Archive of Spotlight Feature Articles

Battling Oyster Disease in the Chesapeake

by Merrill Leffler, Maryland Sea Grant

Skipjacks still sail Maryland waters in search of increasingly scarce oysters. (Photo by Michael Fincham)

Not so long ago Chesapeake Bay was the nation's leading producer of oysters - no more. While most of the bay's legendary reef habitats long ago fell to overharvesting and were then silted over, productivity was still immense - even 15 years ago Maryland and Virginia watermen were bringing up some two million bushels of oysters. This year, however, they may be lucky to haul in 70,000 bushels. Oyster populations in the Bay and other mid-Atlantic estuaries are at the mercy of two parasitic diseases that have devastated them: Haplosporidium nelsoni, which causes MSX disease, and Perkinsus marinus, which causes Dermo disease (the parasite was originally identified as the genus Dermocystidium and the shortened version has hung on).

Though not harmful to public health, MSX is especially virulent to oysters in salinities over 15 parts per thousand salt (ocean water is 32-35 ppt); Dermo has inflicted mortalities in waters even less than 10 ppt salt. Over the years, Virginia populations have been especially susceptible to MSX because of their proximity to the ocean; while oysters in many low salinity regions in Maryland's upper bay were seemingly insulated to MSX, during drought conditions such as we've had over the last several years, ocean waters push further up bay and oyster become more vulnerable. Even in years of average rainfall, they are susceptible to Dermo, which was inadvertently spread by infected oysters throughout the Bay system during the last twenty years by resource management agencies and aquaculturists that were trying to rehabilitate depleted oyster grounds.

There may be a significant number of native oysters that demonstrate some degree of tolerance to disease, though that number is still comparatively small; furthermore, the usefulness of those survivors for large-scale ecological restoration is unproven and currently limited. While oysters over time may evolve greater disease tolerance, such tolerance in Crassostrea virginica has been slow to evolve, at least as evidenced in Delaware Bay and Mobjack Bay in Virginia, which experienced immense mortalities in the mid and late-1950s.

Waterman Alton Brown prepares to lower his tongs over the side and into the Chester River. (Photo by Skip Brown) (right) When Alton Brown culls through this haul, he may be fortunate to retrieve a few live oysters amidst the debris of empty shells. (Photo by Skip Brown)

Recognizing the devastation that MSX, Dermo and other diseases - Juvenile Oyster Disease (JOD) in the northeast and "summer mortality" on the west coast - were inflicting on the industry, coastal communities and the health of ecosystems such as the Bay where healthy oyster populations contribute to maintaining water quality, in 1991 Congress authorized the Oyster Disease Research Program. Referred to as ODRP its major goal, says James McVey of NOAA's National Sea Grant College, which administers the program, has been to develop research-based approaches for restoring the commercial and ecological viability of oysters in U.S. coastal waters.

ODRP has marshaled university-based research in achieving notable successes that are now in use by oyster growers and resource managers. For example, new molecular probes such as those developed by Gerardo Vasta at the Center of Marine Biotechnology, part of the University of Maryland Biotechnology Institute, can rapidly detect as little as one cell of Dermo in an oyster - this capability can alert growers early on of a potential disease problem and give them time to assess remediation strategies. Quantitative shellfish management models developed by researchers Eileen Hofmann and colleagues at Old Dominion University and Rutgers University are now being employed by resource managers in Chesapeake and Delaware bays that enable them to adjust harvest seasons and quotas where disease is a key factor.

While these advances are helping growers manage around disease, they don't counter it head on. Is it possible to restore reproducing populations of native oysters that can resist both MSX and Dermo? There are many who believe that in the near-term it is not possible and are supporting the introduction of a non-native species, Crassostrea ariakensis, which in field trials has given indications that it can resist disease. However, many argue that it is not a matter of either-or and that researchers have been developing disease-tolerant strains that over the long term could help the native oyster C. virginica make a come-back.

ODRP support has played a significant role in underwriting the development of these strains. "We've had pretty decent results with breeding oysters resistant to Dermo and MSX," says geneticist Standish Allen of the Virginia Institute of Marine Science (VIMS), "though getting resistance to both is the trick." Strains such as CROSBreed and DEBYs were originally bred from survivors of disease for use in commercial aquaculture - the question is whether they can be employed for restoring large reproducing populations under the constant pressure of disease. If they can, it will require hatcheries to produce the spat, or seed oysters, perhaps in the billions. There are few hatcheries in the Bay region and certainly none with that kind of capacity.

Seed oysters in these stainless steel cages are ready to be transported onto restored reefs. (Photo by Skip Brown)

(right) Don Meritt picks up a clean oyster shell from a pile that fills a containerized steel cage. Meritt and colleagues will fill these cages with river water and add millions of newly spawned oyster larvae. The larvae will feed in the tanks until they grow large enough to cement themselves to the shell, undergo metamorphosis and become spat that are ready for planting. (Photo by Skip Brown)

Under the direction of Maryland Sea Grant Shellfish Specialist Don Meritt, the hatchery at the Horn Point Laboratory, part of University of Maryland Center for Environmental Science (UMCES), has been responsible for increasingly large amounts of seed for restoration research: from 20 million in 1997 to 75 million this past year; with an expanded hatchery nearly completed, part of a new Aquaculture and Restoration Research facility at Horn Point, Meritt hopes to hike production to 500 million spat a year

He and Allen have been working with researchers at the University of Delaware and Rutgers on field testing disease-resistant strains and comparing them with local oysters and those from elsewhere on the east coast and from the Gulf especially, where Dermo has been present since at least the 1940s. Can these specially-bred oysters survive disease pressure over the long term and be used on a much larger scale that it would likely take to jump start reproducing populations? Can they help to genetically rehabilitate wild populations? "It's possible," says Meritt. "Have we been able to do it," he asks rhetorically. There are many unknowns, he says. For example, will oysters maintain disease resistance, will they interbreed and then be susceptible to a new disease? Will their genes be diluted in mating with local oysters?

It is questions such as these that researchers working with support from the Oyster Disease Research Program will be pursuing next. Stay tuned.

For information on the web:
Maryland Sea Grant, www.mdsg.umd.edu/oysters
Virginia Institute of Marine Science, www.vims.edu
Chesapeake Bay Program, www.chesapeakebay.net/american_oyster.htm

3/10/03

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