December 19, 2018 — SEAFOOD NEWS — Recent research carried out at the University of Rhode Island and the Virginia Institute of Marine Science has found that growing farmed oysters can reduce disease loads in wild oysters.

This counter-intuitive finding is based on the fact that the primary killer of wild oysters is Dermo, a parasite that occurs naturally in the environment, and lives in the tissue of oysters.  The single celled parasite is harmless to humans, and has nothing to do with bacteria such as vibrio.

“The very act of aquaculture has positive effects on wild populations of oysters,” 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. “The established way of thinking is that disease spreads from aquaculture, but in fact aquaculture may limit disease in nearby wild populations.”

Working with colleagues at the University of Maryland Baltimore County, Rutgers University, the U.S. Department of Agriculture, and the Virginia Institute of Marine Science, Ben-Horin integrated data from previous studies into mathematical models to examine the interactions between farmed oysters, wild oysters and the common oyster disease Dermo.

Basically, Dermo is spread through an oyster reef when infected oysters die, and their tissues decay.  But aquaculture, particularly caged or bagged oysters off the seabed, act as filters, and take in the Dermo parasite, but they are harvested and sold before the parasite has any lethal effects.

The net result is that near oyster farms, the incidence of wild Dermo goes down.

According to Ben-Horin, diseases are among the primary limiting factors in wild oyster populations. There are few wild populations of oysters in New England because of Dermo and other diseases, and in the Chesapeake Bay and Delaware Bay, wild oysters are managed with the understanding that most will die from disease.

“As long as aquaculture farmers harvest their product before the disease peaks, then they have a positive effect on wild populations,” Ben-Horin said. “But if they’re left in the water too long, the positive effect turns negative.”

The study’s findings have several implications for the management of wild and farmed oysters. Ben-Horin recommends establishing best management practices for the amount of time oysters remain on farms before harvest. He also suggests that aquaculture managers consider the type of gear – whether farmers hold oysters in cages and bags or directly on the seabed – when siting new oyster aquaculture operations near wild oyster populations.

This story was originally published by SeafoodNews.com, a subscription site. It is reprinted with permission.