Chesapeake Bay Oysters

Credit: Chesapeake Bay Program

Crassostrea virginica, also known as the eastern, American or Atlantic oyster, was once plentiful enough in the Chesapeake Bay that seasonal harvests were in the millions of bushels. As recently as 100 years ago, oyster reefs were so massive that they posed a navigational hazard to ships. The Swiss writer, Michel, wrote in 1701,

"The abundance of oysters is incredible. There are whole banks of them so that the ships must avoid them. A sloop, which was to land us at Kingscreek, struck an oyster bed, where we had to wait about two hours for the tide. They surpass those in England by far in size, indeed they are four times as large. I often cut them in two, before I could put them in my mouth."*

People have long valued this bivalve an important food source, and many consider it a delicacy. These voracious filter feeders perform additional functions that are vital to the Bay’s ecosystem:

• They consume algae and other water-borne nutrients by filtering water at a rate of up to five liters per hour.  Scientists believe that the Bay’s once-flourishing oyster populations historically served as a natural water-cleansing system, filtering the estuary’s entire water volume of excess nutrients every three or four days.  Today’s oyster population–reduced to about one percent of its historic level–requires about a year to filter the Bay’s water volume. Increasing the number of oysters in the Bay would improve the water quality of the entire estuary.
  
• Oyster reefs, with their many folds and crevasses, can have fifty times the surface area of an equally extensive flat bottom.  Its convolutions provide habitat for an enormous range of other animals, such as worms, snails, sea squirts, sponges, small crabs, and fishes.
  
• Oysters that lie exposed on intertidal flats provide food for the American oystercatcher, Haematopus palliatus, a distinctive wading waterbird that inserts its powerful, flattened beak between the mollusk's shells, severs its adductor muscle, and removes the meat.

Habitat and Life Cycle

• The eastern oyster usually lives in water depths of between 8 and 25 feet.
  
• An oyster’s shape and size often depends on the type of bottom to which it originally attached. An oyster orients itself on the bottom with the outer, flared edge of its shell tilted upward. The left valve is cupped, while the right valve is flat. The shell opens periodically to permit the oyster to feed on plankton.
  
• Oysters mature at an early age (usually by age one). They are protandric, which means that in the first year they spawn as males, but as they grow larger and develop more energy reserves in the next two to three years, they spawn as females. Bay oysters are ready to spawn by the end of June.
  
• An increase in water temperatures triggers the male oyster to release sperm and the female to release eggs into the water. This triggers a chain reaction of spawning which clouds the water with millions of eggs and sperm. A single female oyster produces 10 to 100 million eggs annually.
  
• The eggs are fertilized in the water and soon develop into larvae, which are drawn to the chemicals released by older oysters on the bottom and eventually find a suitable site to settle, such as another oyster's shell. Juvenile attached oysters are called "spat."

What’s Threatening Our Oysters?

Oyster harvests are now tallied in terms of thousands, rather than millions of bushels. This is due to several factors, including:

• Historic over-harvest, which removed huge volumes of large oyster shells, destroying reef habitats and suitable sites for oyster spat settlement;
  
• Two parasites that are harmless to humans but lethal to oysters within the 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 therefore is the more damaging to the oyster population; and
  
• Loss of habitat due to environmental pollution:
  
  
  
  • Algal blooms promoted by eutrophication, the process through which the Bay’s waters become over-enriched with nutrients such as nitrogen and phosphorus (which are derived from lawn and farm fertilizers, septic system drainage, farm animal waste, and other runoff), leading to a severe depletion of oxygen in areas. This may hinder the development of oyster larvae.
    
  • Pollutants such as metals are directly toxic to vulnerable juvenile life-stages.
    
  • Siltation from eroded soil at land development sites, farm fields and forestry harvesting, which smothers oysters and prevents them from feeding.
  
  
• Natural predators, such as sea anemones, sea stars, sea nettles and other filter feeders, which eat oyster larvae; and flatworms and small crabs which consume new spat.

All of these factors have contributed to the decimation of the native oyster population throughout most of the Bay.

What’s Being Done to Help the Oyster?

Both Maryland and Virginia have instituted regulations to help protect the oyster from over-harvest. The Chesapeake Bay Program and other institutions support oyster reef restoration, but oyster research, especially on oyster diseases, must continue. Although land development and soil erosion have increased the deposits of silt and chemicals in the Bay, resource managers and scientists are working to solve problems associated with the unique habitat of the oyster.

Due to dramatic declines in the number of oysters living in the Chesapeake Bay and the seafood industry's eagerness to restore a viable local fishery, the possibility of introducing non-native species into the Bay's waters is being explored.

Credit: Chesapeake Bay Program

Posted: 10-29-2002