Crassostrea virginica, also called the eastern, American or Atlantic oyster (but known in the Chesapeake Bay region as the native oyster), once yielded seasonal harvests in the Chesapeake Bay that reached millions of bushels. As recently as 100 years ago oyster reefs were so massive that they posed a navigational hazard to ships. And as one traveler commented in 1701,

"The abundance of oysters is incredible. There are whole banks of them so that the ships must avoid them…. They surpass those in England by far in size…they are four times as large. I often cut them in two, before I could put them in my mouth." *

Today, the oyster population is reduced to about one percent of its historic level.

Background

While not everyone values the peculiar-looking bivalve as essential eating, many consider it a delicacy. But it is the oyster's eating habits that make it so important to its environment: it is a voracious filter feeder that performs several vital functions in the Bay's ecosystem:

• Filtering Water - Healthy oysters consume algae and other water-borne nutrients, each one filtering up to five liters of water per hour. Scientists believe that the Bay's once-flourishing oyster populations historically filtered the estuary's entire water volume of excess nutrients every three or four days.
• Providing Habitat - An oyster reef, with its many convolutions, can encompass 50 times the surface area of an equally extensive flat bottom. Its crevasses provide habitat for an enormous range of other animals-worms, snails, sea squirts, sponges, small crabs and fishes.
• Feeding Bay Creatures - Oysters that lie exposed on intertidal flats provide food for some shorebirds, including the American oyster catcher, Haematopus palliatus, a distinctive wading bird that inserts its powerful, flattened bill between the mollusk's shells, severs its adductor muscle and removes the meat.

Oyster Habitat and Life Cycle

• The native oyster usually inhabits water depths of between 8 and 25 feet.
• An oyster's mature shape often depends on the type of bottom to which it originally attached. It orients itself with its outer, flared shell tilted upward. One valve is cupped and the other is flat. The submerged shell opens periodically to permit the oyster to feed.
• Oysters usually mature by one year of age. They are protandric, which means that during their first year they spawn as males (releasing sperm into the water). As they grow larger over the next two or three years and develop greater energy reserves, they release eggs, as females.
• Bay oysters are usually prepared to spawn by the end of June.
• An increase in water temperature prompts a few initial oysters to spawn. This triggers a spawning 'chain reaction', which clouds the water with millions of eggs and sperm. A single female oyster can produce up to 100 million eggs annually.
• The eggs become fertilized in the water and develop into larvae, which eventually find suitable sites on which to settle, such as another oyster's shell. Attached oyster larvae are called 'spat'.

What's Threatening Chesapeake Bay Oysters?

Oyster harvests are now tallied in terms of thousands, rather than millions of bushels. The sharp harvest decline can be traced to several specific factors:

• Years of over-harvesting, during which high volumes of large oyster shells were scraped from the bottom, destroying reef habitat and potential sites for oyster spat settlement
• Two parasites, harmless to humans but deadly to oysters in their first two years of life:
  • MSX (Haplosporidium nelsoni), which thrives in higher salinity brought on by dry years, and
  • Dermo (Perkinsus marinus), which tolerates low salinity and is therefore the more damaging to the oyster population;
• Loss of habitat due to environmental pollution, , such as:
• Algal blooms deplete the water of oxygen, hindering the development of oyster larvae
• Toxic chemicals and metals threaten the development of juvenile oysters
• Siltation from eroded soil at land development sites, agricultural practices and forestry harvesting smothers oysters.

In addition to these factors, natural predators such as sea anemones, sea stars, sea nettles and other filter feeders consume oyster larvae.

Dramatic declines in the Bay's native oyster population,their important ecological function and the seafood industry's desire to restore a viable local fishery, have generated studies the viability of introducing a non-native oyster species into the Bay's waters.

Restoring the Native Oyster

Both Maryland and Virginia have instituted regulations to help protect the oyster from further over-harvesting. The Oyster Management Plan was revised in 2004. The new plan includes a description of problem areas and strategies for restoring the native oyster. The Chesapeake Bay Program partners are continuing to examine:

• Disease
• Research programs
• Repletion programs
• Habitat and water quality conditions
• Increases in oyster production
• Reduction and control of fishing mortality
• Collection of data

The CBP partners' new Oyster Management Plan was completed and signed in January 2005.

* J. Wharton, The Bounty of the Chesapeake: Fishing in Colonial Virginia. University Press of Virginia, Charlottesville, VA.

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Last modified: 10/4/05