Shawn D Coyle Dallas E Alston and Clia MS Sampaio

Macrobrachium rosenbergii nursery systems are an intermediate stage between larval rearing and grow-out in which postlarvae (PL) are cultured at relatively high densities from metamorphosis to PL, to advanced sizes. For the purpose of this chapter, PL refers to recently metamorphosed juvenile prawns ('juveniles') ranging in weight from 6 to 10 mg. At the conclusion of the nursery period, PL are generally referred to as nursed juveniles. The duration of the nursery period and optimal size of juveniles differs geographically, primarily according to the duration of suitable water temperatures for pond grow-out (the shorter the growing season the larger the nursed juveniles needed).

The nursery phase was first suggested by Ling (1969a). There are several advantages to stocking nursed juveniles as compared to stocking PL directly into grow-out ponds. Survival to harvest is generally improved because larger nursed juveniles are hardier than PL (Willis & Berrigan 1977;Sarver etal. 1982; Alston 1989; Valenti 1996). Juveniles are more resistant than PL to predation, cannibalism and fluctuating environmental conditions (New & Singholka 1985; Ling 1969b; Fujimura &Okamoto 1972; New 2002). In addition, stocking nursed juveniles generally results in larger average individual weight, higher production and a potentially higher value of harvested prawn (Sandifer & Smith 1975; Ra'anan & Cohen 1982, 1983).

The nursery phase is especially important for locations where climatic and water restrictions do not allow continuous culture (New 1990, 1995). The development and use of freshwater prawn nurseries in temperate climates has made it possible to extend the growing season in temperate regions where culture is temperature-restricted to only 4 to 8 months of the year (Sandifer & Smith 1978; Smith & Sandifer 1979b; Cohen & Barnes 1982; Ra'anan & Cohen 1982; Sandifer etal. 1982, 1983; Smith etal. 1983; Ra'anan et al. 1984). In temperate climates, nursery reared animals are required at the beginning of the outdoor season, therefore, production of juveniles occurs during the colder months of winter and early spring. At pond stocking, ju veniles of the proper size and number must all be available at the same time, so that the maximum number of days in production ponds can be performed. The fewer the number of available production days, the greater the importance of stocking nursed juveniles of the appropriate size.

Nurseries are also employed in tropical regions to maximise production efficiency in grow-out ponds, as PL can be cultured at much higher densities for the first 1 to 3 months than usual for grow-out production (Alston 1989, 1991; New 2002). In tropical areas, prawn nursery systems can be incorporated into existing outdoor production systems and can be achieved in either indoor tanks, outdoor ponds or cages suspended in ponds. Compared to stocking PL directly into grow-out ponds, the nursery phase provides more control over water quality, feeding schedule, diet and predators (Malecha 1981; Sarver et al. 1982; New & Singholka 1985; Alston 1989). In addition, the larger size of nursed juveniles allows them to compete more effectively for food with older prawn in continuous culture or selective harvest operations. In tropical climates, the age and size of nursed juveniles varies widely because hatcheries hold them in tanks or 'hapas' suspended in ponds until they are sold (Chowdhury et al. 1993; New 2002). Stocking nursed juveniles in tropical climates makes it possible to obtain up to three yearly cycles in grow-out ponds (Zimmermann & Sampaio 1998).

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