Thursday, March 16, 2017

The Chesapeake Bay - Part 4 - Oysters

The eastern oyster is one of the most iconic species in the Chesapeake Bay.  For more than a century, oysters have been one of the area's most valuable commercial fisheries, and the filter-feeder continues to clean the waters and provide food and habitat to other animals.  However, over-harvesting, disease and habitat loss have led to s severe drop in oyster populations.  Scientists are working to manage harvests,establish sanctuaries, overcome the effects of disease, and restore reefs with hatchery-raised seed in an effort to restore this important bivalve.
    You don't have to like eating this peculiar-looking shellfish to appreciate its vital role in the Chesapeake Bay ecosystem - and its importance to people who live around the Bay.  Oysters are natural filter feeders.  This means they feed by pumping water through their gills, trapping particles of food, as well as nutrients,suspended sediments and chemicals.  In doing so, oysters help keep the water clean and clear for bay grasses and aquatic life.  One oyster can filter more than 50 gallons of water in a single day.
    As oysters grow, new larvae  (or seed) settle on top of adults, forming layers of oysters that spread upward and outward.  With their countless nooks and crannies, these reefs provide habitat to hundreds of diverse water creatures - from small fish and invertebrates seeking shelter to larger fish looking for food.  Oysters do have a number of natural predators:  (1) anemones, sea nettles, and other filter feeders eat oyster larvae, (2) flatworms and mud crabs feed on new spat, (3) blue crabs and some fish species eat older spat and first-year oysters, and (4) shorebirds feed on adult oysters that are exposed upon intertidal flats.
    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.  Oysters have also added to the region's historical and cultural heritage, fueling countless bull and oyster roasts.  Oysters have also inspired the unique design of boats in the area: the log canoe, the pungy, the bugeye, the skipjack, the deadrise and the scow.
    The decline of the Chesapeake Bay's native oyster population can be attributed to several factors, including historic over-harvesting, disease and habitat loss.  The severity of this decline is often illustrated in terms of its impact on water quality: in the late 19th century, the Bay's oysters could filter all of the water in the Bay in three or four days; today's oysters would take nearly a full year to filter the same amount of water.
    In the 17th century, huge numbers of oysters lived in the Chesapeake Bay.  European settlers reported enormous oyster reefs that thrust up from the Bay's bottom, posing hazardous navigation for their wooden ships.  Colonists first used hand tongs to harvest oysters, but by the 1800s, oyster dredges were also in use.  In the 1850s, more than 1.5 million bushels of oysters were harvested from the Bay each year.  In the 1880's, the number jumped to 20 million bushels each year.  At the turn of the 20th century, the Chesapeake Bay oyster fishery was one of the most important assets of the United States.
    Over-harvesting, however, removed huge volumes of oysters from the Bay and led to the end of the Bay's healthy oyster reefs.  Dredging for oysters has scraped the reefs away, and oyster beds are now limited to flat, thin layers of dead shell and live oysters spread over a wide surface of the Bay's bottom.  These damaged habitats offer less surface area for reef-dwelling creatures to inhabit, and can easily be buried by a heavy wash of sediment, or dumping.
    In 1949, scientists discovered Dermo in the Chespeake.  MSX was discovered in the waters ten years later.  Dermo, or Perkinsus marinus, is a parasite that usually infects oysters at the age of two years; it causes slow growth rates and death.  MSX, or Haplosporidium nelsoni,also leads to oster death, but can infect oysters of all ages.  Both diseases are contracted between May and October, and their prevalence can be affected by water temperature and salinity.
    Overcoming the effects of Dermo and MSX has posed a challenge to oyster restoration in the Chesapeake Bay.  It is estimated that by age three, 80 percent or more of a single oyster year class, in a high disease area (like the Virginia portion of the Bay) will die from disease.  Over the past century, the watershed has experienced a change in land use, as urban, suburban, and agricultural areas have replaced forested lands.  This has increased the amount of nutrients and sediments entering our rivers and streams, and has contributed to the poor water quality that affectslife in the Chespeake Bay.   Excess nutrients, for instance, fuel the growth of algae blooms that create low-oxygen dead zones that restrain the growth of oyster larvae; while sediments can suffocate oysters and other shellfish.  Also, poor water quality can cause stress that will make oysters more susceptible to disease.

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