Oysters
  • 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).
  • Harvests of native oysters are now 1% or less of historical levels due to harvesting, disease, changes in water quality, and other factors. Overall oyster populations and the ecosystem services they provide are similarly diminished.
  • NOAA, including the NOAA Chesapeake Bay Office and NOAA Restoration Center, are actively involved in restoring native oysters in the Chesapeake Bay. Efforts to restore native oysters are accelerating, especially in Harris Creek, where 22 acres of reefs were built in 2012, and 34 acres are planned for 2013.

Status

Biomass: Biomass—the total weight of a species in an area, such as a river or bay—is measured by fishery-independent (not collected using commercial or recreational fishing statistics) surveys, which were rare until recently in the Chesapeake Bay. However, historical biomass data are available for selected rivers and bays.

A 2012 paper compared results of fishery-independent surveys of Chesapeake oysters from about 100 years ago to results of more recent surveys. The paper lists the estimated percent of historic extent (area) and biomass remaining for several subestuaries within the Chesapeake and some nearby estuaries. The remaining area values in the Chesapeake ranged from 6% for the Piankatank River/Mobjack Bay to 54% for the James, while the biomass percent values ranged from 3% for the Piankatank River/Mobjack Bay to 37% for the James and 38% for Tangier/Pocomoke Sounds. Most of these are much higher than estimates of remaining oysters based on landings (see Figure 1). They are all higher than the estimates in the same paper for the nearby Chincoteague and Sinepuxent Bays (along the Atlantic Ocean in Maryland and Virginia), but most are lower than the estimates for Delaware Bay. But even 100 years ago, many Chesapeake oyster reefs had been harvested by European settlers for about 300 years and by native harvesters for long before that. Thus, the historical data were not an undisturbed baseline, and a high percent remaining does not necessarily mean that oysters exist now in commercially harvestable amounts.

In addition, data for other tributaries not covered in the paper may be very different.

Oyster biomass data have been reported in Maryland since 1994, most recently in the 2011 fall survey report. The biomass estimated for each year is compared to the 1994 estimate, which is based on samples from 43 bars. That estimate was highest in 2000, at almost 80% higher than the 1994 value. While it was below the 1994 value from 2002-11, it exceeded the 1994 value in 2012. This change was attributed to a strong spat set in 2010 combined with high recent survival.

The latest fishery-independent oyster survey report for Virginia has 2009 data for oyster size composition, mortality rates, and spat set rates for all of the major rivers on the Western shore of the Chesapeake Bay. It does not include biomass estimates.

There are metrics for Chesapeake Bay oyster restoration that specify the amount of oyster biomass in a given area for a reef to count as “restored.”

Overfishing: Overfishing of oysters may be occurring, but no official determination has been made A harvest moratorium has been suggested in some peer-reviewed academic papers (including papers published in 2007 and 2011), but not by any of the fishery management agencies. More information on landings is available via NOAA Fisheries.

The "Chesapeake Bay Oyster Landings by State, 1880-2011" graph shows how Chesapeake landings declined over time to their current level of less than 1% of historical harvests. Two of the dips were probably caused by the spread of two oyster diseases, Dermo (which was first seen in the Chesapeake round 1949) and MSX (which appeared around 1959). 

The "Chesapeake Bay Oyster Landings by State, 1990-2011" shows some of the same data, focused on recent decades so more detail can be seen. The state breakdown shows taht Maryland harvests exceeded those in Virginia for most of this period, but that Virginia started catching up in 2007. Only Maryland data were reported in 2005, and only Virginia data were reported in 2011. Because NOAA Fisheries landinds data are a mix of wild harvest and aquaculture, it can't be determined if the wild harvest changed in Virginia after 2007, but the oyster aquaculture industry there has been growing.

Overfished: See "Overfishing" above.

Value of fishery: The dockside value of oysters landed in 2009-10 (the latest year with data from both states) was $4.4 million in Maryland and $5.1 million in Virginia. Oysters and striped bass have traded places a few times over the last few decades for third- and fourth-most valuable Chesapeake Bay fisheries, behind blue crabs and Atlantic menhaden.

Fishing and habitat: There is evidence that some harvest techniques, especially power dredging, can damage oyster reef structure. In one study, fished reefs were not as tall or complex as unfished control reefs. The lower height of the reef was associated with higher oyster mortality, because the oysters were in sometimes anoxic (essentially no oxygen) bottom waters. It has not been established whether less height and complexity leads to less fish habitat, however. In addition to habitat, oysters provide ecosystem services such as water filtration and nitrogen removal. Harvesting oysters reduces the amount of these ecosystem services that oysters can provide, as well as removing brood stock for future generations of oysters.

