Procurement and stocking of larvae

According to New (2002), berried females can be obtained from captive broodstock or from the wild, and they should be held in slightly brackishwater (~5 p.p.t.) at 25 to 30°C and preferably at pH 7.0 to 7.2 until the eggs hatch. Recently, however, Yen & Bart (2008) have reported that the survival of larvae from females reared at 0 and 6 p.p.t. was higher than those from females reared in 12 p.p.t. This preliminary study, which examined the effect of salinities ranging from 0 to 18 p.p.t., also found that female broodstock were larger, reproduced earlier and produced more offspring when maintained at low salinities. These authors postulated that this fact could significantly impact freshwater prawn hatcheries using estuarine water where seasonal salinity fluctuation is common. Some Indian hatcheries report that larvae obtained from wild broodstock are healthier and show double the survival rate (Balamurugan et al. 2004). Bart & Yen (2003) suggest that domesticated brood-stock produce larvae that present more uniform development and consequently higher survival than wild stocks. This subject remains controversial as there is no scientific confirmation of these observations. Further studies on wild versus domesticated broodstock and the effects of salinity on broodstock and larval performance are required.

Larvae should be healthy and free of debris. Larvae should be enumerated (Chapter 4) prior to transfer to the culture tank. If larvae are hatched in fresh or low salinity water (0-5 p.p.t.), they should be acclimated to the salinity and temperature ofthe larval culture system. Ifacclimation is necessary, a change of 5 p.p.t. or 3°C every 3 to 4 hours is recommended. If females are placed into a hatching system, it is recommended that larvae be hatched at 4 to 7 p.p.t. to facilitate acclimation and transfer to the culture system. Some hatcheries in Brazil hatch larvae at zero salinity, while others use 5 to 6 p.p.t.; the hatched larvae are then transferred to larval rearing tanks at 12 p.p.t. within 3 to 4 days at the maximum. Collection of larvae for any tank should occur over a maximum of1 to 3 days to minimise the number of stages present in the culture tank. Collection beyond 3 days may lead to a disparity in growth rates leading to increased cannibalism.

Levels of sophistication in controlling the density of larvae vary considerably. Where separate hatching tanks are used, the presence of larvae in the hatching tanks is verified by interrupting aeration and using a light source in one of the tank extremities to attract the larvae, since these animals have positive phototaxy. Once hatching has been confirmed, larvae are captured by siphoning into collecting buckets where they are counted and transferred to larval rearing tanks. Larvae may be counted in 250 ml samples taken from the buckets and homogenised by aeration; the sample numbers are then averaged. The average is multiplied by the bucket volume, thus providing a rough estimate of the total numbers present. The larvae are then stocked into the larval rearing tanks. A typical Thai hatchery stocks 10 to 15 kg of berried females (grey-black eggs) in a 3 m3 larval tank. Within three days 5 million larvae can be collected. Some hatcheries put the berried females into the larval rearing tank partially filled with 0.3 m of 3 to 5 p.p.t. water, then simply removing the spent females and raising the level of water to 0.8 to 0.9 m and increasing salinity to 12 to 15 p.p.t. after the larvae hatch.

New (1990) reported that many early Hawaiian hatcheries stocked newly hatched larvae at 60/L, but some practised two-stage rearing with an initial stocking density of 160/L. In 2008, a typical well-developed Thai hatchery using a recirculation system was stocking newly hatched larvae into larval rearing tanks at 100 to 150/L (H. Kongkeo, pers. comm. 2008). However, in Supanburi province, where backyard hatcheries are mainly located, the maximum stocking density did not exceed 100/L and was normally 60 to 100/L

Typically the quantity of larvae stocked is determined by the number/weight of berried females used rather than by a conscious stocking of a defined number of larvae. US hatcheries stock 700 to 800 larvae/L in pre-stocking small tanks (~380 L); after 7 days they are moved to the final tank (C. Upstrom, pers. comm. 2008). Alternatively, larvae are stocked at 50 to 100 larvae/L in monophase tanks (Valenti& Tidwell 2006). Brazilian hatcheries generally stock 80 to 100 larvae/L or stock 400 to 800 larvae/L in pre-stocking tanks for the first 10 days of rearing (Valenti & Moraes-Riodades 2004; Valenti 2007). After that, the density is reduced to 60 to 80 larvae/L by transferring to the final larval rearing tanks. Hsieh et al. (1989) reported that inland hatcheries in Taiwan at that time often used a multi-stage hatchery system with initial stocking rates of 300 to 1000/L in 500 L tanks, with transfers to larger tanks as the larvae grew. A similar effect may be achieved by stocking larvae at a high density in the rearing tanks, at first with a low water level; later, the density is reduced by gradually increasing the water level. This increases feeding efficiency, as well as optimising the use of the stocking area, and eliminates the stress caused by transferring larvae mid-cycle.

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