Oyster farming is an aquaculture (or mariculture) practice in which oysters are bred and raised mainly for their pearls, shells and inner organ tissue, which is eaten. Oyster farming was practiced by the ancient Romans as early as the 1st century BC on the Italian peninsula and later in Britain for export to Rome. The French oyster industry has relied on aquacultured oysters since the late 18th century.
Oyster farming was practiced by the ancient Romans as early as the 1st century BC on the Italian peninsula. With the Barbarian invasions the oyster farming in the Mediterranean and the Atlantic came to an end.
In 1852 Monsieur de Bon started to re-seed the oyster beds by collecting the oyster spawn using makeshift catchers. An important step to the modern oyster farming was the oyster farm built by Hyacinthe Boeuf in the Ile de Ré. After obtaining the rights to a part of the coast he built a wall to make a reservoir and to break the strength of the current. Some time later the wall was covered with spat coming spontaneously from the sea which gave 2000 baby oysters per square metre.
Commonly farmed food oysters include the Eastern oyster Crassostrea virginica, the Pacific oyster Crassostrea gigas, Belon oyster Ostrea edulis, the Sydney rock oyster Saccostrea glomerata, and the Southern mud oyster Ostrea angasi.
Oysters naturally grow in estuarine bodies of brackish water. When farmed, the temperature and salinity of the water are controlled (or at least monitored), so as to induce spawning and fertilization, as well as to speed the rate of maturation – which can take several years.
The first step to cultivating oysters is conditioning broodstock. Broodstock are the "parent" oysters that will provide gametes for larvae. Oysters in the wild are only "ripe" with gametes for a short window. All of the oysters in an area will spawn at the same time to increase the chances that their gametes meet and fertile larvae are produced. To ensure ripe oysters for spawning throughout the season, some growers choose to keep mature oysters in a separate system where the farmer can manipulate the temperature and food within the system. While a recirculating system can be used, a flow-through system is generally better because the natural diversity of phytoplankton is a better diet for conditioning oysters. By setting up this separate system, the farmer can mimic the transition from winter to summer quicker than real-time, and essentially convince the oyster that it is time to spawn whenever the farmer needs more larvae.
When the farmer actually wants to spawn the oysters, he or she will put a batch of oysters in a tray and rapidly heat and cool the water to induce spawning. It is important to have a large number of oysters, because it is impossible to tell if an oyster is male or female from its outer appearance. Once the oysters start to spawn they can be picked up and placed into their own separate containers until they have released all of their gametes. Eggs and sperm can then be mixed together to fertilize.
Larvae tanks should be cleaned and disinfected before putting water in the tanks. Water quality should be tailored for the particular species, but most larvae will generally grow faster in warmer water. After the fertilized eggs and beginning-stage larvae have been added to the tank, they should be fed filtered or cultured algae daily, and have their water changed every-other day. This ensures no pathogens or foreign organisms enter the system and compete with or eat the larvae, and their water quality stays pristine to encourage growth. This is the most fragile stage of an oyster's life history.
After about two weeks an oyster will be ready to set. They will develop a small, round discoloration called an eyespot despite not being used for seeing. Their muscular foot will be visible under a microscope. At this point, the larvae can be put in a system with a variety of cultch options. The best cultch is usually full or ground up oyster shell because oysters are naturally attracted to other oyster shell to ensure their future reproductive success. After the larvae settle, they are considered "spat."
Three methods of cultivation are commonly used. In each case oysters are cultivated to the size of "spat," the point at which they attach themselves to a substrate. The substrate is known as a "cultch" (also spelled "cutch" or "culch"). The loose spat may be allowed to mature further to form "seed" oysters with small shells. In either case (spat or seed stage), they are then set out to mature. The maturation technique is where the cultivation method choice is made.
In one method the spat or seed oysters are distributed over existing oyster beds and left to mature naturally. Such oysters will then be collected using the methods for fishing wild oysters, such as dredging.
In the second method the spat or seed may be put in racks, bags, or cages (or they may be glued in threes to vertical ropes) which are held above the bottom. Oysters cultivated in this manner may be harvested by lifting the bags or racks to the surface and removing mature oysters, or simply retrieving the larger oysters when the enclosure is exposed at low tide. The latter method may avoid losses to some predators, but is more expensive.
