44 MARCH 2018 • WORLD AQUACULTURE • WWW.WAS.ORG Larval rearing and fry production Larvae are reared for 15 days till they grow into fry that resemble adult fish. For the first two days, newly hatched pabda larvae use yolk-sac reserves. After the mouth opens, by the second day after hatching, small quantities of feed should be offered. They must be fed appropriately for good development and growth. As there is likely to be differential growth among fry from the same batch, it is not always possible to standardize the particle size depending on age. Hence, visual observation of food acceptability at different stages is the best way of selecting the appropriate feed type. Feeding larvae with live organisms has proved always best. First feeding can be done with mixed zooplankton from adjacent ponds, Artemia nauplii produced in the hatchery, or finely chopped tubifex worms, four times per day. Larvae are usually raised in a plastic tub (Fig. 7), cement tanks, or aquaria until they are 2-3 cm or more. After collection of live food from the pond, contents must be sieved so that only smaller zooplankton are put in the rearing container. After seven days of feeding, larvae are transferred to a larger nursery tank. Pabda larvae are then provided with other supplemental foods like fish flesh, earthworms, and freeze-dried tubifex. However, ground tubifex worms (Fig. 8) and finely sieved zooplankton are the best foods to control cannibalism among pabda fry. Feed is provided slightly in excess quantity for the larvae to feed ad libitum. It is important to provide a good environment for the larvae. For this purpose, indoor rearing tanks are provided with continuous aeration and water exchange. There is a chance of mortality and poor growth of larvae due to a poor environment and high stocking density during the indoor rearing phase. A stocking density of 1,0002,000/m2 is considered to be optimum for good growth and survival in indoor conditions. In the event of cannibalism, segregation of the stocked spawn based on size is necessary and is done by grading through nets of different mesh size. Pabda prefer darkness, so hiding places should be provided for better survival (Fig. 9). Larvae grow to 15-20 mm fry during 15-21 days of rearing. After a maximum of 21 days of indoor rearing, fry should be transferred to outdoor rearing tanks for fingerling production (Figs. 10 and 11). Nursery rearing Ponds of 800 m2 are suitable for nursing pabda fry. After drying the pond bottom completely, cow manure at 5-10 kg per decimal (ca. 40.5 m2) should be applied. After tilling the pond bottom, lime at 250 kg/ha is added and left to react with pond soils for 1-2 days. The pond is then filled with water. Frogs, snakes and other predators are very harmful for nursing of fry and so it is important to protect the nursery pond from these predators. Small-mesh netting is used to surround the whole nursery pond. Nursery ponds are stocked with 5-d old fry at 15,000/ha. A feed of 50 percent rice bran and 50 percent dried fish powder can be mixed together and used as nursery feed at 20 percent body weight per day. Fish are nocturnal and so feed should be given two times at night. Fry are reared in nursery ponds for up to 40 days, until fry reach 5 cm. FIGURE 9. Survival can be enhanced by providing hiding places to larval rearing units. FIGURE 10. Outdoor cement tanks for nursery rearing of pabda. FIGURE 11. Pabda seed can be reared in cages placed in an earthen pond.
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