Hatchery-raised oysters showing resistance to disease
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In a hatchery located a few dozen yards from the York River, scientists are accomplishing something watermen and biologists have sought for years - a native oyster that can fend off disease. While efforts of the Virginia Institute of Marine Science to evaluate the performance of disease-resistant, non-native Japanese oysters in the Bay have received much attention in the media, biologists at the Institute have also been successfully developing disease-resistant strains of the native oyster, Crassostrea virginica using broodstock from various sources.
More than 80 percent of the third generation of a strain originating from Delaware Bay, which was spawned in 1993, survived to reach market size.
"They really showed a lot of promise," said Lisa Calvo, one of the VIMS scientists overseeing the project.
The oysters were produced in the hatchery in April of that year and placed on trays outside the hatchery in the York River that August. By September 1994, only 10 percent had died while 68 percent reached market size. By the following May, mortality remained at less than 20 percent, and 98 percent of the survivors were market size.
"Disease pressure was relatively high during the period that the performance of the oysters was evaluated," Calvo said. "So it wasn't a case of these oysters doing so well because disease levels were low.
Mortality among non-hatchery oysters that were placed in the river as a "control" group was about 80 percent in each year.
The Delaware oyster strain also outperformed, in both in growth and survival, the third-generation James River strain which was produced and evaluated during the same period. The James River strain did fare better than the non-hatchery oyster control group.
Calvo said the oysters are not immune to the disease. Indeed, after the summer of 1996, about half the oysters had died, but almost all had reached market size by then.
But by breeding those that are able to withstand diseases for longer periods of time, successive generations are produced that are more resistant to the parasites that cause MSX and dermo.
"The whole idea is that with each generation of exposure, the ones that survive are probably the ones that are more robust to that disease for some reason, and have some kind of improved tolerance," Calvo said. "Every year that you expose them, you are selecting for more and more resistance."
At the hatchery, the surviving, most resistant oysters from each generation can be selected as the "brood stock" for the next generation. "This is all mimicking what happened in the field of agriculture decades ago," said Valerie Harmon, who operates the VIMS hatchery.
While to some extent, this "survival of the fittest" is what takes place in nature, the hatchery allows the process to be accelerated dramatically.
In the wild, eggs or sperm from older, more disease-resistant oysters, can mix with that of younger, less-resistant oysters. "It takes a lot longer in nature," Calvo said.
This year, the fourth generation of the Delaware oysters were reared at the hatchery and placed in the river. For comparison, the hatchery also produced young from wild oysters "rescued" from Tangier Sound and from Mobjack Bay.
But this year, in addition to deploying the oysters at the medium-salinity York River site, oysters were also placed in high- and low-salinity sites so that the effect of salinity on the performance of the strains can be evaluated.
Meanwhile, neither Calvo nor Harmon believe the hatchery holds the key to replenishing oyster stocks in the Bay. "It would be really hard to have a large restoration project with hatchery-produced seed," Calvo said.
But the hatchery's success does improve prospects for a hatchery-based oyster aquaculture program in the state, they say.
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