One of the major drawbacks of hatchery production in temperate climates is the asynchrony of egg development among females. Because only 5 to 10% of the females will typically hatch eggs within a 1 to 3 day window oftime, large numbers of broodstock must be maintained over long periods oftime (4-6 months) in temperate climates. This results in high costs and high mortality of broodstock. Methods to synchronise spawning would provide a larger number of females hatching eggs together, thereby reducing the number of broodstock required to produce the same number of larvae. Research to synchronise spawning through the endocrinological control of the timing of reproduction would provide major reductions in costs associated with holding facilities.
Holding broodstock for extended periods oftime indoors in temperate climates will also be necessary as prawns are domesticated and genetically improved. Under these circumstances, broodstock health and subsequent egg and larval quality are highly dependent upon good nutrition. Since these prawns are predominately held in tank systems, all of their nutrition must come from a well-balanced diet proffered by management. Unlike the diets typically developed for grow-out production (i.e. somatic growth), broodstock diets must provide the proper nutrition for optimal egg production (i.e. high-quality yolk deposition) and high-quality sperm production. The use of properly balanced diets may indeed affect reproductive performance. Das et al. (1996) found that a diet containing 40% crude protein with an energy level of 4000 kcal/kg increased egg production. Cavalli et al. (1999) demonstrated that high dietary levels of n-3 highly unsaturated fatty acids (HUFAs) and linoleic acid (18:2n-6) increased not only egg production, but also promoted the tolerance of 8-day-old larvae to ammonia stress. Similar results were observed for ascorbic acid (vitamin C) and tocopherol (Vitamin E) (Cavalli et al. 2003) This clearly suggests the possibility of enhancing not only fecundity, but also larval quality through further improvements in maternal diets.
Last but not the least, New (2005) indicates that freshwater prawns remain undomesticated, although unconscious (and sometimes negative) selection may have taken place in countries where the parent stocks were introduced in small numbers. Conventional selection and breeding for desirable traits or (if consumers will accept it) by gene manipulation and transfer are essential to improve the viability of freshwater prawn farming. Further work on hybridisation to combine the attractive traits of various species is also warranted. For example, it may be possible to combine the delicate flavour of one species, which commands a premium price, with the more rapid growth rate and/or final market size of another.
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