After more than nine years of researching black gill in Georgia shrimp, scientists at the University of Georgia Skidaway Institute of Oceanography are hopeful their work may help the state’s shrimpers deal with the condition, which many shrimpers blame for reduced harvests. They want to develop a forecasting tool that would allow shrimpers to predict what kind of season they may have and prepare accordingly.
Black gill is a condition found in shrimp in the southeastern United States. The Georgia Department of Natural Resources first officially reported black gill in its survey data in 1996, however Georgia shrimpers anecdotally have reported its presence at least since the 1980s. Early on, no one knew the cause of black gill or its effect on shrimp, and the number of infected shrimp has varied widely from year to year—as has the number of shrimp for harvest.
The presence of black gill is seasonal. It visually disappears during the winter. It begins appearing as the water warms in the early summer and peaks in September and October, which is also the height of the commercial shrimping season. That seasonal variability also gave UGA Skidaway Institute researcher Marc Frischer a clue as to why black gill only started appearing in the last three decades. He found a link between warmer winters and a higher incidence of black gill in the shrimping season later that year.
“What happened is that our climate is changing, especially with warmer winters,” Frischer said. “And that’s what is driving this occurrence of the symptomatic black gill.
“The ciliate that causes it has probably been here forever. We don’t think it is an invasive species. We recognize it now as an impact of climate change.”
The link to winter temperatures gave Frischer insight into a way to help the Georgia shrimpers. Since a cure for black gill is not likely, the next best answer might be a forecast tool. Frischer hopes that by providing shrimpers with some idea of what they can expect out of their efforts, it might avoid the devastating consequences the shrimp industry experienced in 2013.
“I think what we really need is a good, accurate forecasting tool that can say how good the year is going to be,” he said. “I think that would be helpful to the industry so they can calibrate their efforts to what their catches might be, and right now, we don’t really have that.”
Frischer was asked to get involved in the black gill project after the Georgia shrimp industry hit a crisis in 2013. Local shrimpers thought they were poised for a great season, as a shortfall of Asian imports drove prices to record levels. When the Georgia shrimpers deployed their nets, however, they discovered there were hardly any shrimp to be caught.
“There was a lot less shrimp on the market, and the prices were triple what they’d been the years before,” Frischer said. “So, the shrimpers put a lot of effort into preparing for the season, and then they didn’t catch enough shrimp to pay for their fuel cost. A lot of shrimpers when bankrupt that year.”
Frischer and his team quickly uncovered the cause: the parasite black gill, a tiny, previously undescribed, single-cell ciliate growing on the shrimps’ gills (the blackening of the gills actually is a defense mechanism shrimp use to protect against the parasite). Subsequent research confirmed the condition affects shrimp in several ways, including reducing their endurance and making them more susceptible to predators. It also probably increased mortality rates, although determining the contribution of black gill to direct mortality rates has been difficult to quantify.
“We’ve learned a lot about the parasite’s life history, what it does and why it causes black gill,” Frischer said. “We think that’s because it feeds off of living gill tissue on the shrimp. That causes the shrimp to respond to turn on its immune system, which is what results in the symptoms of black gill.”
Although the parasite is harmful to shrimp, Frischer stressed that it is not harmful to humans. “If you’ve eaten local shrimp anytime since the 1990s, you’ve eaten shrimp with black gill. I’ve never heard of any health issues for people associated with eating shrimp affected by black gill, and they still taste delicious.”
In December, Frischer hosted the sixth “black gill cruise” on board the Research Vessel Savannah, bringing together a wide-ranging group of stakeholders including, scientists, shrimpers, managers, policy makers, educators and the press to compare notes and establish connections.
“The point of this year’s cruise was to look back and see where we’ve come and chart a path forward, what we’ve learned and what we still have to learn,” he said. “We discussed challenges currently facing the fishery, and what are perhaps new opportunities.
“It’s not wiping out shrimping. But it’s not helping either. And so, the shrimpers are being compelled to learn to live with it.”