6.2.5 Setting the larvae and growing the spat

Oyster larvae attach to the cultch and metamorphose into spat usually within 24 hours from the time larvae were added to the tanks. Some setting can occur on the bottom and lower part of the sides of the tank but this can be avoided by painting these parts of the tank with liquefied wax (paraffin). Loose shell can also be scattered over the bottom of the tank to catch larvae that may settle there.

Once larvae have metamorphosed to spat they must be fed. When remote setting began, hatcheries supplying eyed larvae often also supplied algal paste for use as food. The algal paste was algae grown in a hatchery and centrifuged down to form a disc of concentrated algae about 12 cm in diameter and 3 cm in thickness. A portion of the paste was broken off, placed in a bucket with seawater, stirred briskly to break up the clumps and then added to the tanks. Air was turned on to ensure adequate mixing of the food in the tanks. Species used to make the algal paste were the same as those grown in the hatchery to rear larvae. Algal paste is still used by some growers but it is not as common as was the case previously. Most hatcheries now require their entire algal production for their own use and have none to ship to remote setting sites. There are companies that grow algae for sale as a concentrated slurry and this can be used as food. Many growers now culture their own algal food using standard methods as described previously. Species used vary from site to site but are the same as those used in hatcheries to feed larvae.

Water in the tanks is not exchanged for the first two or three days after setting but after that a slow flow-through of coarsely filtered seawater is begun. The objective is to acclimate the spat to local environmental conditions and also provide additional natural food. If algal food is added to the tanks, the water flow from the open environment is turned off for a short period so that as little as possible of the added food is lost.
The length of time spat are held in the tanks is variable. In early spring and late fall it may be upwards of a month but in summer if can be as brief as one week. It also depends on the schedule used at a grower’s facility as the following example illustrates.

Example:
The grower has 18 tanks.
a) Larvae are added to each of six tanks at the beginning of the week,
b) Another six tanks hold spat from larvae received last week. They are being acclimated ready to transfer to growout at the end of the week.
c) The remaining six tanks are being cleaned and prepared for the next batch of larvae which will arrive at the beginning of the next week.
d) Thus, six tanks of cultch with attached oyster spat are being produced regularly each week. (Spat are kept in the tanks for a minimum period since it is costly to feed them with artificially produced food).

The spat are usually 2 to 3 mm in size when they are transferred to growout. Bags of cultch with spat are placed in the mid- to lower intertidal zone on pallets to keep the cultch out of the substrate and reduce mortalities. In summer, transfer from the tanks to growout generally occurs in early morning or late evening when temperatures are lower. The time taken for the transfer should be kept to a minimum to reduce stress and mortalities. Bags can be stacked to a height of 2-3 m, depending on tidal range. Tarpaulin covers are placed over the bags to keep the spat out of direct sunlight and to reduce settlement of fouling organisms. Bags with spat are left in the intertidal area for varying periods of time and then the cultch with spat is spread either on good growing ground or is strung on ropes or wires for floating culture.
As with hatchery operations, it is important that growers keep accurate records of each set. With acquired experience they can determine optimum conditions to maximise the production of seed from larvae.
The remote setting concept was developed and perfected as a relatively inexpensive way to produce Pacific oyster seed but it could be used for clams, scallops and mussels. To date it has not been used widely for species that do not attach firmly to cultch as do the oysters.
The technology has opened new opportunities for bivalve culture worldwide. If a grower wishes to culture a species of bivalve and cannot obtain sufficient seed from local natural sources or prefers to use hatchery seed, he no longer needs to build an expensive hatchery. Arrangements can be made to produce larvae at any hatchery and ship them to the grower’s site. It is important to realize that the hatchery can be located anywhere in the world since larvae can be shipped great distances and arrive in a healthy state. Hence large, efficient hatcheries can be located at ideal sites rather than at locations that may be politically expedient but are not ideally suited for the purpose.
A distinct advantage in shipping mature larvae rather than juveniles is that larvae are grown in water that is finely filtered and may also have been sterilized with either UV-light or ozone. The danger of spreading diseases or parasites is much reduced compared to shipping juveniles that are generally grown to the desired size in the sea and may have acquired local diseases or parasites.