The ponds used in this experiment are the grow out pond belongs to the farmer. These ponds are located at Banglen District, Nakhon Pathom Province about 60 km. from Bangkok. Two ponds measuring about 3.5 rai each (1 rai = 1,600 sq.m) were subdevided into two compartments using plastic screen. This results in a four unit ponds measuring about 1.75 rai each. These ponds are essentially stagnant ponds, such that water was added to replenish the water loss through evaporation and seepage or water changing in cases at unfavorable pond condition.
Prawn juveniles used for the experiment were about three-months old reared in the pond prior to stocked at the rate of 21,000 and 52,000 per hectare. The four fish species namely; giant gourami (Ospronemous gouramy), silver carp (Hypopthalmichthys molitrix), big head carp (Aristicthys nobilis) and common carp (Cyprinus carpio) had been stocked at the rate of 1,760; 4,400; 1,232 and 792 individuals per hectare respectively. The initial mean weight of the prawn is ranging from 3.5 – 4.7 g, while the initial mean weight of fish range from 1.0 – 6.0 g. The prawns were stocked on mid December 1985, and the fish were stocked one month later. Treatments were conducted in two replicates (Table 2).
Chicken manure in wet state (% dry matter = 39.2%) had been applied to each pond once every week at the initial rate of 125 kd per pond, afterward it would be decreased upto level of 75 kg per pond. Besides manuring, artificial feed (pellets) made by the farmer were given especially for the prawns with the protein content is about 30%. The prawns and fish were fed daily in the afternoon at the feeding rate of 10–30% of the total biomass of both fish and prawns.
Culture period for this experiment was 7 months starting mid December 1985 until mid July 1986. For monitoring the growth of fish, sampling were conducted every two months, at the same time for partial harvest of the prawn, to measure the individual weight and length of the fish from each species. At the end of grow out period, fish production and survival rate were carried out. As for the prawn, partial harvest was conducted three times, started from March 1986 afterward it would be harvested every 2 months by seining the pond (mesh size 2 cm.). Total harvest was made at the end of grow out period by draining the pond, followed by hand picking.
After seining, the marketable sized prawns were sorted out, while the unmarketable ones were returned to the pond. The harvest prawns were divided into 5 (five) groups in accordance with their morphotypes namely; Female with eggs; Female without eggs; Orange claws males (O.C); Long claws (L.C) or blue claw males; soft shell prawns (S.S). The orange claws males were divided again into grade 1, 2 and 3 according to the weight. Finally, there were seven categories (types), where total weight of each category was determined and the number of prawns were estimated by counting from 1 – 2 kg of subsample.
Within grow out period, two times restocking of prawns were carried out that was on March 21, 1986 and on May 20, 1986. The total number of prawns for restocking was judged according to the total number of prawns harvested earlier. For the first restocking, the number of prawns was 50% of the total number of prawns from the first period harvest. For the second restocking, the number of prawns as the same as prawns number harvested from the second partial harvest.
In order to appraise the production cost of the polyculture of prawn and fish, we based on the farm size which has 20 grow out ponds. The area of each pond is about 3.5 rai. From this size the operating cost of each pond of polyculture system with a low density (L.D) and a high density (H.D) of prawn were calculated out.
The experimental design for this purpose is a CRD, with the stocking density as a treatment by two level and each level has two replicates. ANOVA is used for statistical analysis.
The water quality and biological parameters were regarded as a supporting data for this experiment concerned. The detailed methods of these parameters were described in Jayamanne's and Harimurti's research papers.
