Beaufort County shrimpers are reporting smaller catches, and they believe black-gill disease is at least partly responsible.
"The black gill has been terrible," said Craig Reaves, who owns Sea Eagle Market in Beaufort. "The last two years are some of the worst we've ever seen."
The outbreak seemed to peak in August, when 95 percent of the shrimp Reaves and his crews caught had the disease, he said.
Black gill has tended to show up in at least some shrimp each year since it first was detected in South Carolina in 1999. This year has been particularly bad, however, and the outbreak apparently gets worse the farther south you go. Georgia shrimpers are reporting as much as 90 percent of their harvest has black gill.
High percentages are also being reported in Beaufort County.
"God, 60 to 70 percent have it," said Charles Gay, owner of Gay Fish Co. on St. Helena Island. "Some of them are just now getting it. Some are coming in with the majority of the head black."
"I can't say that's causing it," he said of the lower harvests, "but we're seeing more and more black gill and fewer and fewer shrimp."
Black gill isn't lethal to shrimp by itself, but it diminishes breathing capacity, making the shrimp more vulnerable to predators. It's not harmful to humans. Shrimp with the disease are headed and sold.
The disease has infected nearly half the shrimp caught in South Carolina since August, according to the S.C. Department of Natural Resources. It's shown up in a year that already was trending toward a poorer catch. A fair spring crop was followed by a poor summer crop, mostly likely caused by heavy rains flushing shrimp too far and wide to be caught.
The fall crop tends to be the most-sought-after shrimp. The August catch in South Carolina was only one-third of the 2008-2012 average; the September catch less than one-tenth, according to DNR.
Tonya Hudson-DeSalve, manager of Benny Hudson Seafood on Hilton Head Island, said she has seen some improvement in shrimp health lately but not in the harvest -- her shrimp-boat captains say they still catching only about half as many shrimp as last year.
The disease can be caused by a number of things, from algae to heavy metals in the water. This outbreak has been tied to a ciliate, a microscopic parasitic organism. It infects the gills, which respond by building a crust that turns the head black.
Not a lot is known about the ciliate. DNR biologists and university researchers are planning DNA sequencing at the Hollings Marine Laboratory in Charleston to try to find where it came from -- a first step to getting a handle on controlling the outbreaks.
One thing researchers do know, it's extremely infectious. A head removed from an infected shrimp and placed near a healthy shrimp, can infect it.
Shrimpers themselves might be helping to spread the disease by heading the crop on the boat and discarding heads, said Marc Frischer, one of the researchers and a professor at the University of Georgia's Skidaway Institute of Oceanography.
Gay believes the ciliate is introduced by bacteria that is, for example, in fish meal used for shrimp baiting.
Frischer said that can't be ruled out, but researchers so far don't have the technology or the tools to study it anywhere but in the shrimp.
Shrimp get rid of the ciliate by shedding their shells more frequently, and that takes a lot of energy. Shrimpers in both South Carolina and Georgia have reported the catch as more lethargic in the nets and more of the shrimp are soft and mushy -- the sign of a recent molting, Frischer said.
The ciliate can't be blamed with certainty for this year's poor crop, cautioned Mel Bell, DNR fisheries management director. The rains were a factor. They also could have stressed the shrimp, making them more susceptible to the parasite.
But black gill turned up big in Georgia last summer, too, and that crop was poor, Frischer said.
"We can't say with 100 percent confidence that the crop failure this year is because of black gill," he said. "But certainly it did some damage."
Beaufort Gazette and Island Packet staff writer Zach Murdock and The (Charleston) Post & Courier staff writer Bo Petersen contributed to this report.