Water source and treatment

Brackishwater is necessary for prawn hatchery operation. It is obtained by mixing seawater and freshwater, and needs to be of high quality. The freshwater used for this purpose should be as close as is feasible to potable quality and be carefully analysed during site selection concerning its physical, chemical and biological characteristics. This is particularly important in regions where the use of pesticides, herbicides and fertilisers is intensive. However, while potable freshwater is ideal, the cost of pre-treated water supplies may be expensive. In practice, freshwater sources for prawn hatcheries include rivers, irrigation canals and wells.

There are few conclusive studies about the effects of water quality on the larval rearing of M. rosenbergii. The general issues of water quality are discussed in Chapter 13, but this chapter focuses mainly on the grow-out phase. New & Singholka (1985) and Valenti (1985) considered it essential to perform a complete chemical analysis of the water before choosing a hatchery water source. Both freshwater and seawater should be free from all kinds of pollution, such as heavy metals (from industrial sources), herbicide and insecticide residues (from agricultural sources), and biological contamination, indicated by the presence of faecal coliforms, which are common in residential and agricultural areas. For example, Law (1995) showed that the oil-spill dispersant Corexit 9527 found in Malaysian estuarine waters could inhibit the hatching of M. rosenbergii eggs. Tolerance levels for various substances are discussed in Chapter 13, but most of the research has been with juveniles rather than broodstock and larvae.

Some underground water sources are unsuitable because they have particularly high levels of iron and manganese, which are lethal to prawn larvae. Angell (1992) found that untreated deep well water in Bangladesh was toxic to prawn larvae. The cause was unknown but treatment with calcium hypochlorite eliminated the problem. This author also stated that adding 10 mg/L ofthe metal chelating agent EDTA to larval rearing water was beneficial. Ismail et al. (1990) suggested that the copper level in rearing water should not exceed 0.05 mg/L and found that stage V larvae seemed particularly susceptible to copper toxicity. Chan et al. (1992) showed that the eggs of M. rosenbergii, when incubated at 12 p.p.t. and 30°C, have a relatively high tolerance for heavy metals but recommended that the levels of cadmium, copper and zinc in larval rearing water should not exceed 0.1, 0.057 and 2.97 mg/L, respectively. Copper was by far the most toxic of the three metals tested. Larvae affected by heavy metal toxicity were shorter and curved in appearance. Fitzgerald etal. (1991) has speculated that high levels of calcium in water decrease larval survival. The importance of calcium in all stages of the culture of freshwater prawns has been rather neglected, but this topic is further discussed in Chapters 12 and 13.

Table 5.1 shows the physical and chemical characteristics of water supplies that have been found suitable for freshwater prawn hatcheries. It should be noted that the figures in this table are not based on experimental data or toxicity tests; they simply provide records of the incoming water quality in some successful hatcheries. Experimental work has shown that freshwater prawn larvae under rearing conditions can tolerate different levels of some water quality parameters than might be deduced from this table (see section 5.4.4). Except where brine is used, seawater and freshwater are mixed to achieve the hatchery operating salinity of 12 to 16 p.p.t. This mixed water should also be within the range of pH 7.0 to 8.5 and at least 5 mg/L dissolved oxygen (DO2). Observations in Thailand indicate thatwaterpH outside the locally favoured range of 6.5 to 8.5 causes decreased growth rate and, sometimes, mass mortalities. Mallasen & Valenti (2005) observed that M. rosen-bergii larvae do not tolerate pH close to 9 and observed that total mortality occurs after larval stage VIII under such conditions.

When flow-through systems are chosen, the establishment of a hatchery near good-quality freshwater and sea-water sources is essential. On the contrary, recirculation systems need less than 10% of the total water requirements ofopen systems during the whole production cycle and water can be shipped to the hatcheries. Water transportation by trucks is economically and technically feasible and frequently used in India (Prakash 1988), Thailand (Menasveta 1991) and Brazil (Valenti 1993, 1995). Thus, the proximity of hatcheries to grow-out farms or suppliers might be more important than their proximity to water sources.

Table 5.1 Physico-chemical characteristics of water found suitable for use in freshwater prawn hatcheries. (Reproduced from New 2002, copyright 2002 with permission of FAO.)

Variables

Freshwater (mg/L)

Seawater (mg/L)

Brackishwater (mg/L)

Chloride (Cl)

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