Whether wild caught or cultured, oysters face an onslaught by parasites, bacteria and loss of habitat
A resilient parasite, Dermo was first observed in Gulf Coast waters and has been moving up the Atlantic coast.
In these maps dark shading indicates regions where Dermo and MSX diseases are epidemic among a large number of organisms (epizootic) or have occurred in particular localities (enzootic); light shading indicates regions where parasites have been reported, but are not causing recognizable mortalities.
Over the past five years, research on oyster disease has made enormous progress.
For more than a century, commercial oyster harvests have had a major economic influence on many of the nation's coastal communities with annual dockside values totaling in the tens of millions of dollars. These numbers rise significantly when support industries are included -- processing plants, boat building, equipment manufacturing and other services that range from retailing to food service. All told, the oyster business has employed thousands of workers. Along the Atlantic coast, in particular, this is no longer the case.
The wild oyster fishery, which has helped define a way of life in the mid-Atlantic and Gulf Coast regions, saw the first signs of decline in 1956 when a mysterious parasite -- it was initially referred to as MSX because of its multispherical appearance -- killed more than 90 percent of the oysters in Delaware Bay. By the next year, MSX began making its way up the lower Chesapeake Bay, and over the next several years, it spread farther up the Bay into Maryland waters.
On the west coast, where the industry is based primarily on aquaculture, growers do not depend on natural sets of oysters; rather, oysters are spawned in the hatchery and then planted on private grounds. While growers in Washington state, Oregon and California have been successful in developing an industry based on a non-native species (the Pacific oyster, Crassostrea gigas), intensive production has also resulted in increased prevalence of many infectious diseases known as "summer mortality" -- these diseases now threaten the future of the industry.
Early in the 1940s, oysters in Gulf Coast waters began falling to a protozoan parasite, Perkinsus marinus (more familiarly known as Dermo because it was originally classified as a fungal species, Dermocystidium). In the mid-80s, Dermo began killing oysters in the Chesapeake Bay -- over the next decade, the parasite spread throughout the Chesapeake, sometimes inadvertently transported by replenishment programs and commercial operations that moved oysters from reproductively rich bottom grounds to areas more favorable for growout. By the early 90s, Dermo had infested virtually every major oyster bottom in the Bay. A resilient parasite, Dermo has continued to move up the Atlantic coast; it has struck Delaware Bay and been seen as far north as Maine oyster grounds.
Harvest records tell part of the story. In Maryland, for example, the annual catch fluctuated between two and three million bushels a year from the 1960s through the 1980s. Over the last eight years, the annual catch has averaged a mere 150,000 bushels, a 90 percent decline. With declining oyster harvests has come a diminished industry infrastructure: shucking houses, businesses that served harvesters and processors in the fall and winter months, economic changes and something more elusive -- a way of life in which social and work patterns that were clearly tied to the region's historical past and ecological present.
A Keystone Species
The heavy loss of oysters to disease and the dismantling of habitats through more than a century of harvesting and landborne pollution have also impacted water quality that depends, at least in part, on robust oyster populations and the bottom-dwelling communities that form around them. Only in recent years have we begun to appreciate what the loss of oysters and their reef structures have meant to coastal sounds and estuaries from Maine to the Gulf of Mexico.
Oysters are keystone species in a number of aquatic ecosystems, meaning that they play a significant role in converting organic matter, namely single-celled algae, to energy. An adult oyster, for example, can filter 40 to 50 gallons of water daily in warm months -- in so doing, they ingest algae, thus removing that living matter from the water. Though algae (also called phytoplankton) form the base of the food chain and are critical for organisms higher up in the chain, too many algae can present a problem for ecosystems that cannot assimilate them.
A major problem in many estuaries like the Chesapeake is two-fold: nutrients flooding in by land and air and a lack of assimilative organisms to handle them. The result is excessive growth of phytoplankton. Without enough filter feeders like oysters, the algae that go uneaten are left to decay. The dead algae, when metabolized by microbes, contribute to oxygen depletion, a major cause of poor water quality, especially along the bottom, where many species live.
