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Blue crabs inhabit the Bay throughout the year

Bay Journal - Bay Naturalist / By Kathy Reshetiloff

Summer on the Chesapeake -- it's just not complete without the traditional crab feast. For many Chesapeake Bay residents, summer doesn't really begin until they've consumed the succulent meat covered in spicy seasoning. Although these crustaceans receive more attention now, blue crabs are found in the Bay throughout the year in one form or another. No other creature depends upon the Bay's varied habitats and water conditions as much as the blue crab.

The life cycle of a blue crab takes it from the salty ocean water at the mouth of the Bay, north into the freshwater tributaries. The entire Chesapeake Bay ecosystem, with its many habitats, is their home. During different seasons of the year, as water temperatures drop below 50 degrees Fahrenheit, crabs may burrow into the Bay bottom and hibernate. They shed and mate in shallow waters and beds of submerged aquatic vegetation. Blue crabs also swim freely in open water.

As a blue crabs grows, it molts or sheds its hard shell, which is incapable of expanding. When this occurs, a soft, slightly larger shell is exposed. The crab pumps water into its body to enlarge the new shell, and within hours, the new shell hardens.

Mating takes place during the summer. Female blue crabs must shed their shell before mating. The male crawls on top and cradles her between his legs until she molts. Cradling protects the female while her shell is still soft. After mating, the female's shell hardens and the two separate. Males remain in the fresher portions of the Bay and rivers and migrate into deeper waters to spend the winter. The female migrates to saltier spawning areas near the mouth of the Bay. Females that mature early in the summer will spawn before its end, bury themselves in the Bay bottom for the winter and may spawn again the next spring.

A late-maturing female will wait until spring to spawn. The female crab stores the male's sperm in specialized sacs, where it can survive for up to a year before fertilization takes place. Eggs are fertilized during the spring with sperm the female has stored over the winter. Females produce 750,000 to 8,000,000 eggs, which they carry on their abdomens. At this stage, the female is called a sponge crab because of the sponge-like appearance of the egg mass. Eggs hatch from June through September.

The first life stage of a blue crab is the zoea. In this form, the zoea does not even look like a crab. It is microscopic and lives a planktonic, free-floating existence. After several molts, the zoea reaches its second larval stage: the megalops. Still invisible to the naked eye, the megalops looks like a cross between a crab and a lobster. Another molt and a tiny crab form is apparent. The crab quickly begins migrating from the southern part of the Chesapeake to tidal rivers and northern portions of the Bay. In another 12 to 18 months, the crab is mature, measuring about 5 inches point to point across the back.

Crab populations in the Bay fluctuate from year to year. Some fluctuations may be caused by weather conditions on the continental shelf while the crab is in its larval stage. If weather conditions are such that a majority of larvae are carried away from the Bay, subsequent adult crab population will decline. Favorable conditions that bring large numbers of the megalops phase back to the Bay result in higher crab populations for the next year or two. Other natural factors that affect populations include predation, cannibalism, and the availability of food and crab habitat.

This succulent seafood is the pride and pleasure of the Bay. Blue crabs have, so far, endured increasing harvest pressures that would decimate other commercial fisheries. Commercially, the harvest of blue crabs is the most important fishery of the Chesapeake Bay. Total catch outranks all other shellfish species in both weight and value. It is estimated that approximately 75 percent of the adult stock of blue crabs is removed from the Bay each year.

Blue crabs are harvested using dredges, crab pots, trotlines, handlines and dip nets. There is concern that the present population will not be able to sustain itself because of increasing harvest pressures. With the decline of other commercially important species, such as striped bass and oyster, the seafood industry in Chesapeake Bay is becoming more dependent on the blue crab. As the human population in the Bay watershed increases, so will the demand for crabs.

Most agree that sound management of the crabbing industry is necessary. However, the biology of the blue crab and the variety of harvesting methods may hinder efforts. The blue crab needs both the Maryland and Virginia protions of the Bay to complete its life cycle. Egg-laden females winter and spawn in Virginia's waters, and larvae develop there. Males winter in the upper portions of the Bay while females winter in the lower Bay. During the rest of the year, adults are dispersed throughout the Bay and tidal rivers. The Maryland blue crab fishery is active from spring through fall; the Virginia fishery is focused in the winter.

Maryland and Virginia have different guidelines as to size limits, seasons and harvesting methods. To ensure the continuance of a commercial and recreational crab fishery, the needs of each life stage must be met.

If the blue crab and the unique heritage of the fishery is to survive for future generations, management decisions must be determined by the status of the resource.

Kathryn Reshetiloff is with the U.S. Fish and Wildlife Service’s Chesapeake Bay Field Office in Annapolis.

Blue crabs inhabit the Bay throughout the year
Bay Journal - July/August 1995

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