Blue crabs

Abstract

Blue crabs are considered estuarine residents with all life history stages, except for the larval stages, occurring in estuarine waters. Adult females release larvae into the water column near the mouths of estuaries (Millikin and Williams, 1980). Larvae are carried offshore where at least 30 days are required to go through seven zoeal stages (Millikin and Williams, 1980; Epifanio, 2007). As a result, blue crab larvae represent one trophic link between estuarine and oceanic food webs. The final plank-tonic stage (megalopa) returns to the estuary, via wind-driven currents and tides (Epifanio, 2007), where they metamorphose to form the first juvenile stage (<5 mm carapace width) and become benthic. These juveniles grow rapidly, molting on average every 3–4 weeks depending on water temperature (Smith and Chang, 2007), making them important prey for a variety of fish and birds, but they are also important predators on other small invertebrates (Lipcius et al., 2007). Therefore, young juveniles (<20 mm carapace width) occupy shallow, structured habitats that also contain food sources including seagrass beds, macroalgae, and oyster reefs (Lipcius et al., 2007). During the 12–18 months required to reach sexual maturity (at 90–100 mm carapace width), habitat use expands based on size and density-dependent factors (Hines, 2007; Lipcius et al., 2007). Smaller juveniles (20–30 mm carapace width) move to alternative nursery habitats including marsh creeks and marsh-fringed mud flats (Lipcius et al., 2007). Larger juveniles (>20 mm carapace width) begin venturing into unstructured habitats and, as they grow, inhabit deeper areas where they continue to be important predators but are prey to fewer organisms (Hines, 2007). Unlike females that exhibit a final molt to reach maturity, adult males grow throughout their lives (reaching sizes of >200 mm carapace width), molting every 30–40 days depending on temperature, and they typically return to more protective habitats during molting, as they are particularly vulnerable to predators. As a result of ontogenetic shifts in habitat as well as movement into lower salinity areas, blue crabs can be found in a wide array of habitats, throughout the estuarine-to-ocean salinity gradient (e.g., 5–35 ppt) (Hines, 2007; Lipcius et al., 2007). Because they represent both predator (contrary to popular belief, they are not scavengers) and prey in these habitats, blue crabs are a critical component of the estuarine food web both within and between estuarine habitats. Blue crabs have been an important food item for humans since the early 1700s and have supported a commercial and recreational fishery since the 1800s (Kennedy et al., 2007). As a result, blue crabs are part of the historic, economic, and social fabric of communities along the Atlantic and Gulf coasts of the United States. Blue crabs are consumed as “hard crabs” (crabs with a hard carapace, typically in the intermolt stage) and as “soft crabs” (crabs with a soft carapace as a result of recent molting) (Kennedy et al., 2007). The predominant fishing techniques vary with the season and with the sex of the harvested crabs. During the warmer seasons (late spring-late fall), blue crabs are typically harvested with a trap or “pot,” and the catch is predominantly males. During the winter, particularly in the mid-Atlantic region, blue crabs are harvested by a dredge, and the catch is predominantly females.