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Why are oysters important?

While not everyone enjoys eating this peculiar-looking bivalve, we can all appreciate the vital functions oysters serve in the Bay's ecosystem, as well as their cultural and economic importance to the region.

Oysters provide habitat and food

Oysters provide underwater habitat in the form of aquatic reefs. With their many nooks and crannies, oyster reefs can create 50 times the hard surface area of an equally sized flat mud bottom. Hundreds of Bay creatures, including sponges, sea squirts, small crabs and many species of fish, need hard surfaces like those found on aquatic reefs to survive.

In addition to providing habitat, oysters are a source of food for a host of animals.

• Oyster larvae are eaten by anemones, sea nettles and other filter feeders.
• Flatworms and mud crabs feed on new spat.
• Older spat and first-year oysters are preyed upon by blue crabs and some species of fish.
• Oysters lying exposed on intertidal flats are food for some shorebirds, such as the American oystercatcher.

Oysters filter water

Oysters are filter feeders. This means that they feed by pumping large volumes of water through their gills and filtering out plankton and other particles. As they filter water to get food, oysters also remove nutrients, suspended sediments and chemical contaminants, helping to keep the water clear and clean for bay grasses and other underwater life. One oyster can filter more than 50 gallons of water per day.

Oysters are historically and economically important

The eastern oyster is a Chesapeake Bay icon. Since the late 19th century, the oyster industry — including the catch, sale, shucking, packing and shipping of oysters — has contributed millions of dollars to the region's economy and built a rich history and cultural heritage in the Bay region.

What caused the Bay's oyster population to decline?

Scientists attribute the decline of the Bay's oyster population to a combination of several factors, including:

• Historic fishing practices.
• The oyster diseases MSX and Dermo.
• Loss of habitat due to pollution from excess nutrients and chemical contaminants.
• Sedimentation from removal of forests and runoff from development and agricultural lands.
• Natural predators, such as flatworms and cownose rays.

The decline in the Bay's native oyster population is often illustrated in terms of its impact on water quality: In the late 19th century, the native oyster population could filter a volume of water equal to that of the entire Bay every three to four days; today's depleted population takes nearly a year to filter the same volume.

How has harvest a pressure on oysters?

Immense numbers of oysters existed in the Bay during colonial times. European settlers reported that huge oyster reefs thrusting up from the Bay's bottom posed navigational hazards to their ships. At the turn of the 20th century the Bay's oyster fishery was one of the most important in the U.S.

The Bay's oyster population has severely declined over the past century due to over-harvesting, which removed huge volumes of oysters. Over-harvesting also led to the demise of the Bay's healthy oyster reefs, which were scraped away by dredging.

Oyster beds are now usually limited to a flat, thin layer of dead shells and live oysters spread widely over the Bay's bottom. These damaged habitats:

• Offer less surface area for oyster spat and other reef-dwelling invertebrates to attach themselves to. This impacts larger fish and blue crabs that live and breed around oyster reefs and prey upon these smaller species.
• Are easily covered by sediment, which smothers live oysters and can eventually bury a damaged reef.

How do diseases and poor water quality affect oysters?

In addition to harvest pressure, the Bay's oysters face a number of other challenges. One of these is disease. Since the 1950s, the oyster diseases MSX and Dermo have decimated the Bay's remaining oyster population.

The Bay's oysters have also been affected by poor water quality.

• Changes in land use over the past century—more agricultural and urban and suburban areas and fewer forested areas—have increased the amount of nutrients and sediment that enter the Bay.
• Excess nutrients fuel the growth of algae blooms that deplete oxygen in deeper waters and can hinder the development of oyster larvae.
• Oysters that are under stress from poor water quality or burial by sediment are likely more prone to disease.

How are oysters being managed and restored?

In 2005, the Chesapeake Bay Program adopted the 2004 Chesapeake Bay Oyster Management Plan, which provides Bay Program partners with a general framework and specific guidance for rebuilding and managing the Bay's native oyster population. The plan has four components: managing harvest, creating sanctuaries, overcoming the effects of disease, and using hatchery-reared seed.

However, it is important to note that the Bay's poor health is not due solely to a diminished oyster population and, therefore, cannot be corrected by restoring oysters alone. Other pressures on the Bay — such as polluted stormwater runoff from farms, cities and suburbs, and construction sites — must be addressed for oyster and water quality restoration efforts to be successful.

Oyster sanctuaries

Oyster sanctuaries are aquatic reefs where shellfish harvesting is prohibited. Scientists often improve habitat in oyster sanctuaries by cleaning off sediment or adding cultch.

• Scientists also improve habitat in these areas by cleaning sediment off the reefs and adding cultch (clean, empty shells or other hard material) for new spat to settle on.
• By restoring oyster reefs and protecting them from harvest, there is potential to increase populations of spawning adult oysters and, in turn, larval production.
• In the short term, factors like disease and water quality will significantly limit the success of oyster sanctuaries and the increase in oyster populations; however, sanctuaries will become important contributors to oyster restoration if disease resistance is allowed to evolve over time in wild populations and is supported by management practices.

Decisions about where to locate sanctuaries are guided by the Virginia Oyster Restoration Plan, developed by the Virginia Institute of Marine Science (VIMS) and the Virginia Marine Resources Commission (VMRC); and by Maryland's Priority Restoration Areas, developed by the Maryland Department of Natural Resources (MD DNR) and the Maryland Oyster Roundtable Steering Committee.

Managing oyster harvest

The second component of the oyster management strategy implements harvest strategies to build a sustainable oyster industry in both Maryland and Virginia.

The main strategy for regulating harvest and enhancing harvest potential is to establish sanctuaries and special management areas throughout the Bay.

• The ideal situation is to estimate the amount of oysters that can be taken safely from the population while maintaining a sustainable Bay-wide population of oysters.
• A major challenge is to determine what level of exploitation is appropriate and will not compromise restoration efforts.

Management strategies for the Maryland oyster fishery are considered by a number of advisory groups working with MD DNR. In Virginia, oyster harvest is managed on a bar-specific basis.

Oyster disease

The third component of the oyster management strategy recognizes the constraints of disease and implements management strategies that reduce the impact of disease.

• A major challenge to oyster restoration in the Bay is to overcome the effects of the diseases MSX and Dermo. It is estimated that, by age 3, 80 percent or more of a year class in high disease areas (i.e., the Virginia portion of the Bay) will die due to disease.
• Maryland and Virginia confront different problems concerning disease. Virginia oysters are faced with constant disease pressure because MSX and Dermo thrive in warmer, saltier waters. Maryland's situation is more variable depending on weather conditions.
• Research efforts have been underway for a number of years to breed strains of native oysters with greater disease resistance. Current research will give scientists a better understanding of how these disease-tolerant strains could contribute to large-scale oyster restoration efforts.

Recently, it has been found that oysters in areas subject to high exposure to MSX are evolving to resist the disease. Scientists and managers are adjusting harvest and sanctuary management strategies to optimize the long-term benefits of the development of MSX resistance.

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From Around the Web

• Oysters and the Chesapeake - Maryland Sea Grant
• Virginia Oyster Heritage Program - Virginia Department of Environmental Quality
• Oyster Reefs - NOAA Chesapeake Bay Office
• Oyster Restoration in Maryland - Maryland Department of Natural Resources
• Oyster Restoration Programs - Virginia Institute of Marine Science
• Oyster Recovery Partnership - Oyster Recovery Partnership