Black gill is back — the disease has infected nearly half the shrimp caught in South Carolina since August. Shrimpers are blaming the disease for a poor crop. Researchers are scrambling to find a cause.

The disease isn’t likely kill off the shrimp, but it could kill off the beleaguered shrimping industry.

“The shrimp will survive; they will rebound. But this is a fishery on the edge of sustainability economically,” said Professor Marc Frischer, of the University of Georgia’s Skidaway Institute of Oceanography, one of the researchers. “They’re already competing against a stacked market of farm-raised, imported shrimp. This may be the last straw.”

Black gill disease, or syndrome, isn’t lethal to shrimp by itself, but with a weakened ability to breathe, the shrimp are more vulnerable to predators. It’s not harmful to humans.

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.

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 the S.C. Department of Natural Resources.

Black gill has tended to show up in at least some shrimp each year since it first was found here in 1999. But this year has been particularly bad. The outbreak apparently is worse the farther south you go; Georgia shrimpers are reporting as much as 90 percent of the crop has black gill.

“God, 60 to 70 percent have it,” said Charles Gay, of Gay Fish Co., in St. Helena. “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, but we’re seeing more and more black gill and fewer and fewer shrimp.”

Black gill can be caused by a number of things from algae to heavy metals in the water, but this outbreak has been tied to a ciliate, a microscopic parasitic organism. It infects the gills, which respond by building a crust around it that turns the head black.

Not a lot is known about the ciliate. Frischer is working with S.C. Department of Natural Resources biologists and plans 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. It’s possibly an invasive species, he said.

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, Frischer said.

Gay personally thinks 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, S.C. Department of Natural Resources 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.”

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