In the third method the spat or seed are placed in a cultch within an artificial maturation tank. The maturation tank may be fed with water that has been especially prepared for the purpose of accelerating the growth rate of the oysters. In particular the temperature and salinity of the water may be altered somewhat from nearby ocean water. The carbonate minerals calcite and aragonite in the water may help oysters develop their shells faster and may also be included in the water processing prior to introduction to the tanks. This latter cultivation technique may be the least susceptible to predators and poaching, but is the most expensive to build and to operate. The Pacific oyster M. gigas is the species most commonly used with this type of farming.
Oyster culture using tiles as cultch. Taken from The Illustrated London News 1881
During the nineteenth century in the United States, various shallow draft sailboat designs were developed for oystering in Chesapeake Bay. These included the bugeye, log canoe, pungy, sharpie and skipjack. During the 1880s, a powerboat called the Chesapeake Bay deadrise was also developed.
Since 1977, several boat builders in Brittany have built specialized amphibious vehicles for use in the area's mussel and oyster farming industries. The boats are made of aluminium, are relatively flat-bottomed, and have three, four, or six wheels, depending on the size of the boat. When the tide is out the boats can run on the tidal flats using their wheels. When the tide is in, they use a propeller to move themselves through the water. Oyster farmers in Jersey make use of similar boats. Currently, Constructions Maritimes du Vivier Amphibie has a range of models.
The farming of oysters and other shellfish is relatively benign or even restorative environmentally, and holds promise for relieving pressure on land-based protein sources. Restoration of oyster populations are encouraged for the ecosystem services they provide, including water quality maintenance, shoreline protection and sediment stabilization, nutrient cycling and sequestration, and habitat for other organisms. A native Olympia oyster restoration project is taking place in Liberty Bay, Washington, and numerous oyster restoration projects are underway in the Chesapeake Bay. In the U.S., Delaware is the only East Coast state without oyster aquaculture, but making aquaculture a state-controlled industry of leasing water by the acre for commercial harvesting of shellfish is being considered. Supporters of Delaware's legislation to allow aquaculture cite revenue, job creation, and nutrient cycling benefits. It is estimated that one acre can produce nearly 750,000 oysters, which could filter between 15 and 40 million gallons of water daily.
Other sources state that a single oyster can filter 24-96 liters a day (1-4 liters per hour). With 750,000 oysters in one acre, 18,000,000-72,000,000 liters of water can be filtered, removing most forms of particulate matter suspended in the water column. The particulate matter oysters remove are sand, clay, silt, detritus, and phytoplankton. These particulates all could possibly contain harmful contamination that originates from anthropogenic sources (the land or directly flowing into the body of water). Instead of becoming ingested by other filter feeders that are then digested by bigger organisms, oysters can sequester these possibly harmful pollutants, and excrete them into the sediment at the bottom of waterways. To remove these contaminants from the sediment, species of seaweed can be added to take up these contaminants in their plant tissues that could be removed and taken to a contained area where the contamination is benign to the surrounding environment.
Pathogens that can affect either farmed C. virginica or C. gigas oysters include Perkinsus marinus (Dermo) and Haplosporidium nelsoni (MSX). However, C. virginica are much more susceptible to Dermo or MSX infections than are the C. gigas species of oyster. Pathogens of O. edulis oysters include Marteilia refringens and Bonamia ostreae. In the north Atlantic Ocean, oyster crabs may live in an endosymbiotic commensal relationship within a host oyster. Since oyster crabs are considered a food delicacy they may not be removed from young farmed oysters, as they can themselves be harvested for sale.
Dermo disease is caused by a protozoan parasite that infects the oyster's blood cells: Perkinsus marinus. It is spread when infective stages are released into the water column from an infected oyster and siphoned into a new host. It is most common in water above 77 °F.
MSX stands for "Multinucleated Sphere Unknown" and is lethal to C. virginica. It is a single-celled protozoan with an unknown method of transmission between oysters. It does not appear to transfer from oyster-to-oyster like Dermo does. After an outbreak in 1997, a strain of MSX-resistant oysters were developed. MSX can be suppressed by low temperatures and low salinities, but once infected, oysters will die within a month.
MSX and Dermo are both considered to be non-harmful to humans. Vibrio, however, is a disease that is carried by oysters and other shellfish and can make people sick, but is not harmful to the oyster itself. Vibrio can only be passed from oysters to humans if they are consumed raw. Vibrio is more common in warmer waters, and all commercial shellfish must be refrigerated before being served in an attempt to kill the vibrio bacterium.