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P21-P25 is suitable for harvesting from nursery tanks because this size can be stocked directly to the pond and easily be transferred. The larvae in nursery tanks can be harvested by first reducing the water level to about 1/3 of its depth and then can be collected from the bag net positioned at the tip of the drained pipe. This method is efficient enough to collect all the larvae (Fig. 35).

The postlarvae can also be harvested with a scoop net, dip net or seine net after 2/3 of the tank water has been drained. This method however, is time-consuming.

The number of harvested postlarvae is estimated from a single water basin of known volume from which animals within have been individually counted. This basin serves as a constant where visual comparisons are made with the rest of the harvest in similar basins. This method is reliable especially if the size of the larvae is uniform.

Methods of transporting postlarvae:

  1. Tanks - Postlarvae can be transported in plastic, fiberglass or canvass tanks of a suitable transport size (500–1000 liters) and provided with aeration. Temperature of water can be lowered by floating plastic bags with ice. Postlarvae at a density of 200–500/liter can be transported for 10 hours without heavy mortalities.

  2. Plastic bag - Very often, postlarvae are transported in polyethyelene bags provided with oxygen. The bag (60 cm × 40 cm) is first filled with 6–8 liters of fresh seawater and then packed with 3000–5000 postlarvae. The density may be reduced if the expected transport time is longer. After properly tightening the mouths of the bags, they are placed in styrofoam boxes or plastic buckets. Temperature is reduced to about 22–25°C by crushed ice mixed with sawdust on the bottom, side and top of the styrofoam box. Under these conditions, postlarvae may be kept alive for more than 12 hours during transport.

Fig. 35Fig. 35Fig. 35
a. Drain 2/3 of tank water using filter boxb. install bag net outside drain pipe; open drainagec. collect larval from bagnet
Fig. 35Fig. 35Fig. 35
d. transfered harvested basinse: made the head count then distributed in other plastic basins: density compared by occular inspection low water temp. 24°cf: place known amount of larvae in plastic bag
Fig. 35Fig. 35Fig. 35
g. introduce oxygen 2/3 local volume of bagh.fashen upper end of bag with rubber bandi. place bags in styrofoam boxes

Fig. 35. Steps involved in harvesting of Larvae

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