Bivalve aquaculture is increasingly dependent on hatchery production of spat. Environmental disturbances (pollution, ocean acidification, climate change, harmful algae blooms, disease outbreaks) are reducing the reliability of natural spat collection, while the demand for spat is increasing as many bivalve producers struggle to meet growing consumer demand for bivalves. Bivalve hatcheries are therefore expanding capacity to compensate for reduced natural production of spat. But hatcheries are also essential for production of genetically selected lines with improved performance characteristics, for production of triploids, and for production of exotic species in locations where no naturalized populations can supply spat. Hatcheries can artificially create conditions of temperature and photoperiod to enable spawning of exotic species, and to extend the spat production season of native species. Early production of spat makes possible a longer growing season and consequently larger, more robust seed with improved survival and growth capacity.
Hatchery production success is critically dependent on the quality and productivity of the broodstock, which in turn depend on 1) the broodstock genetic background, 2) the physical environment, and 3) the quantity and quality of the feed provided. Hatcheries “condition” broodstock by providing the optimal combination of these factors.
1) Broodstock genetic background
Broodstock animals may be harvested from wild populations near the hatchery to ensure that offspring will be well-adapted to the local environment. Or they may be individuals selected from previous generations reared at the hatchery that exhibit desirable traits such as disease resistance, rapid growth, and preferred shell shape or color. Genetically selected stocks may also be available from breeding programs at universities or fisheries laboratories.
Triploid animals are commercially desirable because they are infertile, and so no metabolic resources are diverted from meat production to gonad development. Triploids are particularly advantageous for oyster growers because they can be marketed at any time of the year, since there is no spawning season when they are unpalatable. To produce triploid oysters, the most reliable method is to purchase male tetraploids from commercial suppliers, whose diploid sperm are used to fertilize eggs from hatchery females.
Whenever broodstock is brought in from a location remote from the hatchery, the animals must be kept in a separate quarantine facility in the hatchery until they can be confirmed to carry no diseases. The quarantine facility must have restricted access by hatchery personnel, with strict biosecurity measures, and effluents must be disinfected.
2) Physical environment
Broodstock should be housed in a quiet area of the hatchery where it will not be exposed to disturbances such as vibration or sudden changes in light due to movements of personnel near the holding tanks. A flow-through water supply can take advantage of natural phytoplankton as food source, but then temperature and salinity control is not practical, and untreated water can bring in harmful algae blooms, parasites and pathogens. A recirculating water system avoids these problems, and also prevents loss of algae feed via outflow.
Control of light exposure and temperature and salinity of the water is required to optimize physiological condition and gametogenesis, maximize synchronization of spawning, or to induce spawning outside the normal spawning season. Gametogenesis can be induced in most bivalves by an increase in temperature, but for any given species the amount and duration of temperature increase required can depend on previous condition, quantity and quality of food supply, and sometimes photoperiod.
For tropical bivalves spawning is often more intermittent, with no distinct seasonal pattern. A chilling strategy can be used to stimulate reproductive development: Broodstock is held at 5 to 10 °C below ambient water temperature, with moderate feed rate, for 4 to six weeks, then temperature is gradually increased to ambient, resulting in increased synchrony of gametogenesis.
3) Food supply
Proper management of feeding is critical for controlling timing of reproduction, for good broodstock fecundity, and for ensuring good survival of larvae. Feed rates must be adequate, but excessive feeding can stimulate somatic growth and suppress gametogenesis. Appropriate feed rates (dry weight of algae/meat dry weight of broodstock) are typically 3-6%/day, with the higher rates being appropriate at higher temperatures. For best results feed should be provided to broodstock without interruption.
Reproduction may be enhanced in colder climates by a two-stage conditioning procedure. Early in the season, before broodstock would naturally begin gametogenesis, the temperature is elevated enough to increase metabolism without stimulating gametogenesis. Relatively high feed rates (e.g. 6%/day) are provided for 4 to 6 weeks to increase glycogen reserves, which will be converted to gamete lipids during gametogenesis. Then feed is reduced to 2% -3%/day and the temperature is gradually raised to stimulate gametogenesis.
Food quality is of course no less important than quantity. The quality of natural phytoplankton from ambient water can be high, but is not reliable, and may not be available in colder seasons. Cultured microalgae used for broodstock conditioning should be the same high-quality strains used by hatcheries for larviculture, most commonly strains of Chaetoceros, Isochrysis (especially the “Tahitian” strain, now known as Tisochrysis), Pavlova, Tetraselmis, and Thalassiosira. All these strains provide significant quantities of essential long-chain highly unsaturated fatty acids (HUFAs), most importantly EPA and DHA. Although it can require significant extra effort for a hatchery to produce multiple species of algae to feed broodstock, a mixture of strains can always be expected to be nutritionally superior to any single strain. Egg HUFA reserves have been shown to be important for larval growth and survival, particularly under stressful conditions.
Fortunately, it is no longer necessary for hatcheries to maintain production of several species of algae for broodstock conditioning in the larviculture off-season. These high-quality algae can be purchased in the form of refrigerated liquid concentrates, such as Reed Mariculture’s Instant Algae products (see http://reedmariculture.com/product_instant_algae.php). These liquid concentrates have specified concentrations and so are suitable for continuous automated feeding to broodstock using a dosing pump. Reed Mariculture’s Shellfish Diet 1800® (see http://reedmariculture.com/product_instant_algae_shellfish_diet_1800.php) provides a blend of 6 species of algae, and is used to feed all bivalve life stages from first-feeding larvae to broodstock.
-- By Eric Henry, PhD, Research Scientist, Reed Mariculture Inc