The experimental was carried out in four ponds, each of 3.5 rai (eq. 5.600 sq. m.) an with a mean depth of one meter. Pond preparation such as clearing, liming, screening of inlets and outlets were carried out except for manure application. All ponds were stocked with Macrobrachium rosenbergii juveniles (each (9 – 12 mg weight). Stocking density for each pond was 20 tails per sq. meter. In addition, two ponds were installed with paddle wheel aerator (two paddle wheel for each pond) and the other two acting as control. Paddle wheel aerator machine capacity is 1 HP, with six paddles and all head spreading water of 70 cm. The juvenil freshwater prawns were stocked into the pond on the 9th December, 1985. During the experiment, the prawns were given pellet feeds (the farmer's pellet composition) once daily as food in the quantum of 5 percent mean body weight. The paddle wheels aerator were operated on 27th March, 1986. They were operated daily three hours during the day and four hours during the night. The experiment was terminated on the 10th June 1986 or two days after the first partial prawn harvest.
1. Zooplankton were sampled every twice a month (biweekly). From ten liters of pond water, taken at five different places in the pond. After filtering through a planktonnet (150 um seiving), zooplankton conentration in 30 ml was preserved by 5 percent of commercial formalin. Three subsamples of 1 ml zooplankton were taken from each of these samples for microscopic examination. The identification and quantitative estimation of zooplankton components were carried out on a Sedwich Rafter counting cell chamber of 1 ml volume, using an inverted microscope equipment (Olympus 226156). Zooplankton genera was then classified into groups such as Rotifer, Cladocera, Copepoda Nauplius and Copepoda Adult.
2. Phytoplankton were sampled the same procedure as zooplankton sample. From two liters of pond water taken at five different places in the pond and collecting in a plastic pail. One liter of the phytoplankton sample was then taken and was preserved by 3 ml Lugol's solution and concentrated to 25 ml. One drop (0.05 ml) of phytoplankton sample was taken from each these samples for examination. The identification and quatitative estimation of phytoplankton components were carried out on a plate chamber (object glass), using a microscope (Olympus 226156). Phytoplankton genera was then clasified into groups such as Chlorophyta (green algae), Chrysophyta (diatoms and golden brown algae), Cyanophyta (blue-green algae), Euglenophyta (euglenoids) and Phyrophyta (dinoflagellates).
3. Chlorophyl a was sampled as same as phytoplankton sampling. From the water sample (in plastic pail) was taken 100 ml. The water sample are added 1.0 ml MgCO3 and concentrated by filtering through a membrane filter. The pigments contained in the phytoplankton were extracted with aceton 90 percent. The concentration of chlorophyll a was determined spectrophotometrically in 665 and 750 wave length absorbant (Boyd, 1979).
4. Benthos was sampled once a month from the bottom soil sample by using Eikman's dredge device, taken at five different places in the pond. The bottom soil sample was preserved in 10 percent commercial formalin. For the purposes of separating macro and microorganisms, the bottom soil sample was processed by means of a washing screening device no. 40 or 350 um sieving. The identification and quatitative estimation was done using a microscope.
5. Organic matter was sampled once a month from each pond. The equipments for organic matter sample with and without were cover placed in the middle of the pond. The organic matter sample equipment were taken out after a duration of one week and the organic matter residue was then filtered. Both the filter and organic matter residue sample were oven dried at 103°C for 24 hours, and then ignited in a muffle furnace for 20 minutes at 550°C. The weight of the filter and residue before and after ignition was recorded. The amount of organic matter in the sample was derived from weight loss on ignition.
6. Stomach content analysis was conducted three times during the experiment. Five sample of prawn were taken from each pond at first (one month culture period) and second (three months culture period) recordings. The third stomach contents sample, on the other hand, was taken at the period of first prawn partial harvest (six months culture period). Sixteen prawns for each pond were taken (two tails of each category female with eggs, female without eggs, male no.1,2 and 3; long claws, shell skin, and petite). The prawns of stomach contents sample were preserved in 10 percent of commercial formalin. Stomach contents of prawns were analysed by Mean Bulk Index Method (Saiki, 1976).
7. Water quality parameters are regarded as supporting this report and detailed discussion is given separately (Jayamanne, 1986).
For the detailed identification of samples of zooplankton, phytoplankton, benthic organisms and stomach contents reference books, such as Boonsom (1984), Needham (1962), Pennak (1953), Shirota (1966), Smith (1950), Ward and Wipple (1945), and Wongrat (1980) were used.
Data of exponential growth rate of phyto- and zooplankton; Chlorophyll a concentration, organic matter particulate and benthic organisms were compared by using analyses of variance. Exponential growth rate (r) of phyto- and zooplankton were derived from the formula: N2 = N1 e-r(t2-t1), where N1 = minimum population, N2 = maximum population, t1 = time of minimum population, t2 = time of maximum population, and e = natural logarithms and r the relative growth constant (Fogg, 1965).