By-catch: The by-catch of oysters is rare, because oyster reefs damage the nets used in other fisheries. Thus, fishers using nets usually avoid oyster reefs. Conversely, the by-catch of other fished species during oyster harvest is also rare, because oyster dredging and tonging only target oyster reefs.

Aquaculture: Aquaculture of C. virginica takes place in Canada, the United States, and Mexico. Oyster aquaculture is extensive in Virginia, and starting to expand in Maryland. In general, saltier waters (like those in Virginia) support higher spawning and growth rates for oysters—but also have higher disease rates, especially for MSX. According to 2012 data from Virginia, growers planted 67 million seed oysters and harvested 28 million single oysters, which were sold for an estimated total value of $9.5 million, up from $6.7 million in 2011. Planting of spat-on-shell oysters in Virginia has also expanded recently, from more than 6,000 bushels planted and 2,000 harvested in 2008 to almost 19,000 planted and 13,000 harvested in 2012, but estimates of their sales value are not available. The industry employed an estimated 176 people; more than half are part time. 

Using a process to ensure that each oyster has three sets of chromosomes (resulting in a “triploid” oyster) instead of the usual two (“diploid”), hatcheries can produce oysters that are sterile. In 2012, oyster hatcheries in Virginia produced about 2.1 billion larvae and seed; 87% of those were triploid, down from 95% in 2011. Triploids are favored over the wild type because they don’t spawn and therefore grow faster and are not affected by reproduction (which leads to losing meat weight and quality) compared to diploids. Hatcheries in Virginia are used largely for aquaculture, usually to produce selected and/or triploid larvae, whereas Virginia restoration efforts usually use natural spat set.

Comparable data on the aquaculture industry are not available for Maryland because the industry is just getting started there. Results of the first reports from growers under a new leasing program are expected in 2013. 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 about 3 billion eyed larvae and about 600 million spat-on-shell annually.

Several methods are used for oyster aquaculture in the Chesapeake Bay.

  • In leased areas, growers may plant oyster shell on the bottom on which to catch settling oyster larvae (in higher-salinity waters) or plant spat-on-shell on the bottom (in lower-salinity waters). The shells on which the larvae set to produce spat-on-shell are a form of cultch (substrate for oyster larvae), while other cultivation methods use so-called “cultchless” oysters that are set on small shell fragments (called “microcultch”) and must be grown in upwellers, which provide a constant flow of water, until they are large enough to put in bags or cages and are called “seed.”
  • 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” are usually grown for the half-shell (raw) market, which typically yields higher prices per oyster. The oysters in bags, cages, or floats are thinned out and redistributed into more containers as they grow to avoid crowding and competition for food.

Science and Management

The oyster fishery in the Chesapeake Bay is regulated by the State of Maryland, Commonwealth of Virginia, and the bi-state Potomac River Fisheries Commission (PRFC). 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 most recent Bay-wide document addressing oyster management is the 2004 Chesapeake Bay Oyster Management Plan, which was intended “to provide both a general framework and specific guidance for implementing a strategic, coordinated, multipartner management effort.”

In the Chesapeake Bay, the oyster fishery 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 Maryland, 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. They may be reopened when the oysters reach a certain size, or after a certain period of time. There were about 2,600 acres in managed reserves in Maryland in 2009. Reserves are usually opened to harvest when 60% of the oysters in them are 4 inches long or more.

In Virginia, only areas legally designated as public grounds can be opened for harvest by the Virginia Marine Resources Commission.

Maryland and Virginia also have oyster sanctuaries, which are closed to harvest indefinitely. In Maryland, sanctuaries cover about 9,000 acres, or about 24% of the mapped oyster bar habitat. There are several oyster sanctuaries in the Potomac River, which is mostly managed by Maryland.

In Virginia, there is one oyster sanctuary (Wreck Shoals) in the James River covering about 90 acres; the other five sanctuaries established by regulation are in the Virginia coastal (seaside) bays. Large areas of bottom are leased for oyster and clam aquaculture in Virginia, where leases (private grounds) cover about 100,000 acres (leased areas are not necessarily used for production). The number of active oyster aquaculture leases in Virginia has not been reported since 2005, when there were 282 leases used for oyster aquaculture, but their area was not reported. There were over 6,500 acres of leases in hard clam culture that year.

