To me, an ecologist is someone who uncovers the connections between the animals, plants, humans and natural forces that shape our world. I’m currently in the fourth year of my Ph.D. in the Odum School of Ecology at the University of Georgia and am a Georgia Sea Grant Research Trainee. The focus of my dissertation is investigating the factors that influence disease and population cycles in aquatic systems, and my primary experimental system is black gill disease in Georgia shrimp.
The shrimp industry in Georgia is both the largest and most beloved fishery in the state. For over a century the iconic silhouettes of shrimp boats draping their nets into near-shore waters have been a part of the culture of the Georgia coast. However, recently the tides have turned for those involved in the industry. A disease, known ominously as “black gill”, has begun to turn up more and more in Georgia’s shrimp. Shrimpers are worried that the reduction in shrimp harvest potentially caused by this disease, in addition to the other economic and social stressors they are facing, will be the death knell to their way of life.
I had never heard of black gill before moving to the coast, but the mystery surrounding its emergence and the potential effects on the fishery provoked me to investigate this disease. It was like a crime novel: the mysterious disappearance of shrimp from coastal waters, a list of suspect reservoir species, and a black spot left on the disease’s victims. Additionally, the disease seemed to disappear from the commercial shrimp every fall, only to suddenly reemerge the next summer once the water warmed.
I began my investigation by “interviewing” the primary suspect: a species of marsh-dwelling shrimp, known as grass shrimp, whose population suspiciously peaked in black gill prevalence just as the commercial shrimp were losing the disease, and vice-versa. Could the grass shrimp be harboring the infection while the commercial shrimp are far away from the coast, maybe even amplifying it, and then re-infecting the commercial shrimp once they returned to the coast to spawn? To answer this question, I first had to prove that it is possible to transmit the infection between these two species of shrimp. I took commercial shrimp that had been treated to eliminate the possibility that they were infected with black gill and cohabitated them with untreated grass shrimp. I then harvested tissue from these shrimps and used DNA analysis to determine if the commercial shrimp had acquired black gill from the grass shrimp. Success! One of my commercial shrimp had contracted the infection, indicating the possibility for this disease to be passed from one species to another.
However, like a good detective, I must follow all my leads. What if the infection is primarily in the water, and it just takes the right temperature to initiate the infection? What if another species of crustacean is responsible for the infection, and the poor grass shrimp are being framed? How likely is it that black gill is causing significant mortality in the shrimp, resulting in the reduction of harvest that the shrimpers are reporting? These questions led me to my field experiments.
I, along with other researchers at Skidaway Institute of Oceanography, developed a protocol to medicate wild shrimp to effectively cure them of shrimp black gill disease. The process involved adding over-the-counter aquarium medications at half the dosage to wild-caught shrimp over two week period. At the end of two weeks, we would test the shrimp to make sure our treatment was effective. Then, I would take these cured shrimp and place them in individual cages into the Skidaway River. After ten days, I would remove the shrimp and check them for reinfection of shrimp black gill disease. I did this seasonally for a year. Surprisingly, 100% of the shrimp deployed in the summer acquired the infection, while none of the shrimp deployed in the winter were infected. This points to differences in transmission being the primary driver between the seasonal patterns of disease prevalence.
Like a good detective novel, the disappearance of commercial shrimp and the emergence of black gill provides a lot of intriguing clues: some of them genuine and others that point to red herrings. It will take time and a thorough investigation to unravel the mysteries of this disease, but by understanding more about black gill we can help protect Georgia’s cherished shrimping industry.