- Dissolved oxygen/water quality. Blue crabs tend to move out of deep waters affected by low dissolved oxygen and into shallow areas. Concentrated in the shallows, blue crabs are more susceptible to fishing gear, density-dependent predation and agnostic interactions (or social behaviors related to fighting). Low dissolved oxygen may also interfere with the dispersal of juvenile crabs into the estuary and with the migration of adult females into the mouth of the Chesapeake Bay to spawn.
- Habitat loss. Juvenile crabs find refuge from predators in underwater grass beds. The loss of these grasses is a loss of nursery habitat, which concentrates recruiting crabs (or those new crabs that are entering the adult population) in the limited nurseries that remain. The loss of shallow water habitat along marshes and forested areas also impacts young crabs, who use these areas to find refuge and benthic prey.
- Predator abundance. Predation affects blue crab population dynamics, survival and reproduction. Predation varies seasonally, spatially and among habitats and most greatly affects the smallest and earliest life stages. Cannibalism is also a source of mortality, especially among juveniles.
- Prey availability. Blue crabs feed on benthic prey, whose abundance can be limited by long periods of severe hypoxia. Eutrophication, habitat alteration and the abundance of crab competitor species can also impact the abundance of benthic prey and may limit their availability to blue crabs.
- Disease. Blue crabs harbor a range of fatal pathogens. Hematodinium perezi often infects crabs in the high-salinity waters of the lower Chesapeake Bay during the fall. Reo-like virus is found Bay-wide and often infects crabs that die in shedding facilities. The effects of these and other diseases on blue crabs remain poorly understood.
- Climate change. Climate change is predicted to have a range of impacts on blue crabs. Changes in water temperature could affect predator abundance, prey availability and winter mortality rates. Sea level rise could affect the habitats that serve as nursery and foraging areas. Changes in precipitation could increase areas of low dissolved oxygen, which could reduce foraging resources and limit the amount of available suitable habitat for blue crabs.
- Environmental variability. The survival of blue crab larvae in coastal waters and their settlement in grass beds and other habitats depend upon oceanic flow, storm events and weather patterns. Tides, salinity and other chemical cues also impact larval settlement, while seasonal fluctuations in temperature affect the timing of other life history events. Extremely cold winters can cause significant blue crab mortality in the Bay, especially in low-salinity waters.
The blue crab exhibits highly variable population dynamics. Blue crabs produce many offspring in multiple broods, but larval survival in the ocean is unpredictable. Juvenile recruitment into the Chesapeake Bay fluctuates from year to year, and is influenced by a range of oceanic and climatic conditions. Once young crabs return to the Chesapeake Bay from the ocean, many factors continue to influence their survival, including environmental variability, water quality, habitat quality, predator and prey abundance, disease, competition, and overwintering mortality.
Blue crabs support both commercial and recreational fisheries. Because of the complex ways blue crabs move throughout the Chesapeake Bay, fishing practices and harvest amounts vary from region to region. Understanding fishing effort and harvest levels is critical to informing management decisions. Improved harvest reporting would provide managers with more accurate harvest estimates.
Resource managers make decisions with the best available data. When data are unavailable, managers must make assumptions. Data gaps related to blue crab management include summer abundance and spawning stock data, natural mortality, and reproductive limitations and success. Filling these gaps and improving commercial and recreational harvest accountability would reduce uncertainty associated with management decisions. Funding and resources related to data collection and analysis will affect how and when these data gaps can be addressed.
Blue Crab Abundance outcome:
Factors Influencing Progress
A range of factors—including harvest, habitat loss and predation—impact the Chesapeake Bay’s blue crab population.