Bivalve hatcheries vary considerably in terms of size and design depending on what species are cultured and on the size of the operation. Other structural differences between hatcheries occur in relation to geographical differences.
A hatchery capable of producing over 50 million or more seed per year does not necessarily require much waterfront land. A site adjacent to seawater is usually preferred, but the hatchery can be operated some distance away provided the seawater intake and discharge lines can be secured.
Good water quality is obviously essential for the successful operation of a bivalve hatchery. Estuaries or other coastal waters receiving industrial, urban or agricultural effluent are to be avoided.
Excessive sediment load is also undesirable because often responsible of endless problems in the seawater delivery system. Areas subjected to heavy storms would expose the hatchery to damage as well as stir up sediment. Bays and estuaries subjected to periods of very low salinity are also not desirable. If high salinity occurs, it is easy to remedy but there is no known method of increasing salinity for bivalve larvae culture.
The suitability of an area can also be partly determined by observing the presence of resident bivalve species and by carrying out a bioassay of seawater from the proposed site, using the 48 hours larvae technique. Survival of 48 hours larvae held in water from the site is compared to seawater of known high quality or artificial seawater.
The design of a pilot or commercial hatchery must include the following facilities:
Seawater extraction and treatment facilities
The seawater intake system can take a number of configurations depending upon the site and seawater quality. A bivalve hatchery does not require running seawater, so that the intake can be located in the intertidal zone which would have the advantage of simplifying maintenance. Seawater can be collected either by sub-sand extraction or surface intake (Fig. 1). The former method is usually highly desirable, as most of the fouling organisms are excluded (Fig. 1 A). The latter method is usually required if green-water culture is used (Fig 1 B).
Centrifugal pumps of the enclosed impeller type are suitable. They should have a fibreglass or plastic housing and impeller. With regard to pressure filters the ones used for swimming pools are good. The housing is of fibreglass and the filter is provided with a backwash. All sand filters, whether gravity or pressure, should have a backwash.
For finer filtration, such as required for large algae tanks, diatamaceous earth is an excellent medium, with an effective pore size of about 1–2 micron. These filters can be purchased, but can be easily home-made, requiring purchase of the filter elements only. Diatomaceous earth is suspended in water and then introduced into the tank. When the pump is turned on, the diatomaceous earth coats the filter elements. After back-washing, the elements must be re-coated.
Ultraviolet treatment of seawater has been found necessary at many locations and is particularly useful in preventing fungal infections of larvae. Various types of units are available on the market, but is easy to fabricate them yourself at considerable savings. Water entering a diatomaceous filter or ultraviolet unit must be pre-filtered, usually through a sand filter.
Snap ring filter bags are useful in the hatchery, particularly for filling larval and large-volume algal tanks. They have an effective pore size of 5 microns.
A hatchery requires large amounts of low pressure, clean air. Rotary vane or lobe blowers are most suitable. Air pressure should be around 6 psi. Output depends upon the size of the hatchery, but is always better to overestimate.
Air system piping should be of PVC. Wherever possible, branch lines should be in the form of loops so that pressure is balanced in the system.
Figure 1. (A) Sub-sand seawater extraction and (B) surface seawater intake for hatchery use.
Most commercial hatcheries filter the air at the blower intake only.
The size of the algal culture unit depends on the projected output of the hatchery, and the particular system of phytoplankton culture employed in the unit. The system is composed of five sections:
The stock culture, inoculation and carboy room must be air conditioned at 25 °C for most of the species of algae used in bivalve culture. The large tank room or area can be simply roofed over with a transparent roof in order to allow penetration of sunlight.
The inoculation room should be separated from the stock culture room by a door, preferably a double door, which should be kept closed at all times. The inoculation room should have no direct connection with the carboy room or large tank area. Submersible pumps are used to carry algae to the larvae.
Broodstock conditioning and spawning units
This is the only part of the hatchery which should have running seawater available. But since only a small amount is needed, a header tank can be used to supply the broodstock troughs (Plate 1). A flexible hose or peristaltic pump, operated by a variable speed motor can be used to feed the broodstock on a continuous basis. An alternative way is to stop the flow from the header tank and add the algae to the broodstock troughs and once the water has been “cleared”, the seawater flow is resumed. If temperature manipulation is desired, water from the header tank can be made to flow through a simple heat exchanger. For continuous flow use, the heating element should be equipped with a simple thermostatic switch.
Bivalve broodstock should be held in elongated troughs with a suitable depth. Oyster broodstock should be held on plastic mesh trays to facilitate cleaning. Clam species should be kept in trays filled with clean sand. There should be sufficient trough space for several hundred specimens of each species being reared in the hatchery; each species is held in its own trough.
