Farming freshwater prawns

tions, where a breeding stock of both males and females was kept, it has been noted thategg incubation time averaged 20 days at 28°C (range 18-23 days). Ovaries frequentlyripened again while females were carrying eggs. Pre-mate intermoults were separated byas little as 23 days (i.e. females on some occasions hatched two batches of eggs within aone-month period). It is unlikely that this would happen under natural conditions but itdoes show the potential fecundity of the animal.As the eggs hatch, a process which is normally completed for the whole brood withinone or two nights, the larvae (free-swimming zoeae) are dispersed by rapid movementsof the abdominal appendages of the parent. Freshwater prawn larvae are planktonic andswim actively tail first, ventral side uppermost (i.e. upside down). M. rosenbergii larvaerequire brackishwater for survival. Those which hatch in freshwater will die unless theyreach brackishwater within a few days. There are a number of microscopically distinctstages during the larval life of freshwater prawns, which lasts several weeks (Annex 1).Individual larvae of M. rosenbergii have been observed, in hatchery conditions, to completetheir larval life in as little as 16 days but reaching this stage may take much longer,depending on water temperature and other factors. The importance of this observation isfully discussed later in the manual. Larvae eat continuously and, in nature, their diet isprincipally zooplankton (mainly minute crustaceans), very small worms, and the larvalstages of other aquatic invertebrates.On completion of their larval life, freshwater prawns metamorphose into postlarvae(PL). From this point onwards they resemble miniature adult prawns and become mainlycrawling rather than free-swimming animals. When they do swim it is usually in a normal(dorsal side uppermost) way and in a forward direction. Rapid evasive movement is alsoachieved by contracting the abdominal muscles and rapid movement of the tail fan.Postlarvae exhibit good tolerance to a wide range of salinities, which is a characteristic offreshwater prawns.Postlarvae begin to migrate upstream into freshwater conditions within one or twoweeks after metamorphosis and are soon able to swim against rapidly flowing currents andto crawl over the stones at the shallow edges of rivers and in rapids. They can climb verticalsurfaces and cross land, provided there is abundant moisture available. In addition tousing the foods available to them as larvae, they now utilize larger pieces of organic material,both of animal and vegetable origin. Postlarval freshwater prawns are omnivorousand, as they grow, their natural diet eventually includes aquatic insects and their larvae,algae, nuts, grain, seeds, fruits, small molluscs and crustaceans, fish flesh and the offal offish and other animals. They can also be cannibalistic. Further reading on this topic maybe found in Ling (1969).1.4 Sources of further biological informationThe polymorphism of male prawns, which is particularly relevant to the management ofprawn farming, is covered in Annex 8 of this manual. However, the internal morphology,reproductive physiology, and osmo-ionic regulation of freshwater prawns and the nature ofecdysis (moulting), autotomy (shedding of parts of the body) and the regeneration ofappendages, are topics that are beyond the scope of this manual. New and Valenti (2000)have provided a review of these subjects.FRESHWATER PRAWNS10