6.3.5 Operation of closed downwelling systems
Downwelling tank systems without continuous water exchange are dealt with following the same procedures as described above. The only major difference is that the biomass of spat is spread over a much greater surface area than in upwelling systems because the juveniles – most commonly scallops – are sensitive to overcrowding. Thus, they are maintained with sufficient spatial separation to permit growth as a single layer so that individuals are not in immediate contact with adjacent spat.
Methods to maintain spatial separation vary from hatchery to hatchery and where the spat have been set on cultch procedures outlined in section 6.2.2 apply. If not set on cultch but on the mesh bases of cylinders or trays as shown in Figure 88 (section 5.4) and Figure 92 then system design and operational management details are different. The holding tanks supplied from the reservoir will need to have a large enough area to fit the numbers of trays or cylinders required to hold the biomass of spat appropriate to the total tank unit volume. For this reason, holding tanks of the kind shown in Figure 92 are shallow and are often stacked one above the other.
As in closed upwelling systems, water quality is maintained by complete water changes twice or three times each week. The trays or cylinders containing the spat are removed and each is sprayed with a seawater jet to dislodge and remove detritus adhering to the spat and to the mesh of the containers. The reservoir and holding tanks are cleaned and refilled before returning the spat containers. Seawater used may be finely or coarsely filtered depending on the size of spat. It is usually filtered to 1 to 2 ?m for early-stage spat and is sand-filtered for larger spat about to be transferred to the sea. Spat are gradually acclimated to ambient sea temperature before they are transferred.
Scallop spat are not as amenable to removal from the containers for grading and size determination. Their shells are more fragile and care needs to be exercised not to damage their byssus gland or displace their shell valves and damage the resilium during removal. Gentle water jets may be used but it is more appropriate to count them if necessary in situ. This can be done as shown in Figure 88B by using a plastic sheet marked with a grid (1 cm squares) under the mesh base of a random selection of trays or cylinders. Means calculated from counting the numbers per cm2 in random squares over 10% of the surface area of a selection of containers multiplied by the total area occupied by spat will give a good approximation of total numbers. Small samples can be removed to weigh and measure to track growth and biomass.