• The native Eastern oyster—Crassostrea virginica—plays an important role in the Bay's ecosystem as both a habitat for a variety of sessile plants and animals and free-swimming fish and shellfish, and as filter feeder because they feed upon phytoplankton (algae).

Status

Biomass: Not measured directly. The Chesapeake Bay Program set a goal for a ten-fold increase in biomass via restoration, but only estimates are available.

Overfishing: Probably occurring, but no official determination. More information on landings is available via NOAA Fisheries; data is also available as an Excel file.

Overfished: See graphs above.

Fishing and habitat: There is some evidence that harvest, especially power dredging, can damage the reef structure, but this is controversial.

By-catch: There are a number of associated organisms that attach to oysters, including several species of mussels, and most of these are by-catch of oyster harvest. Their biomass is not known.

Aquaculture: Extensive in Virginia, starting to expand in Maryland. Virginia waters, with higher salinity, tend to have higher spawning and growth rates, but also have higher disease rates. According to the latest data available in Virginia (2008, Virginia Sea Grant report), growers planted almost 28 million oysters and harvested almost 10 million oysters, with an estimated employment of about 150 people. Hatcheries in Virginia are used only for aquaculture, because restoration efforts can use natural spat set. Virginia hatchery production in 2008 was about 500 million larvae sold, with 2009 production expected to be two to three times higher.

Comparable data on the industry are not available for Maryland yet because the industry is just getting started there. The one large hatchery in Maryland, the University of Maryland Horn Point Hatchery in Cambridge, produces larvae mainly for restoration as spat on shell; it currently produces 4-5 billion eyed larvae and up to 300 million spat on shell annually. Several methods are used. Leased bottom may be used to plant oyster shell on the bottom (to catch settling oyster larvae in higher salinity waters) or planting spat-on-shell on the bottom (in lower salinity waters). Oysters can be grown off the bottom in cages or rack-and-bag systems, in bags on long lines, or in floats that hold bags of oysters near the surface. In cages or bags, where they are protected from predators, cultchless oysters or singles may be grown for the half-shell (raw) market, which typically yields higher prices per oyster. Cultchless oysters are set on small shell fragments and must be grown in upwellers until they are large enough to put in bags or cages.

Science and Management

The oyster fishery in the Chesapeake Bay operates as a public fishery and as a leased bottom fishery, in which people can lease an area of bottom from the state to grow oysters. In the public fishery, during the winter harvest season, oysters can be harvested from any area that has not been set aside as leased bottom, a managed reserve, or a sanctuary, or closed because of public health concerns due to bacteria levels. Maryland has managed reserves, in which areas are closed and then reopened to harvest after a certain period of time, usually several years.

Fishery regulations are established with respect to season and time limits, catch limits, and gear restrictions. These regulations are based on management strategies designed to conserve the oyster resource. The Maryland Department of Natural Resources and Virginia Marine Resources Commission have more information about oyster fishery regulations.

Landings are the number of pounds of fish or shellfish brought to dock or shore by commercial and/or private fishermen. Although it is not necessarily the most accurate measurement of population size, landings statistics give an idea of how the oyster population in the Bay is doing because they are the easiest and least expensive measurements to obtain. Other oyster surveys are done, but do not estimate the population size.

Life History and Habitat

Life history, including information on habitat, growth, feeding, and reproduction of a species, is important because it affects how a fishery is managed.

Geographic range: The Eastern oyster’s range encompasses the east coast of North America from the Gulf of St. Lawrence in Canada to Key Biscayne, Florida and south through the Caribbean to the Yucatan Peninsula of Mexico and to Venezuela. the Chesapeake Bay provides good environmental conditions for the species; however, oyster productivity varies within the Bay system, depending on salinity, water quality, habitat conditions, and disease.

Habitat: Found in areas of hard bottom. If placed on soft bottom, oyster shells tend to become buried and the animals die.

Life Span: Varies greatly depending on tidal height, salinity, disease prevalence and virulence, and predator and parasite prevalence and effectiveness. Usually up to several years.

Food: Most of the filtered particles are about 3-10 microns in diameter, about the same size as the single-celled phytoplankton (algae) that make up most of their diet.

Growth Rate: Spat grow at the rate of about an inch per year, and sometimes faster. Growth rates can be affected by temperature, food quantity, salinity, and disease. Shell growth usually occurs in the spring and soft body tissue growth occurs after spawning. Oysters usually reach market size (3 inches) three to five years after spat settlement, but in warmer and saltier waters, they can reach that size in as little as a year. Sterile (triploid) oysters usually grow faster because they are not expending energy on reproduction.

Maximum Size: Approximately 8" long.

Reproduction: Eggs and larvae are released into the water, usually by the end of June as water temperature rises to about 18-20° C (64-68° F). Spawning is also salinity dependent, and is less common when the salinity is below 10 psu. Like some other animals (including some fish) in which large body size is an advantage for females (so they can produce more eggs), they start life as males (after a brief bisexual period), called protandry, and most become females by their second winter. The fertilized eggs develop into free-swimming larvae which eventually develop a small foot used to attach to a hard substrate, often another oyster shell. Young attached oysters are called spat and the process of attachment is called setting.

Migrations: The larvae are planktonic or free swimming. They have limited local movements depending on tidal currents.

Predators: The main predators on larvae are other filter-feeders, especially comb jellies. Predators on adults include cownose rays and oyster drills. Various parasites can lead to death of the oyster, such as boring sponges (Cliona sp.)

Commercial and Recreational Interest: Both, but mainly commercial due to the specialized gear that is usually required.

Distinguishing Characteristics: Flattened shell with many ridges. Shape varies greatly depending on where the oyster grows. The left or bottom valve tends to be deep and cup shaped, while the right or top valve tends to be flatter.

Diseases: MSX (caused by Haplosporidium nelsoni) was first found in the Chesapeake Bay in 1959, two years after it was first found in Delaware Bay. It is more common in higher-salinity waters. MSX probably arrived with the Japanese oyster, Crassostrea gigas, which was intentionally introduced into Delaware Bay to test its growth there (it did not grow well). Dermo, caused by Perkinsus marinus, was first recorded in the Chesapeake Bay in 1949, and is more prevalent than MSX in lower-salinity waters of the Bay.

Role in the Ecosystem

Oysters are filter feeders, consuming phytoplankton (free-swimming algae) and improving water quality while filtering the water for food. As generations of oysters settle on top of each other and grow they form reefs that provide structured habitat for many fish species and crabs.

Human activities both positive and negative impacts on oyster populations and thus on their ecosystem roles, shown in the conceptual diagram below made by Eco-Check (click on it for a larger image). Ecological impacts of oysters (their benefits and stressors) are summarized in a similar conceptual diagram at the bottom of the Oyster reefs page.

• It is frequently reported that "Chesapeake" meant "Great shellfish bay" in the Algonquin language, but it probably meant "big water" or "village on the big water," according to Algonquian linguist Blair Rudes.