Aquaculture can help reduce the spread of disease in wild oyster populations, a fisheries researcher at the University of Rhode Island recently concluded. The discovery contradicts the long-held belief that aquaculture has an inherently negative effect on nearby shellfish wildlife.
Written by: Philip Cozzolino / The Narragansett Times
“Aquaculture typically gets a bad wrap–it’s often thought of as the source of disease,” said Tal Ben-Horin, a postdoctoral fellow at the URI Department of Fisheries, Animal and Veterinary Sciences in the College of the Environment and Life Sciences. “But we know that oysters are filtering the water they live in and they provide that ecosystem service. So we built mathematical models that took everything we knew from everything that’s been studied on oysters, and this was across the board– from the Gulf of Mexico to the Chesapeake Bay–and we asked the simple question of how does the interaction between aquaculture and wild populations look?”
“The very act of aquaculture has positive effects on wild populations of oysters,” Ben-Horin continued. “The established way of thinking is that disease spreads from aquaculture, but in fact, aquaculture may limit disease in nearby wild populations.”
Ben-Horin was joined in the project by colleagues at the University of Maryland Baltimore County, Rutgers University, the U.S. Department of Agriculture and the Virginia Institute of Marine Science. Together, researchers integrated data from previous studies into mathematical models to determine the relationship between aquaculture and the local wild population of oysters, with the resulting data showing a revelatory conclusion: when conducted with the necessary considerations, aquaculture at large can be of benefit to wild shellfish.
The findings, part of a research project at the National Center for Ecological Analysis and Synthesis, was published this week in the journal Aquaculture Environment Interactions.
According to Ben-Horin, diseases are incredibly common in wild oyster populations. New England suffers a lack of wild oyster populations due to the disease, dermo, a sickness caused by a naturally occurring, single-cell parasite that proliferates in the tissue of host oysters and spreads to nearby wild populations through the water column. dermo, while not harmful to humans, typically kills infected oysters in about two or three years, and as the deceased shellfish decay on the seabed, the parasite spreads into the water column, to be absorbed by neighboring oysters, which naturally filter water. If oysters are held on farms long enough to naturally filter the disease from the water column, but not so long that death occurs from the dermo infection, the balance can tip, and the aquaculture operation can have a positive effect on the nearby wild oyster populations.
“If you talk to any grower, they don’t think aquaculture helps spread disease in shellfish populations,” said Ben-Horin. “As long as aquaculture farmers harvest their product before the disease peaks, then they have a positive effect on wild populations. But if they’re left in the water too long, the positive effect turns negative. From the grower’s perspective, we’re pretty much confirming the hunch they’ve had for a long time, and we’re putting numbers on that hunch.
Ben-Horin said growers should harvest oysters once about every 18 months to simultaneously limit the spread of disease and maximize filtering and purifying potential before dermo kills the shellfish. The study also found that aquaculture operations can negatively impact wild populations when the proper considerations are not heeded. For example, an aqua farm that doesn’t harvest its product regularly will contribute more to the spread of disease than the water body in its natural state would.
“We got outcomes across the board,” said Ben-Horin. “You can have aquaculture having a positive effect on wild populations, enhancing the abundance of wild populations. But the area does exist where aquaculture can have a negative effect. Typically when that happens, it’s with high densities of aquaculture that are left in the water for a really long period of time.”
Still, says the URI researcher, it makes fiscal sense for growers to adhere to some kind of timely harvest schedule.
“But the situation exists where you can have oysters and culture that you harvest on a regular schedule like most growers do because they don’t want their product dying from disease, and it ends up having a positive effect on wild populations,” he said.
Ben-Horin said, though typically thought to be a driving factor behind the spread of disease on aqua farm operations, the density of product has little to do with the prevalence of infected oysters.
“This is a mathematical model that is approximating reality,” he said. “The result I think is most important is that it’s often thought that density is often thought to be the driving variable behind the spread of disease. But we’re finding it’s not really about density. It doesn’t matter, it’s really the harvest schedule that matters most.”
As a result of the new research, Ben-Horin recommends establishing best management practices in regard to duration of oyster products being grown on aqua farms.
The next step in Ben-Horin’s research is to gain a better understanding of the reach of dermo by studying infected models in relation to ocean-circulating specimens.
“Everything that happens in the water is connected,” he said. “There’s a close relationship between the wild and farmed oyster populations and their shared parasites. Sometimes ecosystem level effects are overlooked, but in this case they’re front and center.”