SRAC Publication No. 432

August 2001 VI PR Revision

Cultivating the Eastern , virginica

Richard K. Wallace*

Oysters have been cultivated in spawning occurs varies from Sperm and eggs are released syn- one form or another for more than north to south. Northern chronously and fertilization 2,000 years. Early efforts involved spawn at temperatures between occurs in the water column. A fer- little more than transplanting 60 and 68 oF (15.5 and 20 oC), tilized egg develops rapidly into a small oysters from one area to while southern oysters spawn at microscopic swimming tro- another area where they would temperatures above 68 oF (20 oC). chophore (Fig. 1). After 24 to 48 grow better, be better protected Spawning can occur throughout hours, the non-feeding tro- from predators and disease, or be the warm months. chophore develops into the feed- easier to harvest. This simple method of cultivation is still wide- ly practiced today and is a major veliger way of producing the eastern oys- ter, Crassostrea virginica. pediveliger The eastern oyster occurs natural- ly from the Gulf of St. Lawrence in Canada to the , the Caribbean, and the coasts of SETTING Brazil and Argentina. It has been introduced on the west coast of North America and in other areas of the world. In recent years the fertilized early spat total U.S. harvest of oysters has egg been 30 million pounds of meats; about 75 percent of the total is the eastern oyster. About 18 million pounds of total oyster production later spat (all ) is by cultivation. unfertilized egg (several days old) Oyster biology An understanding of basic oyster biology is essential to any success- ful culture operation. Under nat- SPAWNING ural conditions, oysters spawn as water temperatures rise in the spring. The temperature at which Adult Oysters

*Auburn University, Marine Extension and Figure 1. Life cycle of the eastern oyster, Crassostrea virginica. Research Center ing veliger stage. At this stage the After oysters are harvested, addi- Shell bags are moved from the larva has a thin shell and feeds tional cultch is planted to provide hatchery to a nursery area in nat- primarily on tiny algae. After 12 substrate where more oyster lar- ural waters. The nursery area to 20 days, the larva develops a vae can set. should be easily accessible for the foot and eye spots and is referred The most intensive culture meth- equipment needed to deploy the to as a pediveliger or “eyed ods involve spawning oysters in a bags. It should also be a site larva.” Pediveligers settle to the hatchery and growing free-swim- where poaching can be kept to a bottom and can crawl short dis- ming larvae in large tanks sup- minimum and, most importantly, tances to find suitable sites for plied with specific algae (e.g., where oysters will grow rapidly. setting. Setting occurs when the Isochrysis, Chaetoceros, or Shell bags can be strapped to a larva cements itself to a hard sub- Tetraselmis) that are known to be pallet and placed in natural strate (usually oyster shells) and nutritious for larvae. Much of the waters for the nursery phase. Or, metamorphoses into a tiny oyster effort and space in an oyster bags can be placed on a hard bot- called a spat. hatchery is devoted to producing tom or suspended from floats or “Spat” usually refers to a recently the algae. When larvae are ready racks. When oysters grow to metamorphosed oyster, but the to set (14 to 16 days), they are about an inch along the longest term may be applied to any small sieved from the large tanks and axis, they are emptied from the oyster. Similarly, the term “seed added to tanks that contain whole shell bags onto the growing area. oyster” may be given to oysters oyster shells in large mesh bags. There can be considerable loss that are too small to harvest, but An alternative method is to set the from predators such as crabs, oys- it generally refers to juvenile oys- larvae on microscopic pieces of ter drills and flatworms, particu- ters larger than spat. oyster shell (microcultch) that are larly if seed is damaged in plant- Spat are mostly male and grow held on fine screens in bucket- to ing. Under good conditions three rapidly. Sexual maturity can occur barrel-size containers. Generally, to five spat per shell should sur- within 4 months in southern only one larva sets on a piece of vive to reach market size. Oysters waters. Some males change to microcultch. This technique pro- are harvested according to local females, usually after the first or duces a crop of single oysters gear and size regulations. second spawning, and some which are desirable for the oys- females can change back to males. ters-on-the-half-shell market. Single oysters Growth to harvestable size (3 Oyster larvae also can be shipped Single oysters are produced by inches, 75mm) can take 12 to 36 to locations far away from the introducing ready-to-set larvae months, depending on tempera- hatchery and set. This process is (600 per square inch, 236/cm2) ture, water salinity and food sup- called “remote setting.” into containers with fine mesh ply. Oysters do best in areas (150-micron, 0.006-inch) bottoms where the bottom is relatively Oysters set on shells that have been covered with finely firm and stable, salinities are from Oyster larvae may be set on ground and sieved oyster shells 10 to 30 ppt (15 to 18 ppt is con- (250-micron diameter, 0.01-inch). sidered optimal), water flow is whole oyster shells, fragments, or other types of shell (e.g., ). The containers are usually sus- adequate to bring food, sediment pended in a larger tank or trough does not smother oysters, and Typically, well washed oyster shells that have been aged at least of filtered seawater. An airlift oxygen concentrations remain pump on each container lifts greater than 3 ppm (greater than 6 months or more are loaded into 9 water from the tank into the con- 5 ppm most of the time). large mesh ( /16-inch, 1.4-cm) bags that hold about 40 pounds tainer; the water flows out through the mesh bottom. This Oyster culture (18.1 kg) of shells (approximately 215 shells). Bags of shells are arrangement is called a down- Methods of oyster culture can placed in light colored, aerated weller; the downward action of range from very simple, with lit- tanks containing filtered seawater the water keeps the larvae in the tle input or control, to very inten- (50-micron, 0.002-inch filters) with container until they set. As with sive, with much input and con- a salinity greater than 10 ppt. larvae set on whole shells, the trol. The simplest form of oyster Oyster larvae are added at a rate containers are covered and left for culture is to place (plant) oyster of 100 per shell with a goal of get- 48 hours while larvae set. shells, clamshells or other appro- ting an initial set of 20 to 30 spat After the larvae set, the flow of priate materials (cultch) in an area per shell. Tanks are covered with the airlift is reversed to create an where oyster larvae are likely to a tarp to block out light and left upweller (Fig. 2) that pulls water settle. Oysters are then harvested for 48 hours. After setting, filtered through the bottom mesh of the in 1 to 3 years, depending on seawater water can be run container and out the top. The their growth rate. In some cases, through the tanks until the spat source water is usually filtered to small oysters are moved to areas are moved to a nursery area. keep out larger organisms and where growth and survival are Hatchery-produced algae or com- reduce fouling on the screen bot- expected to be better than in the mercial algae paste can be used as toms. As the oysters grow, they location where the larvae set. supplemental feed. are usually moved to upwellers Suspended belt culture system Air inflow Piling Float Water outflow PVC Water spacer level

