Survival has been reported to be typically 40 to 50% in flow-through systems (New&Singholka 1985, Chakraborty etal. 1999; New 2002) but Thai backyard hatcheries now routinely achieve 60 to 80% survival. Survival rates above 60% in experimental recirculating systems have been reported (Ra'anan & Cohen 1982; Ong 1983; Mallasen & Valenti 1998a). High survival rates (75-80%) have also been obtained in a commercial recirculation system with low-tech biofilters in the USA (C. Upstrom, pers. comm. 2008). Several factors may affect survival, such as larval, water and feed quality, and general management but it seems that both commercial and backyard hatcheries that follow good hatchery management protocols can achieve good survival rates. However, these generalisations need further experimental examination (see the final paragraph ofthis section).

Normally, survival is measured only at the end of the culture period because it is difficult to estimate survival/mortality by sampling. Daniels etal. (1992) suggested that survival during the rearing period can be estimated daily from the number of dead larvae siphoned with debris. This amount is subtracted from the number of estimated larvae in the culture tank on that day to obtain a new estimate. However, cannibalism on dead or weak larvae may reduce the accuracy of this estimation. Usually, this method underestimates mortality but can be a good indicator of increasing mortality and problems. In flow-through systems, New & Singholka (1985) observed that dead larvae are rapidly devoured by the remaining larvae and when dead larvae appear in the tank the problem is already severe. These authors suggested that if large mortality is observed, aborting the larval rearing cycle was better than trying remedial actions.

Quality control of the larvae stocked is very important to maximise survival. Research efforts have been made to find ways of separating weak from healthy larvae, employing their negatively rheotactic and photopositive characteristics (Singh & Philip 1995). Such methods are potentially useful in research work but when larvae are observed to be weak in a commercial situation it is recommended that the whole batch be discarded rather than separating the strong from the weak. Some crustacean hatcheries use a stress-evaluation test for quality control of PL. Such methods can also be used in research, but need standardisation (Dhert et al 1992). Cavalli et al (2000) have described a test for ammonia tolerance as a means of detecting different nutritional levels in M. rosenbergii larvae. This short-term ammonia toxicity test was shown to be a sensitive and reproducible criterion for larval quality and is therefore a potentially useful tool for hatchery operators.

Final survival to metamorphosis may be variable, even in different tanks in the same hatchery. Several management practices can increase survival rates. For example, Abreu et al. (1998) demonstrated enhanced survival rates when substrates (habitats) were placed in larval tanks. Keysami etal. (2007) observed that adding Bacillus subtilis to Artemia nauplii (108 cells/ml) when feeding M. rosenbergii larvae produced higher larval survival and a faster rate to metamorphosis. Greenwater systems were used in pioneering work to scale up experimental larval rearing to a commercial scale in Hawaii in 1969/1970 (Fujimura&Okamoto 1972). However, initial survival rates were low - about 15% in 1969 (attributed to water pollution due to over-feeding and over-exposure to sunlight of late-stage larvae) and nearly 21% in 1970. Twenty-five years later, Phuong et al. (2006) compared a recirculating clearwater system (recycling rate of 100-200% daily) and a modified stagnant greenwater system, both under different stocking densities of 30, 60, 90 and 120larvae/L. The greenwater system yielded significantly better, but highly variable, survival rates (32.392.3%) compared to the recirculating system (27.4-52.5%). According to these authors, the presence of algae in the rearing water may help to stabilise water quality and enhance the nutritional effects from the feeding of Artemia, hence shortening the time required to complete the rearing cycle. This in turn may lead to decreased cannibalism of larvae and PL, increasing survival. Maciel et al. (2004) also observed an increase in the larval development and health of another freshwater prawn species (M. amazonicum) cultured in greenwater in comparison with clear water. Conversely, Silva et al. (2007), who reared M. rosenbergii larvae in a 150 L cylindrical-conical tank for 28 days with minimal water exchange (150% during the whole culture period), changing 25% of the water volume every 3 days after the first 10 days, reported only 39% survival. However, Aquacop (1983) and Cohen & Ra'anan (1989) reported average survival rates greater than 75% in large recirculating systems. The situation therefore remains unclear and the controversy between the proponents of greenwater versus clearwater systems continues. Meanwhile, as noted in the introduction to this chapter, most producers, large and small, no longer use a greenwater system for rearing freshwater prawn larvae, no doubt because of the complexities of effectively balancing and managing several differing biological processes.

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