An Examination of the Frequency of Black Gill Disease in White Shrimp in South Carolina Using Samples Collected by Commercial Shrimp Fishermen - Year II
The condition known locally as black gill (brown gill, brown lung) has been persistent in white shrimp in South Carolina since 1999. Black gill is characterized by a darkening (melanization) of the gill surface in Penaeid shrimp and is easily observed through the exoskeleton of the carapace. The "disease" has routinely appeared in August and manifests itself in mid to late September before becoming scarce in late October or November as water temperatures decline below 72° F. It is important to note that black gill is not transmissible to humans and does not affect the taste of affected shrimp.
Commercial shrimpers report that buyers, both retail and wholesale, complain about the appearance of the shrimp. Some shrimpers have reported that the exoskeletons are soft and tow duration mush be reduced to prevent damage to the shrimp. Some believe that shrimp die quickly in the nets and come aboard the boat in a state of decomposition, although DNR staff has not observed this directly. Anecdotal reports from commercial shrimpers suggest the disease "moves" up and down the coast, seemingly starting near the Charleston area each year. During one year, the disease appeared to spread from central South Carolina through the Georgia coast in about 6-8 weeks.
While gill discoloration can be caused by mishandling of the shrimp after harvesting, fouling of the gills with debris, or infection by viruses, bacteria, or parasitic isopods, the primary causative agent in South Carolina shrimp appears to be one of several Protozoans (Hyalophysa Chattoni, Gymnodinioides inkystan and Synophrya hypertrophica) that colonize the gills during a portion of their normal life cycle.
During this dormant stage, the protist has formed a cyst attached to a gill surface. The supposition is this reduces the transfer of oxygen from water to the shrimp, and the shrimp respond by forming a barrier of melanin between healthy gill tissue and the encysted protist. Eventually, the shrimp molts in an effort to remove the cyst. At this time, the cysts metamorphose into an active feeding stage and feed on a waxy fluid the shrimp secrete to aid in discarding the old shell. Repeated molting of the shrimp results in a thinner shell and makes the animal more susceptible to injury.
Five (5) commercial trawlers representing different areas of the South Carolina coast were contracted to provide data to DNR biologists. This work began in August and extended through October 2006. On each day that shrimpers worked, they recorded the percent occurrence of black gill from a sample of approximately 100 randomly selected shrimp. These data were then provided to DNR on a weekly basis. In addition, about 50 shrimp from the consolidated daily catch were frozen and provided to DNR (picked up weekly). These samples were used to establish shrimp size and species composition and verify the black gill infection rate reported by shrimpers with microscopic analysis of the sub-sample. When required, shrimp were forwarded to experts for other analysis. This operation continued through October, at which time the incidence of black gill fell below 5 percent.
Preliminary results were presented at the Shrimpers Association meeting this past February.