Wild Larvae Collection
Most oyster farmers obtain their seed by collecting wild set larvae. Special collection materials, generically known as “cultch,”are placed out when large numbers of larvae appear in the plankton. Monitoring of larval activity is helpful to determine where and when to put out the cultch. It can be difficult to discriminate between different types of bivalve larvae in the plankton to ensure that it is the required species that is present but recently, modern highly sensitive molecular methods have been developed for this.Various materials can be used for collection. Spat collected in this way are often then thinned prior to growing on (Fig. 3). InChina, coir rope is widely used, as well as straw rope, flax rope, and ropes woven by thin bamboo strips. Shells, broken tiles, bamboo, hardwood sticks, plastics, and even old tires can also be used. Coatings are sometimes applied.
Restricted by natural conditions, the amount of wild spat collected may vary from year to year. Also, where nonnative oyster species are cultivated, there may be no wild larvae available. In these circumstances, hatchery cultivation is necessary. This also allows for genetic manipulation of stocks, to rear and maintain lines specifically adapted for certain traits. Important traits for genetic improvement include growth rate, environmental tolerances, disease resistance, and shell shape.Methods of cryopreservation of larvae are being developed and these will contribute to maintaining genetic lines. Furthermore, triploid oysters, with an extra set of chromosomes, can only be produced in hatcheries. These oysters often have the advantage of better growth and condition, and therefore marketability, during the summer months, as gonad development is inhibited.Techniques for hatchery rearing were first developed in the 1950s and today follow well-established procedures.Adult brood stock oysters are obtained from the wild or from held stocks. Depending on the time of year, these may need to be bought into fertile condition by providing a combination of elevated temperature and food (cultivated phytoplankton diets) over several weeks (see Fig. 4). Selection of the appropriate brood stock conditioning diet is very important. An advantage of hatchery
polyvinyl chloride (PVC), layers of shell chips and particles prepared by grinding aged, clean oyster shell spread over the base of settlement trays or tanks, bundles, bags, or strings of aged clean oyster shells dispersed throughout the water column, usually in settlement tanks or various plastic or ceramic materials coated with cement (lime/mortar mix).For some of these methods, the oysters are subsequently removed from the settlement surface to produce “cultch less”spat. These can then be grown as separate individuals through to marketable size and sold as whole or half shell, usually live.Provision of competence to settle Pacific oyster larvae for remote setting at oyster farms is common practice on the Pacific coast of North America. Hatcheries provide the mature larvae and the farmers themselves set them and grow the spat for seeding oyster beds or in suspended culture.In other parts of the world, hatcheries set the larvae, as described above, and grow the spat to a size that growers are able to handle and grow.Oyster juveniles from hatchery-reared larvae perform well in standard pumped upwelling systems and survival is usually good, although some early losses may occur immediately following metamorphosis. Diet, ration, stocking density, and water flow rate are all important in these systems (Fig. 6). They are only suitable for initial rearing of small seed. As the spat grow, food is increasingly likely to become limiting in these systems and they must be transferred to the sea for on-growing.The size at which spat is supplied is largely dictated by the requirements and maturity of the grow-out industry. Seed native oysters are made available from commercial hatcheries at a range of sizes up to 25–30 mm. The larger the seed, the more expensive they are but this is offset by the higher survival rate of larger seed. Larger seed should also be more tolerant to handling