UGA researchers explain recent decline in Georgia’s blue crab population

Two researchers at the Skidaway Institute of Oceanography in Savannah have offered an explanation for the recent decline in Georgia’s blue crab population that has devastated one of the state’s most important coastal fisheries.

In an article in the November/December issue of the American Scientist, Richard F. Lee and Marc E. Frischer, working on a grant from the Georgia Sea Grant Program at the University of Georgia, say their research shows that Georgia’s recent drought, working in conjunction with an opportunistic parasite, is ultimately to blame for the decline in blue crab numbers.

Prior to the recent drought, the 45-year average for blue crab landings in the state was 8.6 million pounds per year, but at the height of the drought landings fell to 1.8 million pounds. This sharp and sustained reduction in yearly catch drove most of the state’s blue crab fishermen out of business and many into bankruptcy. Lee and Frischer discovered that many of the crabs caught during the drought seemed to be suffering from a parasitic infection. The infection was determined to be Hermatodinium perezi. Though crabs infected with Hermatodinium were not unknown to the Georgia coast, Lee and Frischer set out to determine why the parasite crashed the local population so suddenly.

According to the researchers, an important part of the puzzle is the effect drought has on the delicate balance of salt water and fresh water that exists in coastal estuaries. Blue crabs spend most of their lives in this brackish water. During a drought, less fresh water comes down the state’s rivers to mix with salt water that tides bring in from the Atlantic Ocean. This raises the salt content in coastal estuaries. And it is in water with this higher percentage of salt that Hermatodinium thrives and spreads from crab to crab.

Lee and Frischer found a direct correlation between the state’s recent drought and the prevalence of the disease in the state’s blue crab population. They also cite data showing that during times of high river flow, when the percentage of salt in the estuaries is lower, blue crab landings historically have increased. That the blue crab fishery is suffering from a weather-related problem could be good news for Georgia crabbers. With the drought officially at an end, anecdotal evidence seems to suggest that blue crab populations are on the rise, and the prevalence of the Hermatodinium infection is decreasing.