Larval rearing unit
For a production goal of 100 × 106 juvenile bivalves per year, 15 tonnes tanks are usually required (Plate 2). Ten of these will be used for larvae up to abut 300 microns and five will be used for eyed larvae ready to set. The tanks must be elevated above the floor level so that they can be easily drained by siphoning. They can be set on wooden platforms, which give flexibility to the layout, or raised concrete platforms can be built into the floor. An ultraviolet unit should be placed in the incoming seawater line so that water used to fill the larvae tanks can be irradiated, if desired.
The design of the ultraviolet unit must be large enough to handle the flow rates needed to fill the larvae tanks within a reasonable amount of time. The floor of the larval rearing area must be equipped with adequate drainage. Water should not be allowed to accumulate anywhere on the hatchery floor.
All valves and piping must be of PVC. Bivalve larvae are sensitive to copper, so there should be no brass anywhere in the hatchery plumbing. Each tank will also have a single air-stone. Bivalve larvae require only very gentle aeration.
Setting facility and nursery
If several different bivalve species are reared, different setting systems and materials are required for each species.
The most efficient type of nursery for single seed is the up-welling system.
Some of the equipment can and must be constructed in-situ, either because suitable equipment often is not available or the cost of manufacture or shipping may be high.
Pumps. The pump components in the pump should not be manufactured in copper, brass or bronze, since these can leach copper ions into the seawater. A high concentration of such ions can be toxic to larvae. A cast iron pump is usually the least expensive and works very well. Insert materials such as stainless steel or plastics will also work, but are usually more expensive.
Plate 1. Cyster broodstock conditioning tanks. Visible in the foreground are two troughs placed in parallel and in the background one header tank and two tanks filled with algae.
Plate 2. Oyster larvae rearing tanks. Bivalve hatchery at the Brackishwater Station in Prachuab Khiri khan in Thailand.
Pipe. The seawater lines should be PVC or flexible plastic pipe. Drains can be plastic or concrete troughs. Points to remember: (1) the fewer the bends and the shorter the run, the easier it will be to clear the drain if it becomes clogged: (2) the larger the diameter of the drain, the less change it will become clogged; (3) troughs with removable covers are usually the most trouble-free.
Valves. One of the most serviceable types of valves is a plastic ball valve. Some bill valves have removable components and can be easily repaired using readily available spare parts. This type is usually recommended.
Containers of various sizes will be required for the culture facility. Commercially available polyethylene containers are safe. Seamless containers in light, non-metallic colors should be used. Always wash the containers carefully before use, and rinse with filtered seawater to remove any toxic substance residue.
Glass containers, after a wash and rinse, are always safe to use. Fiberglass containers must be flushed with filtered seawater before use. It is often desirable to soak new fiberglass containers for a few days to eliminate any plasticizer or release agent residue. Metal containers or un-coated wood should be avoided. Ceramic materials work well if they are coated or glazed with nontoxic substances.
Commercially available geologist sieves can be employed, but the finer mesh size sieves are extremely expensive. Sieves can be constructed using Nitex nylon mesh or polypropylene mesh fused to the bottom of plexiglass tubing (Fig. 2). Be sure to inscribe the sieve size on the side of the new sieve with some type of permanent market.
Plungers can be constructed by cutting a 15–18 cm diameter disk from a 0.5 mm thick sheet of plexiglass with a jig or coping saw. Drill a series of 1.5 cm holes inside the periphery. Cut a center hole in the disk. Place a short length of plastic pipe with a cap on one end through the hole. The pipe can then be cut so that a coupling will secure the pipe and cap section to the disk once the parts have been glued together. Add about a 45 cm or longer section of pipe to the coupling for the handle. Cap this section to keep water out (Fig. 2)
Graduated pipettes can be purchased from a scientific equipment supplier. An automatic pipette with disposable tips is easier to use but it is more costly. Disposable tips can be rinsed and reused. Use fresh water to prevent cross contamination.
A microscope is necessary to inspect clam eggs, embryos, larvae and post-set stages until the bivalve larvae exceed 0.5 mm in size. There is no substitute for a good, easy to use, modern compound microscope. Many problems in bivalve culture can be corrected if observed early enough. A convenient, easy to use microscope will help insure frequent observations.
It is recommended that a microscope have the following features:
A microscope is a precision instrument and should be handled accordingly. If seawater is spilled over the instrument it should be sponged off and wiped with distilled water and dried.