4.3 Harvesting postlarvae
New postlarvae (PL) are about 7-8 mm long. Although PL can withstand the physiological shock of sudden transfer from 12 ppt water into freshwater, it is not recommended to harvest them from the larval tanks and transfer them directly into holding tanks containing freshwater.
The animals are best acclimatized to freshwater in the larval tank. Once the majority of larvae have metamorphosed (at least by day 32-35), reduce the water level in flow-through system tanks to about 35 cm by using the turn-down drain. Then gradually flush the tank with freshwater over a period of 12 hours. Always continue to provide aeration during this process. The PL can then be harvested and transferred, or the larval tanks refilled to 70 cm with freshwater and the animals temporarily held in them. If the latter is done, the PL should only remain in the larval tanks for a few more days, with frequent water exchange, before transfer to a larger holding tank. If you do not transfer them quickly enough, the biomass will become too high, and water quality deterioration and cannibalism will occur.
The best way to harvest PL from the larval tanks is to reduce the water level and then remove them in dip nets. Some hatchery operators use larval concentration systems which are similar to the larval siphon filtering device shown in Figure 35. However, this causes more stress than dip-netting and is therefore not recommended. During the harvesting process, cover most of the tank and allow the PL to concentrate in the illuminated area. The last few remaining PL can be obtained by removing the tank filter sock and flushing them to the exterior by lowering the turn-down drain and catching them in a net.
Take care that animals do not become stranded, or dissolved oxygen levels fall too low, during these procedures. Estimate the number of PL for stock record purposes for every cycle (Annex 6). Quickly transfer your PL in any suitable container to holding tanks that contain pre-aerated freshwater. These temporary transport containers (e.g. buckets) should not be overcrowded with PL. Postlarvae should not be left too long in them or oxygen depletion will occur. However, no aeration or special packing is necessary unless the holding tanks are on another site.
Most flow-through hatchery operators harvest their postlarvae only once, at the end of the production cycle. Others, especially in research and recirculation hatcheries, prefer to make several harvests of PL before each cycle is terminated. If intermediate harvesting is practised, the first is made when about 25-30% of the larvae have metamorphosed. This usually occurs around day 23-28 after stocking. Then, two or three subsequent harvests are made at three-day intervals until the final harvest. The method is described by Valenti and Daniels (2000) but is not recorded here because a single, complete harvest is recommended for commercial freshwater prawn hatcheries in this manual.