Issues: Oysters

When Europeans first arrived in the Chesapeake, oysters were so prolific that they created navigational hazards and they provided a good food source for both Indians and settlers. In the 19th century, canning technology and mechanized harvesting methods led to intensive harvesting, using hand tongs in shallow water and dredges towed behind skipjacks on deeper oyster bars. Exploring some of the Severn's tidal ponds in a kayak, one can see places where eroding banks contain large numbers of deeply embedded oyster shells. It is unclear if these middens were created by Indians or later settlers, but they indicate that the 14 oyster shucking houses in 19th century Annapolis were not the first centers of local oyster consumption.

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After decades of overharvesting, Chesapeake oyster harvests began a decline in the late 19th century. At the start of the 20th century, the state of Maryland undertook an official mapping project of all commercial oyster bars, and the Severn is well represented. Although the largest oyster bars were in the adjacent Chesapeake, others extended well up the Severn into Round Bay. In the 1920s, poor local sanitation resulted in a permanent ban on oyster harvests in the Severn, with the ironic result that recently restored oyster reefs are not under the harvest pressure existing in some of our neighboring tributaries.

In spite of the harvest ban, the Severn's original oyster beds have largely disappeared, apparently overwhelmed by siltation. However, the Severn has been the site of major efforts to restore oyster reefs.

While the Severn’s salinity is fresher than optimal for oyster reproduction and growth, the major oyster diseases that have devastated Chesapeake oyster harvests since the 1970s are even more salinity-limited than oysters. For that reason, both the Oyster Recovery Partnership (ORP) and the Chesapeake Bay Foundation (CBF) have considered the Severn and nearby Chesapeake as a priority area for oyster restoration.

The first step in building a restored oyster bed is to establish a firm base on the bottom so the new oysters will not be buried in silt. This has been done with huge loads of old oyster shells until recently, when they became unobtainable. Various other hard substrates are being used, including concrete from broken highway roadbeds.

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One such example in 2008 was organized by the Chesapeake Bay Foundation near the mouth of the Severn’s Asquith Creek, as shown in the figure above. A high-pressure hose is being used to blast chunks of concrete removed from the Bay Bridge off a barge into ~12 feet of water to form the base of the new oyster reef. Several weeks later, three million baby oysters (spat on shell, about the size of a quarter) were laid down on top of this base to form the restored reef.

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As shown at left, a more gentle system was used by the CBF vessel Patricia Campbell to deposit the baby oysters on top of the base. Other CBF restored reefs use 1 year-old oysters that have been grown under local piers by volunteers starting from spat. These larger oysters have higher survival rates after planting on the bottom because their thicker shells make them more resistant to predators such as crabs.

Oyster reefs have been restored by ORP and CBF in the Severn since 1998, and monitoring since then has shown these oysters have good growth and low mortality due to disease. The oysters on these reefs benefit the local water quality because their constant filter feeding removes phytoplankton from the water. The hard bottom and shells provide substrate for a variety of encrusting organisms, and these in turn provide food for fish such as white perch.

The total number of oysters deposited on restored beds in the Severn and adjacent Chesapeake is an impressive 68 million, including about 40 million in 2008. The Severn was chosen for this large effort because of the favorable local conditions. The map below shows the distribution of these restored reefs.

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