Oyster bag PVC bag closure Anchor

Figure 3. Example of a flexible belt system with flotation. Figure 2. Airlift upweller. (Illustrated by F. Scott Rikard) with larger mesh bottoms to lines (polypropylene ropes) and 2. Substrate conditions (soft, increase flow and reduce clogging uses PVC pipes to spread lines hard, shifting, stable) in the mesh. and attach bags to the lines. The 3. Salinity flexible belts are deployed on suit- Grow-out takes place in natural 4. Prevalence of diseases waters where single oysters are able hard bottoms or suspended placed in polyethylene mesh bags horizontally in the water column 5. Tidal range that are typically 36 x 18 x 3 inch- with floats (Fig. 3). Another 6. Sedimentation rate method is to put a float in each es (91 x 46 x 7.6 cm). Single oys- 7. Water flow ters are at great risk from preda- bag and link the bags together. As tors if not protected in bags. the oysters grow heavier, a con- 8. Oxygen concentration siderable amount of flotation is Oysters do not all grow at the 9. Algae concentration (food needed for suspended oyster supply) same rate, so they are sieved in bags. order to be placed in containers of 10. Prevalence of predators Oyster bags often become over- the appropriate mesh size. 11. Fouling organisms Representative mesh sizes and grown with marine organisms stocking densities are shown in such as barnacles, , bry- 12. Accessibility and security ozoans, etc. Regular air drying for Table 1. As oysters grow, they are Most potential sites within the several hours may help, or bags moved to larger mesh containers southern U. S. are in estuarine may need periodic pressure wash- at lower densities. areas where conditions are highly ing. variable. For example, water salin- Table 1. Typical mesh size of Many intensively cultivated oys- ity may be within an acceptable oyster growing containers ters come from areas where they range for parts of a year or for (bags) and suggested stocking are completely uncovered at low several years at a particular site, tide. This regularly dries the oys- densities. but may be outside that range for ters, gives easier access to oyster other periods of time. Under- BAG MESH OYSTERS/BAG bags, and makes it easier to main- standing such variability can help (inches) tain supporting structures and in selecting a good site. Further- carry out the harvest. 0.75 (19 mm) 250 more, several factors may be closely associated, such as salinity, 0.50 (12.7 mm) 500 Site selection disease and predators. Seek help 0.25 (6.4 mm) 1,500 from local experts and the appro- While some oysters have been priate state agency when assess- 0.13 (3.3 mm) 4,000 grown to harvestable size in ing the characteristics of a particu- 0.08 (2 mm) 10,000 ponds or in the effluent from lar site. 0.04 (1 mm) 50,000 other culture operations, most production takes place in natural To use a site in public waters you waters. This means that there is must obtain permits from various Single oysters are generally grown little control over the many vari- agencies. This may involve a lease off-bottom so they will not be ables that affect growth and sur- from the state or other formal smothered by soft sediments. vival. Therefore, selecting a site arrangement to use water bottoms Oyster containers can be placed that has favorable conditions for or the water column. Each state on racks attached to the bottom, oyster culture is essential. These has its own requirements; at a on racks suspended from struc- factors should be considered: minimum, permitting generally tures, or on floating long lines. A involves the U.S. Army Corp of 1. Status or classification, by a flexible belt apparatus developed Engineers, the state natural state agency, of the water for at Harbor Branch Oceanographic resources agency, and the state safe harvesting Institution, Inc. places a large public health department. number of bags on two parallel Marketing Oysters can be sold to existing Significance markets at prevailing prices. Oyster production in the U.S. has Some producers have been able to The eastern oyster is important declined from 40 years ago, while market their oysters under brand both economically and ecological- inflation-adjusted prices have names or regional names and ly. Oysters help filter estuarine remained flat or increased only obtain a premium in specific mar- waters, which are habitat for hun- slightly. This indicates that con- kets. A number of post-harvesting dreds of species of marine organ- sumer demand has decreased techniques that reduce human isms. Factors such as declining over time, possibly because of health concerns are being tried, water quality, disease and over- concerns about the safety of eat- and these may help rebuild mar- harvesting have greatly reduced ing raw oysters and general kets. However, markets should be the economic and ecological bene- changes in people’s eating habits. realistically assessed before fit of oyster reefs in some areas. resources are committed to oyster Cultivation can increase oyster cultivation. production and restore the ecolog- ical role of oyster reefs.

SRAC fact sheets are reviewed annually by the Publications, Videos and Computer Software Steering Committee. Fact sheets are revised as new knowledge becomes available. Fact sheets that have not been revised are considered to reflect the current state of knowledge.

The work reported in this publication was supported in part by the Southern Regional Center through Grant No. 98-38500-5865 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service.