The present experiment was conducted over a period of five months, from December 1975 to April 1976, in four ponds at Nimgachhi (Pabna), under the North Bengal Fish Culture Project, Begra. The ponds were numbered as Pond II, Pond Ill, Pond IV and Pond V respectively for the purpose of the present study. The. ponds were identical in size, and being newly constructed, were found to be almost identical in all other respects. Each of them was 8,992 em long and 1,890 em wide with a water surface measuring approximately 0.42 acre. The depths of water in Pond II, Il I, IV and V were 140 cm, 157 em, 157 em and 155 cm respectively in December, while in next April the depths of water in the same ponds were 41 cm, 48 em. 43 cm and 41 cm respectively. All the ponds .wcrc rainfed, weedfree, completely independent, and well-exposed to sunlight and wind.
Preparation of ponds: After construction in 1974 the ponds got rain-filled during the rainy season the same year. A few carp fry had already been released there in June 1975. Prior to start of the experiment, these fishes and other aquatic organisms were removed from the ponds by repeated netting, and the ponds, subsequently on 5 November 1975, were treated with endrin at a concentration of 0.01 ppm approxi- mately.
Fertilization: Fertilization of ponds was done with both organic and inorganic fertilizers. While the inorganic fertilizers used were urea, triple super-phosphate and muriate of potash, the only organic fertilizer used was mustard oil cake. Out of a total of 800 kg of inorganic fertilizers an initial dose of 125kg per acre of water area with urea, triple-super-phosphate and muriate of potash were applied in the ratio of 4:8:1. The rest was applied in 10 equal doses at monthly intervals. Mustard oil cake was used at a rate of 12 maunds per acre of water area, and the initial dose was 75 kg, applied 15 days alter the first application of inorganic fertilizers, The rest of the organic fertilizers was divided into 10 equal doses again, and applied some 4 or 5 clays after application of the inorganic fertilizers. No supplementary feed was added to the ponds
Stocking: The ponds were stocked with fingerling; of two species of major carps, viz., catla (Catla catla) and rohu (Labeo rohltay. The rate of stocking was 1,500, 3,000, 4,500 and 6,000 fingerlings of catla and rohu in ponds II, III, IV and V respectively, the two species being in equal proportions. The average weight and average length of the fingerlings of both catla and rohu were recorded before releasing the finger- lings into the ponds. The fry were taken from the rearing pond nearby, and the specimens in every single catch from the same were immediately released in the four ponds in the desired ratio, i.e., 1:2:3 :4.
Growth rate: Before stocking the fry into the ponds? representative sample of some 10 to 12 specimens each time was kept aside, and the mean length and mean weight of a total of some 100 specimens of each species were calculated. The initial total weight of the stocked fish for each of the ponds was found out by multiplying the average weight of the fish by the corresponding stocked number. Thereafter monthly check on the growth was made by random sampling of at least 30 per cent of the stock. The fry we re collected by an ordinary drag net of suitable mesh size, taken in an enamel bowl, weighed with a spring balance and the average weight calculated.
Collection of water samples : Samples of water were collected from near the surface of each pond once at the beginning of the experiment, and subsequently during the experimental period on every sampling day between 1600 and 1700 hours in blackened bottles of 500 ml capacity.
Collection of soil samples: Soil samples were collected at the end of the experimental period from zero to 15 cm depth from some 25 different points of the bottom, and thoroughly mixed up to obtain a representative sample of bo.torn-soil for each. pond. The samples were air-dried, ground, sieved through a l0-mesh seive, and bottled for laboratory analyses, and then the percentage of moisture in the air-dried samples determined by drying the soil samples overnight in an oven at 105°c. Analyses were made from air-dried samples but results were expressed on oven-dry basis, corrections having been made for the percentage of moisture.