Leasing for aquaculture is now allowed in sanctuaries in Maryland, as long as it is outside the mapped “natural oyster bars” (based largely on maps drawn by C. C. Yates, who surveyed oyster reefs in 1906-12) and the area leased does not exceed 10% of the sanctuary area. There are currently over 3,600 acres in over 310 shellfish bottom leases in Maryland, with additional “water column” leases for oysters grown in floats. There is no leased bottom in the Potomac, because PRFC has never been authorized by Maryland and Virginia to allow leasing.


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 native range encompasses the east coast of North America and Central America from the Gulf of St. Lawrence in Canada south through the Gulf of Mexico and Caribbean.

A different oyster species, the Caribbean or mangrove oyster (Crassostrea rhizophorae), is also found around the Caribbean islands; it resembles the Eastern oyster and uses similar intertidal habitats, like mangrove roots.

The Chesapeake Bay provides good environmental conditions for the species; however, oyster productivity varies greatly within the Bay system, depending on factors such as salinity, water quality, habitat conditions, food supply, disease prevalence and virulence, and suitable attachment substrate (or “cultch”).

Habitat: Eastern oysters are usually found on hard bottom areas. If placed on soft bottom, oyster shells tend to become buried, and the animals die. Natural oyster bars are often located on the edges of channels, probably because they have good water flow, which may bring them more food and provide more larvae from up-current areas. Specific habitat tolerances, as described in the 2007 status review, are:

  • Depth: 2-26 feet (0.6-8.0 meters) in the Chesapeake Bay, except oysters near the mouth of the Bay (and farther south, and rarely north) may be intertidal. Winter mortality limits intertidal oysters (exposed out of the water at low tide) to the southern, warmer parts of their range, generally south of Maryland, except oysters may be intertidal in locations farther north that are not subject to ice scour.
  • Salinity: varies by life stage. Larvae need 10-27.5 practical salinity units (psu), adults can tolerate 5-40 psu, but optimum range is 14-28 psu. Adults have little growth below 5-10 psu.
  • Temperature: optimum for larvae is 68-90.5o F (20-32.5oC), for adults 68-86oF (20-30oC); adults can tolerate 35.6-96.8oF (2-36oC) and up to 120.2oF (49oC) for short periods. Larvae can grow in water as cold as 63.5oF (17.5oC).
  • pH: larvae are the most sensitive; tolerable range is 6.75-8.75.
  • Dissolved oxygen: >20% saturation, which corresponds to 2.3 milligrams/liter at 50oF (10oC) and 1.5 mg/l at 86oF (30oC). Hypoxia for most marine life is considered <30% dissolved oxygen saturation, so oysters are more tolerant of low dissolved oxygen than are many Bay animals. Subtidal oysters can close their shells and use anaerobic respiration for several hours if the water has low dissolved oxygen, just as intertidal oysters close their shells at low tide.

Life Span: Varies greatly depending on tidal height, salinity, disease prevalence and virulence, and predator and parasite prevalence and effectiveness. Oysters usually survive up to several years, but they have lived up to 20 years in captivity.

Food: Most of the particles filtered by oysters are about 1-10 microns in diameter, about the same size as the single-celled phytoplankton (algae) that make up most of their diet. Filtration rate estimates range from 1.5 to 10.0 liters of water filtered per hour per gram (L h-1g-1) dry tissue weight, which is higher than the filtration rate of most other bivalves. Some of the particles filtered (including sediment and food) are ejected as “pseudofeces” without passing through the gut, because oysters need to ventilate their gills for more time than they need to feed.

Growth Rate: Spat grow at the rate of about an inch (25 millimeters) per year, and sometimes up to 10 millimeters per month in their first six months. Growth rates can be affected by temperature, food quantity and quality, salinity, boring organisms, and disease. Shell growth usually occurs in the spring, and soft body tissue growth occurs after spawning (usually in the summer). Oysters usually reach market size (3 inches or 76 millimeters) three to five years after spat settlement, but in warmer and saltier waters such as those in the Gulf of Mexico, they can reach that size in as little as one to two years. Sterile (triploid) oysters usually grow faster because they are not expending energy on reproduction, enabling them to reach market size sooner (one study indicates five months sooner) and to weigh 29-60% more than diploid oysters grown for the same length of time in the same place.

Maximum Size: Approximately 8 inches long (20 centimeters). Fossil shells may be longer.

Healthy native oysters

Reproduction: Eggs and sperm are released into the water as water temperature rises to about 64-68°F (18-20°C), usually by the end of June. Spawning is also salinity dependent, and is less common when the salinity is below 10 psu. It is estimated that females produce from 2 to 115 million eggs. Like some other animals (including some fish), for oysters, large body size is an advantage for females that enables them to produce more eggs.

