Seed supply has historically been one of the most significant limiting factors to the development of shellfish aquaculture industries. However with several hatcheries in operation in British Columbia, Washington State and California supplying the growers, there is usually seed available for the grower who plans carefully and orders from the supplier well in advance of the expected delivery date. Ordering several months in advance is essential for the hatchery to be able to condition broodstock during that period so that enough seed can be produced to meet growers demands. Most growers want to have seed delivered in the spring and early summer to take advantage of the growing season and to ensure that seed grow enough to reduce the risk of mortalities during winter. For the hatchery that means peak production in spring and summer.
There are some basic principles concening seed purchase and production for the grower to consider:
- The first and cardinal rule is that scrimping on seed is an economic mistake!
- Second, the price of the larvae or seed will depend upon what services go into it prior to purchase.
- Third, be sure of the quality of the seed. Poor seed cannot be turned into good product even with the best care and conditions. Good seed translates into high growth rates and and a high quality product.
The grower has several seed purchase options to choose from:
- Obtaining seed from natural sets.
- Purchasing eyed larvae and doing the setting at the growing site (Remote Setting)
- Purchase eyed larvae and contracting someone to do the setting.
- Purchase seed that has already been set (on cultch or as single seed.
The costs and benefits of each of these options must be carefully weighed. Natural sets are not always successful and are usually feasible only if the cultch can easily be transported to the setting site. For planned production, seed or larvae purchase is a necessity. Ordering larvae and doing the setting requires facilities, experience and attention to detail. Ordering set seed is more expensive but a little less risky, and, depending on the seed size and growing methods, means a product to market in less time.
Technically speaking, bivalves that have not yet metamorphosed and set are called larvae. Oyster larvae for example will set at somewhere between 16 and 20 days post-fertilization. The presence of the eye spot and foot indicate readiness to set and are called “eyed larvae”. Setting happens when the oyster contacts a suitable surface with its foot, a gland secretes cement and the left valve is attached. Immediately afterwards anatomical changes occur: the eyspot, velum and foot disappear and the whole body rotates in side the shell. Once this process is complete, they are usually referred to as seed.
Clams undergo a slightly different process. Velum and foot probing activity indicate readiness to set but unlike the oyster, the clam has no visible eyespot. When it is ready to metamorphose, the velum is lost and with it the ability to swim. The clams search available surfaces and move around by the foot until a suitable spot is found. Then a byssal thread is secreted and the calm attached. If it does not like the chosen spot it can sever the thread and move to a new spot.
Growers that intend to do their own setting will order larvae by the millions. A batch of 2.5 million oyster larvae will be about the size of a golf ball. How many to order at a time and when to schedule delivery of the larvae depends on the capacity of the grower’s setting facilities. Observing larvae under a microscope shortly after arrival will give the grower some indication of the quality and health of the larvae.
Shipping larvae from the hatchery must be carefully coordinated and precisely timed. Especially if larvae will be air freighted across international boundaries, freight carriers and customs officials must be fully briefed as to the nature of the shipment. Anything beyond 24 hours after shipment time from the hatchery becomes critical for larval survival. The larvae should be carefully inspected for quality. If they are poor or damaged, the shipper or hatchery need to be informed. Once they have been received by the grower, they can be revived in warm water in preparation for introduction to the setting tank.
Larval quality checklist:
- Packaging: Well packed with good padding. The gel pack should be 1/3 to 1/2 thawed. If not it may have been too hot or too cold.
- Larval bundle: Should not be off-smelling, not run when opened (too wet) nor fall apart (too dry).
- Larvae should be uniform size, around 300 microns. There will be some variation in size depending on the hatchery. Reliability of the larvae is what is most important.
- Eyespot should be visible on all larvae.
- Larvae should have food in the gut.
- Cilia on the velum should be active and distinct. The foot should also be active. This should be observed in one or two individuals of a sample of the larvae when they revive in warm water after shipping.
- There should be very few mortalities and those that are should be the runts.
- Larvae should not have cracked or broken shells.
(Checklist provided by J. Stacey of Fanny Bay Oysters Ltd.)
Successfully setting clam and oyster larvae requires a great deal of preparation and care. Controlling the process is the key to successful setting. Oyster larvae can be set on cultch such as tubes or large pieces of oyster shell. Density and distribution of the set is controlled in setting tanks by properly arranging cultch in the tank, aerating to disperse the larvae and shielding from light. Producing single oysters is also controlled by setting: the larvae set on fine pieces of substrate such as ground and screened shell where each larva settles on one bit of shell. Clam larvae, on the other hand do not require cultch but can be set on screeens in large numbers in an upweller or flow through system as is commonly done now in hatcheries. Recent innovations introduced by Island Scallops Ltd. allow for mass remote setting directly on the intertidal beach or in floating enclosures in deep water avoiding the need for temperature-controlled setting tanks. Setting can also be done on contract with companies eqipped with setting tanks. The grower supplies the cultch, and the contractor supplies the larvae, setting tanks and expertise. For more details on setting technology see the setting page.
Oyster Seed: Cultched
Growers can avoid setting altogether by purchasing seed after it has set. For seed on cultch such as shell or tubes, the bulk and weight of this precludes shipping over any great distance but set and conditioned seed is available from some suppliers. Here the suppliers order the larvae and do the setting on their own shell cultch in vexar bags. The seed are then “hardened” on nursery beaches for three or four months and then sold to growers as conditioned seed. The fastest possible turn-around time is to set in April and have conditioned seed available for delivery to the grower in July or August. This is possible only in good years when conditions are optimum for high juvenile growth rates.
Oyster Seed: Singles
Single oyster seed is available from many sources and is usually available in a range of sizes. In a commercial hatchery such as Coast Oyster Company, single seed are produced by inducing the larvae to set on tiny cultch fragments. This is usually made from grinding shells and then screening them to obtain uniform fragment sizes. Optimum size is large enough for one larva to settle on it but small enough so two or more cannot. Once they have been set this way, the single seed can be grown out further. Seed is now commonly boosted in size using nursery rearing systems such as FLUPSY’s (Floating Upwelling System) growers are now able to purchase single oyster seed as large as 3/4″ (19mm).
For further details see the section on nursery culture.
Triploid Oyster Seed
Normally oysters will prepare to reproduce as water temperature rises in the spring and summer. During the summer much of the oysters’ resources and body matter is diverted into the reproductive process and they spawn, lowering the market value of oysters in summer. This may also contribute to higher levels of mortality in summer. Manipulating temperature and food levels in the hatchery has allowed them to control spawning and produce larvae and seed according to a schedule. Because of the spawning problems, research and development was done in the 1980’s to produce oysters with three sets of chromosomes (3n or triploid) rather then the normal two (2n or diploid). Triploid oysters do not undergo much of a reproductive process and gonads are significantly reduced in mass compared to normal diploids. This should mean a better oyster in summer. Triploid oyster seed is now commercially available from most hatcheries.
Clam seed is sold by most hatcheries that sell oyster seed. They are sold in similar size increments.