East coast shellfish aquaculture industry (Photo: ECSGA)
Oyster aquaculture may prove helpful to stop wild stock disease
UNITED STATES
Tuesday, December 18, 2018, 03:20 (GMT + 9)
Contrary to long-held beliefs, a team of scientists has found that oyster aquaculture operations can limit the spread of disease among wild populations of the resource.
Photo: FDA aquaculture management guide - Oyster Farm
Working with colleagues at the University of Maryland Baltimore County, Rutgers University, the US Department of Agriculture (USDA), and the Virginia Institute of Marine Science (VIMS), fisheries researcher at the University of Rhode Island, Tal 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.
The host and parasites. (A) The eastern oyster, Crassostrea virginica. (B) examples of the effect of disease on oyster condition.
Note the creamy appearance of the healthy oyster, which is due to the accumulation and storage of glycogen, vs. the emaciated, glassy appearance of the diseased oyster, which has been unable to accumulate glycogen. The dark-colored digestive gland is visible through the almost transparent flesh of the diseased oyster
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.
Dermo is caused by a single-celled parasite that occurs naturally in the environment and proliferates in the tissue of host oysters, which spread the parasite to other oysters when they die and their parasite-infected tissues decay in the water column.
However, Ben-Horin explains that it takes two to three years for the parasite to kill the oysters. As long as the oysters are held on farms long enough to filter disease-causing parasites from the water, but not so long that parasites develop and proliferate and spread to wild oysters nearby, aquaculture operations can reduce disease in wild populations.
The disease does not cause illness in humans.
“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.”
He said that several factors can confound the positive effect of oyster aquaculture. Oyster farms that grow their product on the bottom instead of in raised cages or bags, for instance, are unlikely to recover all of their oysters, resulting in some oysters remaining on the bottom longer. This would increase rather than reduce the spread of the disease.
“But when it’s done right, aquaculture can be a good thing for wild oyster populations,” Ben-Horin said. “Intensive oyster aquaculture – where oysters are grown in cages and growers can account for their product and remove it on schedule – is not a bad thing for wild populations.”
The study’s findings, published in the journal Aquaculture Environment Interactions, have several implications for the management of wild and farmed oysters.
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