Eastern oysters actually all start life as males, then most become and remain females by their second winter. Oysters release eggs and sperm into the water, where fertilization occurs. The fertilized eggs develop into nonswimming larvae called veligers, 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. Larvae usually go through settlement, metamorphosis, and attachment within two to three weeks after fertilization occurs, but the process may take a month or more under certain conditions.

Due to circulation patterns, some subestuaries tend to trap and retain oyster larvae (these areas are called “sinks”) while others may have most of the larvae produced from their oysters moved elsewhere (called “sources”). Because external fertilization is a function of oyster density, among other factors, it is important to have enough oysters of different ages (and thus sexes) near each other on a reef for successful reproduction and recruitment.

Migrations: Very limited. The larvae are planktonic or nonswimming (though they drift in current), so offspring may settle onto another nearby reef, but rarely travel very far from the reef where they were spawned. Unlike some bivalves (e.g., clams, scallops), oysters cannot move once they have attached to a substrate.

Predators: The main predators on oyster larvae are other filter feeders, especially comb jellies (Mnemiopsis, Beroe). The main predators on adults in the Chesapeake Bay include cownose rays, black drum, and oyster drills. Crabs, worms, and boring sponges may also prey on adults.

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

Distinguishing Characteristics: Oysters are bivalves—they have two shells, or “valves.” The left (or bottom) valve tends to be deep and cup shaped, while the right (or top) valve tends to be flatter. Shape varies greatly depending on where the oyster grows. Once an oyster animal is removed from its shells, a dark area is visible on the inside of the shell. This is the adductor muscle scar, showing where the adductor muscle (which an oyster uses to close its shell) was attached to the valves. In Eastern oysters, the adductor muscle scar is purplish, distinguishing it from similar species.

Diseases: Dermo, caused by the pathogen Perkinsus marinus, was first recorded in the Chesapeake Bay in 1949, and is more prevalent in lower-salinity waters of the Bay. Another disease, 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).

Of the two diseases, mortality is generally higher with MSX, which killed most of the oysters larger than 2 inches (51 mm) in higher-salinity waters (>15 psu) when it reached the Bay. Because most of the higher-salinity waters are in Virginia, this had a dramatic effect on the Virginia fishery. Many of the surviving high-salinity oysters recovered in the 1970s with wetter weather and thus lower salinity, which reduces the virulence of MSX as well as Dermo. Wet years tend to have higher oyster survival (because disease intensity is reduced) but lower oyster reproduction (because spawning and settlement require water that has 10-12 psu).

For oysters grown in aquaculture from hatchery seed, there are two main ways to reduce disease effects. One is to use artificially selected strains that have been selected for disease resistance. The other is to grow triploid oysters, which are produced in hatcheries to have three sets of chromosomes, rather than the naturally occurring pair of chromosomes. Triploid oysters grow faster than diploids, and usually reach market size before they succumb to one of the diseases. About 95% of Virginia oyster growers now grow triploid oysters.


Role in the Ecosystem

Oysters are filter feeders, consuming phytoplankton (free-floating microscopic algae) and improving water quality while filtering the water for food. One oyster can filter more than 50 gallons of water in 24 hours. 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.

Oysters alter the sediments below them through their biodeposits, which add organic matter. Biodeposits consist of feces (partially digested food) and pseudofeces (undigested food that has not passed through their gut). The biodeposits are then subject to decomposition by aerobic bacteria (if an aerobic sediment layer is present). The resulting dissolved nutrients may be recycled back into the water, assimilated by benthic microalgae (if present), or settle into the anaerobic sediments are under the aerobic ones. Once dissolved nitrogen reaches the anaerobic layer, bacteria there may use it in denitrification, which converts it to nitrogen gas, removing it from the aquatic system. While denitrification is hard to measure in the field, some researchers remove cylinders of oyster reef along with their sediments and overlying water and put the contents in chambers, then monitor the cylinders in the lab where denitrification rates can be measured.

Human activities can have both positive and negative effects on oyster populations, and thus on their ecosystem roles, as shown in this conceptual diagram below made by EcoCheck (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 NCBO’s Oyster Reefs page.

Direct human impacts on oysters. Diagram produced by EcoCheck.


Did You Know?

  • 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.
  • The flavor of raw oysters varies according to where they are grown. Site-specific food flavors are called "terroir," a term originally applied to wine. This is sometimes called "merroir" for oysters. Salinity and what the oysters eat contribute to their flavor.
  • Oysters aren't the only thing produced as a triploid for market reasons: Triloid crops (such as bananas, citrus, and watermelon) are often produced to get seedless fruit.