The original range of the blue crab is from Nova Scotia and throughout the Gulf of Mexico to northern Argentina, including Bermuda and the Antilles. The blue crab is seldom found north of Cape Cod, but has been recorded in Maine and Nova Scotia following consecutive warm years. The blue crab has been introduced, probably via ship ballasts into Europe, north Africa, and southwest Asia. Introductions into the Mediterranean Sea and adjacent waters have produced breeding populations whereas others were probably temporary occurrences. The blue crab also has been introduced into Japan.
The life history of the estuarine dependent blue crab involves a complex cycle of planktonic, nektonic, and benthic stages, which occur throughout the estuarine-nearshore marine environment. A variety of habitats within the estuarine environment are occupied depending upon the particular physiological requirements of each life history stage. Spawning females and early zoeal (larval) stages inhabit lower estuarine and adjacent marine waters where salinities are >20 ppt, a level necessary for zoeal development. Later stage zoea exist mainly in the open gulf. The megalopal stage reenters the estuaries and adopts a largely benthic existence.
Distribution of juvenile and adult blue crabs within estuaries appear to be determined by a complex interaction of abiotic, trophic, and other biotic factors. Juvenile blue crabs exhibit wide seasonal and areal distribution within the estuary but are associated with waters of low to intermediate salinity and soft-mud sediment bottom types, often adjacent to vegetated habitats. Adult blue crabs are widely distributed and occur on a variety of bottom types in fresh, estuarine, and shallow oceanic waters. Large (ie., 80-85 mm carapace width, CW) blue crabs are more prevalent in larger bays and bayous. Although adult blue crabs are ubiquitous throughout an estuarine system, they are distributed seasonally with respect to salinity and sex. In general, males predominate in low salinity areas while females predominate in high salinity areas. In Louisiana, blue crabs have been reported 305 km (189 mi) upstream in the Atchafalaya River.
Blue crabs are one of the most common estuarine macroinvertebrates and are generally abundant throughout the system. Peak abundance of adult crabs occurs during the warmer months. During winter crabs concentrate in areas of tidal exchange in the lower estuary. Juvenile blue crabs are most abundant in estuarine waters of low to intermediate salinity during the winter months.
Louisiana averaged over 70% of the total Gulf of Mexico blue crab landings during the 1990s and lead the country in 1987, 1988, 1991, and 1992. The high productivity of blue crab and other species in Louisiana estuaries can be attributed to several factors:
- Near subtropical climatic regime and abundance rainfall.
- The broad near-sea level plain that has resulted from land building by the Mississippi River.
- The large input of fresh water and nutrients by the Mississippi River.
- The low coastal wave activity.
- Daily tidal flushing which is so important in all coastal marshes.
- Low tidal amplitudes.
- Large amount of marsh edge.
Blue crab population abundance can fluctuate dramatically from year to year. The blue crab is an “r” (reproductive)-selected strategist species, which is characterized by production of large numbers of young, rapid growth, early attainment of sexual maturity, high mortality rates, and short life span. Such species exhibit large interannual fluctuations in abundance because physical, chemical, and biological factors strongly influence abundance. In addition, blue crabs populations are cyclic. Five-year and longer cycles have been identified.
Blue crabs are migrants that occupy various estuarine and nearshore habitats, according to the physiological requirements of each life history stage. After larval development in high salinity waters and megalopal recruitment into estuarine waters, early crab stages (5-10 mm CW) of both sexes begin an up-estuary migration to shallow areas of low to intermediate salinity. Female crabs mate at the pubertal molt and move to more saline waters to spawn while males tend to remain in brackish areas. Five general migration patterns have been identified: 1) spring up-estuary migration of large juveniles and adult males; 2) recruitment of small juveniles to the upper estuary; 3) return of spawned females from offshore to the lower estuary in the summer; 4) upper-to-lower estuary and offshore migration of gravid females in autumn (the fall run of females); and, 5) down-estuary migration of large juveniles and adult males from the upper estuary in November and December.