Harvesting

Newly metamorphosed prawn PL are about 7 mm long and begin to swim and crawl like adult animals. In some hatcheries, habitat substrates (e.g. pieces of screening material) are put into the larval tanks when PL are observed, to provide increased surface area and reduce cannibalism.

Newly metamorphosed PL are quite salinity-tolerant. In Thai commercial hatcheries, 60 to 70% of the larvae have normally metamorphosed by day 28. At this time, the remaining larvae are transferred to another tank for further rearing rather than being discarded as was previously the practice (New & Singholka 1985). The salinity of the original tank is then reduced to 4 to 5 p.p.t. (rather than totally freshwater; this is simply to keep the Artemia alive, not because of any requirement of the PL) and dry inert feed is introduced, in addition to Artemia and the egg custard diet, thus acclimatising PL to 'grow-out feed'. After approximately 90% of the larvae have metamorphosed, the Brazilian practice is to gradually add freshwater to the tank over a period of some hours to 2 days until the salinity reaches zero. New (1990) reported that this process can be achieved within 2 to 3 hours without stressing the PL; current practice in Thai hatcheries is often to complete this within 3 to 4 hours. PL are typically acclimatised to freshwater (<0.5 p.p.t.) in Chinese freshwater prawn hatcheries within 6 to 8 hours (Miao Weimin, pers. comm. 1999). PL may remain in this environment at high density for a maximum of 5 days or are transferred to PL holding tanks. During this holding stage, some hatchery operators feed their PL five or more times a day with an inert food (such as egg custard, spleen, cow liver or a formulated aquafeed). Feeding is adjusted to ensure sufficient food to reduce cannibalism. Tanks are cleaned by siphoning and approximately 30 to 50% of the water is replaced daily.

As noted above, Thai practice is to move late metamorphosing larvae to another tank and to reduce the salinity for the PL to 4 to 5 p.p.t. If PL have to be kept more than 5 to 7 days before transfer to grow-out ponds, they are transferred to a 10 to 20 t holding tank into which nets or branches are introduced as habitats where moulting PL can shelter. Formulated aquafeeds, sometimes floating fish feeds (New 1990) and egg custard (but no spleen or liver) are fed. The density in these holding tanks, for periods of up to 1 month, is 1500 to 2000/m2 of tank bottom. Tank cleaning (siphoning) and a 40 to 50% water exchange are carried out every 2 or 3 days. An alternative method would be to install a simple biofilter to avoid water exchange.

Daniels etal. (1992) recommended several harvests ofPL before completion of the production cycle. The first is performed when about 25 to 30% of the larvae have undergone metamorphosis. It occurs, generally, between day 23 and 28. Two or three subsequent harvests are carried out at three-day intervals until the final harvest. Most authors report only one harvest, when more than 90% of the larvae have undergone metamorphosis (Smith et al. 1976; Ra'anan & Cohen 1982; Malecha 1983; New & Singholka 1985; Valenti 1985, 1996; Cohen & Ra'anan 1989; New 1990). Aquacop (1983) recommended cropping ofPL when postlarval density on the bottom of the tank is higher than 2/cm2. Delay in harvesting may decrease productivity because of the potential loss ofPL due to cannibalism by larvae.

Careful harvesting and counting of PL for packing and shipping are important facets of successful hatchery operation. Smith & Hopkins (1977) described an apparatus to separate postlarval prawns and Martinez-Palacios et al. (1985) presented a method based on the rheotactic behaviour exhibited by PL; this avoids the stress claimed to be caused by the dip-net harvesting technique described by New & Singholka (1985). However, neither method is commonly used in commercial hatcheries. Aquacop (1983) and Daniels et al. (1992) described harvesting methods in recirculation systems. Aeration is discontinued and the water is stirred by hand. Most of the PL cling to the tank side. Larvae are dipped from the tank and transferred to another larval culture tank for continuation of the larval culture or into a temporary tank and then returned to the original tank. For economic reasons, most commercial hatcheries perform only a final harvest and all remaining larvae are discarded. Culture water can be pumped out through a small mesh screen into a holding tank for future use. The number of harvested PL can be estimated by counting them in a known volume of water (Aquacop 1983) or weighing the total biomass (Aquacop 1977).

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