Monoculture of prawns posses a number of problems. Most significant of all are resulting over fertile water in pond, where plankton or algal blooms might be occurred and eventually leading to a total ecological collapse (Abeliovich, 1969; Barica, 1975 in Cohen et al., 1983). And the high stocking rate which usually practiced results on a wide range of sizes although high yield can be achieved, causing up to 50% of males to remain below market size during six months grow out period (Cohen et al., 1983). Also, it seems that physical contact between the prawns might play an important role in the establishment of the social hierarchy (Cohen et al., 1981). Following the experiment carried out by Ra'anan (1981) under lower density of prawns, such contact is reduced resulting in a break down of the characteristic social structure and in the relative uniformity of prawn weight.
To reduce the widespread of prawn size or to increase the mean weight of prawn, the stocking density should be reduced by integration of prawn production into various fish species in polyculture system. In general, the danger of unicellular algal blooms is reduced due to the grazing of the silver carp and the water turbidity caused by stirring up pond bottom mud by the common carp (Cohen et al., 1983; Malecha et al., 1981). The interesting thing in prawn polyculture with fish is shown by Cohen et al. (1983) which the presence of fish did not interfere with the prawn's social structure, in more details, Wohlfarth et al. (1985) had proved that the growth and survival of fish and prawns were independent. Prawn were only influenced by their own stocking density. Prawns were not influenced by the species of fish co-stocked with them, stocking rate of fish nor by differences in feeding or manure regimes. Growth of fish was affected only by their own stocking rates and not by prawn stocking rates.
The density of prawn is found to be a major factor affecting the size of males and females whose growth characteristic differently to yield (Broody et al., 1980). The stocking densities of prawn and fish in polyculture system, where prawn as a main species, are various range from 15,000 – 80,000 tails per ha and 5,500 – 13,000 tails per ha for prawn and fish respectively (Malecha et al., 1981; Cohen et al., 1983; Schroeder, 1983; Wohlfarth et al., 1985). However, the optimal stocking densities of prawn range from 20,000 – 40,000 prawns per ha (Malecha et al., 1981; Wohlfarth et al., 1985; Karplus et al., 1986).
The secondary fish species for this experiment are selected based on their capability to utilize the plankton in the pond, namely silver carp, bighead, and common carp would also be introduced although it is a bottom feeder primarily to stir up the bottom and by doing so turbidity would be increased and therefore light penetration would be less. Other species to be stocked is the giant gouramy which is also important commercially.
Macrobrachium rosenbergii are characterized by development of a high size distribution culminating in a well-defined social structure and morphotypic variation exhibited by mature population (Ra'anan and Cohen, 1983), which consists of three distinct sub-population, each of which differs in its growth characteristics and the distribution (Cohen et al., 1983) ;
female, characterized by a fairly tight, normal distribution of sizes.
a group of small males (S.M) that do not reach marketable size under any growth condition.
a group of rapidly growing large “blue claw” (B.C) and “orange claw” (OC) males showing a broad almost rectangular distribution of size.
So, it is clear that the economic yield characters of Macrobrachium rosenbergii are determined by the morphotypic variation and the population structure, namely BC, OC, SM, female with egg and female without egg. From economic point of view, this biological characteristic present a problem, since about 25% of the individuals in the mature population would remain below market size during a single grow out period of 6 months, (Broody et al., 1980).
Individual male prawns undergo a serial change through the different morphotypes, from SM to OC to BC (Ra'anan, 1982), and these three different morphotypes change with density but those of female morphotypes did not (Karplus, 1986). Furthermore, Sagi et al. (1986) has shown that by a periodic selective removal of the OC and BC males throughout the grow out season results in the SM promptly transforming into the CC males, and shifting into the rapid growth pattern. An this sometimes accompanied by restocking the pond with juveniles in order to maintain a constant population size (Fujimura, 1974; Malecha, 1983).
