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Oysters were important to the diet of local Indians in precolonial times, as can be seen by the large numbers of oyster shells in middens on the banks of the Severn. Oysters grow on "bars" which are reefs made up of oyster shells rising up off the naturally silty bottoms. Because the optimal salinity for oyster growth and reproduction is approximately 15 parts per thousand, the Severn with its lower salinity has not been a traditional prime Chesapeake oyster location. However, historic oyster bars charted in surveys made early in the 20th century, show extensive bars in Annapolis outer harbor and up the Severn into Round Bay.
Due to the danger of infectious disease, public health authorities have closed the Severn proper to oyster harvests for several decades. Since some of the many homes lining the Severn utilize septic systems in questionable repair, and leakage of raw sewage into the estuary can cause the filter feeding oysters to concentrate bacterial and viral pathogens, prudence dictated a closure of the Severn bars. Nevertheless, the nearby Chesapeake continues to be a site of oystering activity. At one time, the Eastport section of Annapolis was home to 18 oyster shucking houses, and watermen could be seen unloading their oyster catches there until the 1990s.
Oyster harvests in the Chesapeake flourished after the Civil war, peaked in the 1880s, and suffered a massive decline due to overharvesting by the turn of the century. The 20th Century has seen a further decline due to overharvesting and diseases, with an all-time low harvest in 1993-1994. The decline of the oyster fishery has been a serious problem for the Chesapeake's watermen, who have depended on oystering for income during the winter.
Biologists now recognize that a healthy oyster population can significantly benefit water quality. Oysters filter the water to capture relatively small planktonic organisms, including some of the same phytoplankton whose excessive growth drives eutrophication, and they can clarify water to the benefit of submerged aquatic vegetation. In colonial times the Bay's oysters filtered the entire volume of Bay water in 3-4 days, while the current oyster population requires close to a year. In addition, healthy oyster beds provide a hard surface for the growth of many organisms, leading to a flourishing bottom community including crabs and bottom-feeding fish such as croakers. Thus, oysters have been considered a "keystone" species critical to a healthy Chesapeake ecosystem.
Efforts to restore oyster populations for better harvests have been hampered by the rise of two oyster diseases in the last 50 years, with the disease known as Dermo (caused by an oyster-infecting dinoflagellate) particularly destructive in the Maryland portion of the Bay. Neither of these organisms causes human disease, but they are a major problem for oysters on the East Coast. Ironically, Dermo has made the Severn region a prime Chesapeake site for oyster restoration efforts, since the Dermo organism is even less tolerant of fresh water than the oyster. However, the vagaries of rainfall feeding the estuary alters its salinity from year to year and no one location has seen really good oyster growth.
The Chesapeake's watermen have joined forces with oyster biologists in new efforts to promote oyster reef restoration, and the year 2000 saw a substantial  budget increase for such efforts. The idea behind the creation of sanctuary reefs where harvesting is not allowed is based on several aspects of oyster biology. Older and larger oysters in a natural oyster population dominate reproduction because they produce many more eggs and sperm than younger oysters; Dermo does not kill oysters until they are ~3 years old, which is about when they typically reach the legal size for harvesting (3"); the current intense harvest pressure prevents reproductive dominance of older oysters, since most oysters are harvested when they are 3" (check your seafood market); the older oysters in a natural reef are naturally selected for Dermo resistance since the susceptible individuals are killed off by the disease. Sanctuary reefs should re-establish the natural selective mechanism for Dermo resistance, and indeed for resistance to future similar oyster pathogens. Since the larval stage of oysters is pelagic (freely drifting in currents), Dermo-resistant oysters will repopulate natural bars around the sanctuary reef, including those open to harvesting. It will take some years to test the validity of this oyster restoration approach, but it is based on sound science and seems to be working at the pioneering sites in Virginia. The mouth of the Severn River has seen several restoration reefs established by various government agencies as well as one by the Chesapeake Bay Foundation. These reefs occupy a total of 35 acres, on which about 15 million spat have been placed. These dime sized young oysters are produced in large quantities by state-sponsored hatcheries to help repopulate depleted oyster beds.
To supply restored oyster reefs with young adult oysters, the Chesapeake Bay Foundation and other organizations have organized programs of oyster gardening by waterfront residents. By enclosing oyster spat in mesh containers and periodically dessicating them to prevent predation by crabs and flatworms, spat survival is far higher than in the wild. Restored reefs at the mouth of the Severn and elsewhere are currently being annually supplied with hundreds of thousands of young oysters from such projects, and with time a natural population of Dermo-resistant oysters should form the basis for a rich local habitat.