Getting

around

Juvenile

Oyster

Disease

"JOD was like a wild fire spreading through my racks – I ended up with 93 percent mortality out of my first batch of oysters." This is Rob Parrino of Paradise Point Oyster Farms describing the impact that Juvenile Oyster Disease had on his oysters in 1988. Parrino was not alone: until these last several years oyster farmers from New York to Maine were losing nearly entire crops of oysters to disease.

We have turned that situation around now, say Jay Lewis and Austin Farley, researchers at the National Marine Fisheries Service Cooperative Oxford Laboratory in Maryland, and growers are again experiencing good survival of hatchery-reared oysters. Though the JOD organism has yet to be identified, scientists have worked with growers to apply techniques from observational and experimental research for circumventing disease.

Juvenile Oyster Disease has been confined to cultured oysters in the northeast with very little evidence of infestations in wild oysters.

The first known outbreak occurred in 1984, in oysters that were spawned in Maine. By 1990, the disease was seen in first-year oysters throughout the Northeast, from New York to Maine. Growers found that affected oysters generally ranged in size from five to 30 millimeters and had a noticeable ring of conchiolin on the inner shell surface(s). Survivors often had "shell checks," marks that indicated where JOD first occurred.

The disease usually strikes after water temperatures rise and remain above 20°C and in salinities above 18 parts per thousand. Oysters that grew larger than 30 millimeters in length without succumbing to JOD generally grew to maturity.

These observations were the first clue in one potential way to circumvent the impact of JOD. Researchers reasoned that by spawning oysters in heated hatchery water early in January through March, they could jump start growth and get juvenile oysters to a length larger than 30 millimeters before the onset of high summer temperatures. The approach worked. As Robert Rheault of Moonstone Oysters says, "we always try to get our seed oysters as early in the spring [as possible]. As soon as the water temperature hits 10°C we like to have our seed in the pond."

Shell checks indicate when surviving oysters first contracted JOD.

Meanwhile researchers came up with another successful strategy for beating JOD, one that Dave Relyea of Frank M. Flowers and Sons, Inc. says is probably the most effective, namely to produce resistant oysters from broodstock that has survived the disease and that has shown strong evidence of shell checks.

Because of the disease-resistant broodstock, the Flowers hatchery has produced record numbers of juvenile oysters without significant mortality; the company has also helped other local growers by providing them with resistant seed. Relyea says that "some of the work the scientists have done seems to prove that our native oysters that have been exposed to JOD have much more resistance than animals that have not been subjected to it." This is one reason oysters produced in local waters may survive better than those developed and brought in from other locations. As Aaron Rosenfield of the Cooperative Oxford Laboratory says, "the most fundamental way of avoiding JOD is to not bring in oyster seed from JOD-infected regions or locations. This helps prevent contamination."

Also, research by Gregg Rivara, Cornell Cooperative Extension Laboratory in Southhold, New York, has shown that increased water flows in upweller culture systems reduce JOD mortality.

While managing around JOD through early spawning and employing disease-resistant stocks has been extremely successful, scientists are still trying to identify the organism that causes the disease, and to cultivate it in the lab. If successful, their work will lead to the development of reliable molecular tools to screen for diseased animals; this capability could give growers a means for selecting broodstock to maintain resistance in subsequent generations.

Frank M. Flowers & Sons on Long Island Sound has been one of the beneficiaries of research on Juvenile Oyster Disease (JOD)