Nursery rearing of shellfish begins immediately after setting of the larvae and extends to the point at which the on-grown seed are put into grow-out systems. The transition from nursery to growout is a fine line. Single oyster seed can be grown to 20mm or larger in nursery systems. It may be desirable to grow Manila clams as large as possible in nursery systems and continue to grow them in trays or other off-bottom systems. Clams will only grow well without substrate until they reach 20-25mm at which point they must be out-planted to a beach. If not, deformed shells and mortalities will result. The time frame and size of the seed is dependent upon the type of culture system used. Nursery rearing is carried out in special systems to achieve the highest survival rates possible. During the juvenile stage, most shellfish are particularly vulnerable to water quality, disease, parasites and predators.
Nursery rearing systems are either intertidal, suspended in deep water, or built on land or floating with seawater flow-through (e.g upwellers/downwellers).
Nursery Systems: Cultched Oyster Seed
Once seed has adequately set the bags of cultch or tube modules are moved to carefully selected areas for nursery rearing. An intertidal nursery beach site provides relief from predators such as starfish and most crabs, reduces fouling and results in hardened seed. On the other hand, special care must be given to seed put in an intertidal nursery to avoid seed drying out especially in hot summer weather. This is usually done by having a layer of burlap fastened down on top of the seed, keeping it moist, shaded and cool during periods of exposure to air. The mesh bags containing seed on shell cultch are spread directly on the beach, at a low enough tidal level to maintain good growth but high enough to spare the seed predation and fouling. Tube modules are similarly anchored into place at the appropriate tidal level on the beach. Where the average tidal range is 5 meters, as it is in the Strait of Georgia, oyster juveniles are usually placed between the 1.5 and 2.5 meter level above datum.
Growers who choose not to rear their seed on the beach or those who do not have a beach lease usable for a nursery will rear the seed in deep water. Tube modules, for example, can be suspended from longlines or rafts. Duration of the nursery rearing period varies, however. Achieving maximum growth over the summer (by setting before the end of June) means that tubes can be hung for grow-out in the fall when the seed may already be 1cm in length. Since the oysters are continuously submerged, predation is more likely. Perch are particularly adept at picking the oyster seed from the tube surfaces. At the end of the nursery period, tubes may be hung from rafts or longlines or stripped for further growout as single oysters on the beach or in trays.
See the page on setting and nursery rearing on tubes and the section on suspending tubes from longlines.
Nursery Systems: Upwillers for Clam and Single Oyster Seed
Designing a nursery system for clam and single oyster seed must be done according to type and size of seed with which the grower wishes to begin culture. Nursery rearing immediately after setting is more difficult and may involve higher mortalities than purchasing boosted seed and rearing it through a late nursery phase. Hatcheries and growers that undertake nursery rearing of seed from post-set to the 1410 micron sieve size (about 2mm) do so using upwellers.
In British Columbia several growers now operate what is called a FLUPSY (Floating Upwelling System). In essence it transfers the technique of the tank-enclosed upweller to a much larger scale by moving the upwellers into a floating structure that draws natural seawater through the system. Large bins with screened bottoms are lowered into openings in the frame and suspended in the seawater. Several bins are placed in a row on either side of a central enclosed channel that ends at a paddlewheel. The rotation of the wheel draws water out of the central channel creating an inflow through the bottom of the seed bins. The outflow from the bins is through a drop section on one side of the bin facing the central channel.
This technology has allowed for large scale clam and single oyster seed nursery production without requiring a land-based facility. With a FLUPSY single oyster seed can be grown economically to much larger sizes (e.g. to 3/4″) than is feasible for land-based upwellers.
For most growers building and maintaining a FLUPSY is not feasible. It requires large capital investment, full-time attention and must be large enough to realize economies of scale. What FLUPSY technology has done is to make high quality boosted seed available to other growers.
The grower now must weight the costs and benefits of buying small hatchery seed at a lower price per thousand and undertaking the nursery rearing of the seed, or buying boosted seed from a FLUPSY system that can go straight into a grow-out system such as trays. There will be a far greater amount of handling and labour when starting from smaller seed and the losses from mortality will likely be higher. The seed must be either graded or densities split and fouling must be controlled on a regular basis. For those who choose to develop a nursery system, it can be done on a smaller scale with trays and/or bags.
Nursery Systems: Trays and Bags for Clam and Single Oyster Seed
Clam and single oyster seed that is large enough can also be nursed in mesh or screened enclosures either intertidally or subtidally. Bags constructed of vexar or similar material as well as trays are commonly used.
Normally trays or bags will have to have some kind of fine mesh material in which to enclose seed. One option is to put seed into a bag constructed of a fine mesh which is then put into the tray or vexar bag. The principle is to utilize as large a mesh size as possible to maximize water flow but small enough that the smallest seed will will not escape. Another option is to line trays with fiberglass window screen material. When stacked, each tray acts as a nursery rearing chamber. By avoiding major fouling events of the spring, seed can achieve significant growth in these nursery systems between May and October. Clam seed, for example, has been reported to gain up to 1g during this period (starting at .05g).