Clearly, more oysters and improved habitat in the Chesapeake could help remove algae and contribute to improved water quality; meanwhile, improved water quality could also lead to an invigorated oyster fishery, based on sustainable harvesting.
Until recently, the prospects of any restoration were hardly conceivable in the mid-Atlantic region. Not only had Dermo and MSX decimated virtually every major oyster ground in Chesapeake and Delaware bays, but except for ad hoc actions to try and manage around disease, a full-scale plan to combat these diseases was virtually non-existent. Such a comprehensive plan is essential. Scientists have lacked a fundamental understanding of the biology of the parasitic organisms, let alone knowledge about the factors that make oysters so vulnerable. Though researchers at the Rutgers University Haskin Shellfish Laboratory had bred oysters resistant to MSX, these oysters were not resistant to Dermo. When Dermo began spreading through mid-Atlantic waters, the Rutgers oysters proved to be vulnerable, thus precluding their use in commercial hatchery operations.
A Coordinated Counterattack
While research on oyster disease through the 70s and 80s increased the scientific understanding of such issues as the oyster's immune system, that research was piecemeal, consisting of individual studies largely funded by the National Science Foundation and individual Sea Grant programs. There was no coordinated, nor consistent long-term support that would enable scientists to develop innovative research efforts - nor did those piecemeal efforts focus on the critical relationship between scientific findings and their practical application by growers and management agencies.
In 1989, however, Congress enacted legislation which recognized that if there was to be any real chance of returning oysters to coastal ecosystems, then it would take consistent support for research and, equally important, education efforts to demonstrate the application of that research. The Oyster Disease Research Program (ODRP) is the result -- its aim, the development and application of a body of knowledge to help restore oysters to coastal systems in the United States.
Over the past five years, this research has made enormous progress that has already put surviving oysters back in the water -- ODRP has significantly improved approaches to managing around disease and has furthered the scientific understanding of the dynamics of disease. In addition, new molecular probes are now coming on line that will soon give east coast oystermen a rapid means to test for threats of Dermo and MSX and west coast growers diagnostic tools to detect Pacific oyster nocardosis, the widespread disease that periodically causes significant mortalities, especially in cultured species during periods of warm temperature.
Scientists have been breeding oysters that are more tolerant of both MSX and Dermo -- several seasons of field trials are pointing to new directions that commercial growers, private and public, are already beginning to employ.
Will we see successes in oyster restoration in the next several years? The answer is yes. In addition to improved prospects for disease-resistant oysters, educational efforts by public agencies, citizens groups and Sea Grant programs focusing on the key role of oyster habitats for aquatic health have excited public participation in oyster gardening in the mid-Atlantic. Citizens in Maryland and Virginia are growing oysters along docks and on leased grounds that can then be planted and used as broodstock for new generations of oysters.
Will these and other efforts restore oysters to levels of the mid-80s in Chesapeake Bay, let alone to what they were fifty years ago? The answer is no. It has taken more than a century of steady harvesting and rising pollution to eliminate oysters -- it may take decades more to even approach large-scale restoration.
While restoration capabilities will depend on laboratory and field research, such as those supported by the Sea Grant Oyster Disease Research Program, widespread restoration will also depend on other actions, from rebuilding oyster habitats to ongoing efforts to slash contaminant and sediment runoff from the land. Even more, bringing back the eastern oyster will take continued public recognition that oysters are keystone organisms and an understanding that if there is to be any chance of sustaining the health of coastal systems, oyster restoration will be critical.
This page was last modified June 24, 2003
Restoring Oysters To U.S. Coastal Waters:
Contents • Introduction • Breeding Disease Resistance
Prospects • Modeling Around Disease • Oyster Foes
Combatting Disease• Juvenile Oyster Disease • Tools for Diagnosis
Glossary • For More Information
Learn more about Oysters
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