Environmental Factors – AL Blue Crab

Habitat Loss, Restoration, and Freshwater Inflow

oil canalNatural and anthropogenic alteration of habitat in the Gulf of Mexico impacts every stage of the crab life cycle. Activities that can contribute to habitat loss include but are not limited to pollution, eutrophi­cation, and alterations in freshwater and sediment flow (Guillory, Perry and VanderKooy 2001). Blue crabs depend on the quality and quantity of estuarine marshes, mangrove areas, submerged vege­tation, and nearshore soft sediment habitats to successfully reproduce and grow (Guillory, Perry and VanderKooy 2001).

Habitats may be altered by human activity through freshwater control. Wetlands are created by and maintained through nutrients and sediments transported to them by river systems; the damming, channelization, and leveeing of rivers can affect the timing and flow of freshwater to a wetland estuary and greatly affect the composition of the habitat (Guillory, Perry, and VanderKooy 2001).

The Mobile Bay watershed is the fourth largest drainage basin in North America that covers 65% of the land area in Alabama (Mobile Baykeeper), and is an active area for blue crab settlement. In general,  settlement is higher in intermediate salinities, though a study by Rablais et al. (1995) showed higher settlement in a lower salinity year. Because of the size of the watershed, and the amount of water flowing through the basin, Mobile Bay is subject to threats from pollution including stormwater runoff, litter, and industrial waste. Organizations that are working to address habitat restoration and water quality improvement in Mobile Bay include The Nature Conservancy-Alabama, Mobile Baykeeper, and Alabama Coastal Foundation.

Current research in the Gulf of Mexico has identified freshwater inflow to be a critical driver of blue crab population dynamics (VanderKooy 2013). Low flow conditions hav ebeen found to negatively affect juveniles blue crabs in teh Apalachicola River estuary (Wilbur 1994). Sanchez-Rubio et al. (2011) related abundance of juvenile blue crab to the influence of global climate factors on regional hydrology and how climate and hydrology structure habitat. Riedel et al. (2010) highlights the correlation between significant down­ward trends in abundance of juvenile blue crabs and a period characterized by drought and habitat changes from both natural and man-made alterations to coastal wetlands. High river flows have also been linked to increased commercial landings in Florida and Texas (More 1969, Wilbur 1994).

Fluctuations in freshwater inflow and resulting changes in salinity may also have secondary effects on blue crab abundance due to predation (Bourgeois, Marx, and Semon 2014). Predation is a factor influencing blue crab abundance, and changes in salinity influence the presence or absence of predators. During periods of high freshwater inflow and low salinity, many predators remain offshore in higher salinity waters, while low rainfall will increase salinity, causing predators to move inshore and overlap with blue crab habitats.


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