need more information about the life history of the crabs to help ensure the long-term sustainability of the blue-crab population and harvest.
A pair of marine biologists at North Carolina State University is working to provide those details by exploring the spawning migrations of adult female blue crabs. They especially hope to find out whether the crabs actually move through a deepwater migration corridor sanctuary established to protect the females before they produce their offspring.
Currently, scientists understand very little about this important facet of crab biology, explains Dr. Donna Wolcott, NC State associate professor in the Department of Marine, Earth and Atmospheric Sciences.
"We suspect that cold stimulates female crabs to go deeper and deeper, but we don't know that for certain," she said. "We'd like to know when they set out; we'd like to know if they're swimming or walking. We'd also like to know whether they're foraging the whole time, or high-tailing it down the bay."
Dr. Tom Wolcott, her research partner and spouse, says their preliminary results indicate that female blue crabs don't begin their migration to the deep and salty waters of the Chesapeake until October, after the sanctuary is reopened to fishing.
"As yet, there's no evidence that females actually migrate in the deep channels of the Bay," said Tom Wolcott, also a professor of marine, earth and atmospheric sciences. "The data show only that females are caught there by fishing dredges during the cooler seasons."
Beginning next summer, the Wolcotts will capture and fit a handful of adult female blue crabs with ultrasonic transmitters that will relay continuous information about the crabs' location and depth in the Chesapeake Bay. A team of researchers will then spend days at a time on a boat following each crab.
The researchers will also attach small devices to a larger number of female crabs – about 500 each year for the next three years – to record information on water temperature, depth and salinity as the crabs make their spawning journey. They won't be able to analyze that information, however, until fishermen capture the fitted crabs and return the devices to the Wolcotts – in exchange for a small reward.
Specifically, the Wolcotts are hoping to discover which environmental conditions in the Chesapeake and other estuary systems, such as Delaware Bay and the Albemarle and Pamlico Sounds of North Carolina, initiate the females' migration. "There may be multiple cues" to begin the journey, Donna Wolcott said.
The Wolcotts' research is funded by the Chesapeake Bay Stock Assessment Committee, an arm of the National Oceanic and Atmospheric Administration that evaluates the health of commercially and ecologically valuable species in the nation's largest estuary. In response to declining populations of adult blue crabs in recent years, the Virginia Marine Resources Commission set aside the sanctuary to protect females as they travel to their spawning grounds.
Once the crabs reach those grounds, they extrude egg masses – called "sponges" – which they carry under their abdomens; each brood may contain 2 to 3 million eggs.
Until recently, marine biologists assumed that the female crabs swim out into the bay with ebb tide currents, then avoid the incoming flood tide by sitting on the bottom. But research the Wolcotts conducted with the Smithsonian Environmental Research Center found that few female crabs are near the surface during nighttime ebb tides, and that the speed at which tagged crabs migrated could be accomplished by walking. The new research aims to help resolve this question.
Female blue crabs mate only once in their lives, typically during the summer or early autumn of their second year. As early as October of the same year – although scientists think they may wait a year – the females make the trek to their spawning grounds. They probably remain in those saltier waters for the rest of their lives, producing a series of broods during the warmer months.
When the eggs are ready to hatch, the females migrate farther down the bay, and even into the Atlantic Ocean near the coast, to release the microscopic crab larvae. The larvae go through a series of developmental stages in coastal waters before traveling to the estuaries.
Blue crabs, which can grow to about 9 inches width, live in coastal waters of the Atlantic Ocean from Nova Scotia to Argentina. In the United States, about half of the commercial harvest of the species occurs in the Chesapeake Bay; blue crab is also popular among recreational fishermen. The species' scientific name, Callinectes sapidus, means "beautiful, savory swimmer."
In addition to being a delicacy, blue crabs are an important part of the food web in estuaries along the Atlantic Coast. While small, they serve as a food source to many species, and are opportunistic feeders. They consume a wide variety of live and dead animals and plants – including smaller, weaker and injured members of their own species.
Editor's Note: For a color digital copy of a photo of Dr. Thomas Wolcott examining a female blue crab fitted with an ultrasonic transmitter, contact News Services at 919-515-3470 or email@example.com.
NC State University News Services Campus Box 7504 Raleigh, NC 27695-7504 firstname.lastname@example.org www.ncsu.edu/news Media Contacts: Dr. Donna L. Wolcott, 252-222-6332 email@example.com Kevin Potter, NC State News Services, 919-515-3470 or firstname.lastname@example.org