When one hears the words, “Chesapeake Bay,” one creature quickly comes to mind: the blue crab.
Displaying an almost arrogant attitude, the tenacious blue crab will take on any opponent, including humans. Watch a crab as it tries to escape from a net or pot. With the gantlet thrown, the crab extends its claws up at its predator while scuttling sideways across a dock or kitchen floor, daring its opponent to follow.
Blue crabs have, so far, endured increasing harvest pressures that would decimate other commercial fisheries. This succulent seafood is the pride and pleasure of the Bay. The scientific name for blue crab, Callinectes sapidus, illustrates the love for this delicacy. Callinectes means beautiful swimmer and sapidus means tasty or savory.
An intricate part of the Chesapeake ecosystem, crabs feed on just about anything, living or dead. No love lost here, crabs are also cannibals, devouring other adults and larvae. Blue crabs eat benthic (bottom-dwelling) organisms, which helps to regulate the latter’s populations.
An important component in the cycling of energy in the Bay, blue crabs also serve as food for rays, striped bass and bluefish. Because they do not possess an internal skeleton, crabs grow by molting or shedding their shell. While in this molting phase (soft crabs), they can be consumed by wading birds and mammals. Humans, of course, eat huge quantities of both hard and soft crabs.
Blue crabs use a variety of aquatic habitats, from the salty waters near the mouth of the Bay to the fresher reaches of rivers and creeks. Depending on the season, crabs may burrow into the Bay bottom or swim freely in open water.
Scientists have found that Bay grasses are particularly important to crabs. Immediately after molting, crabs are particularly vulnerable to predators because of their soft shells, so they often hide in grass beds for protection. Young crabs use Bay grass beds for nursery areas, and crabs of all sizes forage for food there. Thirty times more juvenile crabs are found in Bay grasses than in areas without grasses.
Mating occurs during the summer months. Before mating, female blue crabs must shed their shell. 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 fresher portions of the Bay and rivers and migrate into deeper waters to spend the winter. The female migrates to spawning areas near the mouth of the Bay. Females that mature early in the summer will spawn before the 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. Eggs are fertilized during the spring with sperm the female has stored over the winter. Females produce 750,000 to 8 million eggs that 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 microscopic and free-floating zoea. 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 12–18 months, the crab is mature, measuring about 5 inches point to point across the back.
Commercially, blue crabs from the Chesapeake Bay account for about one third of the U.S. harvest. Commercial harvests from the Bay between 1968 and 2005 averaged around 73 million pounds. Most recent harvests have been about 60 million pounds. Scientists expect the 2007 Bay harvest will be comparable to the 2006 harvest, which preliminary estimates put at 51.8 million pounds. Scientists have noted that the low abundance of juvenile crabs may lead to decreased harvests during the 2007 fall crabbing season.
With the decline of other commercially important species, such as the 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 does the demand for crabs.
In addition to harvest demands, nutrient pollution affects blue crabs. Excessive amounts of phosphorus and nitrogen from sewage plants, farm fields and lawns promote massive algae blooms. These blooms block sunlight from reaching the grass beds, which in turn, reduces shelter and feeding areas for crabs. Also, when the algae die and decompose, they use up precious oxygen needed by Bay grasses to make food as well as by other aquatic creatures, including crabs, to survive. Crabs often die from low oxygen levels when trapped in crab pots under these conditions.
If the blue crab and the unique heritage of the fishery are to survive for future generations, management will have to include more than determining seasonal harvest limits. We must also preserve its habitat by increasing Bay grass beds and reducing nutrient pollution running off the land. Restoring grasses relies upon the natural expansion that comes with improving water quality.
Kathryn Reshetiloff is with the U.S. Fish and Wildlife’s Chesapeake Bay Field Office in Annapolis.
Save a Crab
- Limit fertilizer use and apply at appropriate times. Nutrients in chemical fertilizers can run off yards into local waterways and eventually drain into the Chesapeake Bay.
- Plant native plants on one’s property. This reduces lawn areas that require fertilizing.
- Conserve water. The more water that is used, the more that must be treated through septic systems or water treatment plants.
- Maintain septic systems. Without regular pumping, septic tanks can fail.
- Drive less. Nitrogen oxides from fossil fuel combustion are a major source of nitrogen entering the Bay.
- Be a responsible boater and pump out wastes. Raw sewage causes pollution that harms human and aquatic health.
- Get involved in local watershed groups.