Blue Crab Populations Decline in Saltier Water
Although blue crabs (Callinectus sapidus) are tough, hardy predators, they don’t thrive in all coastal waters. Blue crabs prefer a certain amount of salt in their habitats—not too much but not too little, either. They grow best in a zone of brackish estuary between higher salinities of ocean water and lower salinities farther up coastal rivers and creeks.
Over the past decade, however, the state’s estuaries have turned saltier, and blue crabs are finding it more difficult to adapt to new conditions.
Sea Grant researcher Michael Childress, a marine ecologist at Clemson University, is studying how environmental changes are influencing this multi-million-dollar fishery in South Carolina.
The salinity of estuaries is the most important factor in predicting abundance of crabs in South Carolina, Childress said. When salt concentrations increase in estuaries, crab populations fall. This has implications for the future as sea-level rise continues to drive salty ocean water farther inland. Droughts, coastal development, and upland water usage also affect estuarine salinity.
From 1995 to 1998, South Carolina experienced higher-than-average rainfall totals, and crab landings increased to a peak of 7.5 million pounds. But over the next four years, 1999 to 2003, the state experienced a severe drought, and crab landings fell to 4.2 million pounds. Crab landings still have not recovered to pre-drought levels.
Childress is studying two hypotheses to explain the relationship between increased salinity in South Carolina estuaries and crab catches.
Crabs seasonally migrate upriver to find optimal salinity in locations beyond the state’s regulatory boundary for the commercial fishery. So do crab landings decrease because commercial fishermen can’t catch them? Or do landings decrease because crab populations decline throughout estuaries?
To answer such questions, Childress developed a computer model called the South Carolina Blue Crab Regional Abundance Biotic Simulation (SCBCRABS). This model attempts to address complex interactions between various habitats and life stages of the blue crab.
Childress uses the model to follow simulated individual blue crabs through time as they occupy habitats and encounter changing environmental conditions. The model is based on theoretical principles of population biology and environmental data.
In computer simulations, the population densities of blue crabs decreased in higher salinity waters. The animals had less prey to eat and greater crowding in the fewer sites of optimal salinity, so more crabs ate each other.
Do crabs permanently escape the fishery by swimming upriver? Probably not, Childress said.
“There is only a two to three month period during the late summer and early fall when crabs move far enough upriver that they can’t be caught,” he said. Usually by December, they move downriver to locations where they can be legally harvested.
In 2010, the S.C. Sea Grant Consortium awarded Childress a two-year research grant to conduct quarterly surveys of the blue crab population in the ACE Basin National Estuarine Research Reserve (NERR). The project is a collaboration between Clemson University, S.C. Department of Natural Resources, ACE Basin NERR, S.C. Sea Grant Extension Program, local fishers, and the public.