The experiment was conducted in 12 experimental ponds situated in the Field Laboratory Complex, Faculty of Fisheries, Bangladesh Agricultural University (BAU), Mymensingh, for a period of 4 months from May to August, 2006. The experiment was carried out with three treatments viz.,T, providing Saudi Bangla tilapia feed, T2 providing mixed feed (rice bran : mustard oilcake= 1:1) and T3 no feed but fertilization was done. Two sets of experiment were conducted. One for normal GIFT and another for monosex GIFT tilapia was assigned in duplicate.
Twelve rectangular-shaped experimental ponds each of 80 m 2 with an average depth of 1.0 m were used for the study. All ponds were prepared by completely drying by draining out the water and then ponds were treated with lime at the rate of 250 kg/ha. After 7 days, the ponds were filled with water. Three days after liming, the ponds were manured with cowdung at the rate of 1250 kg/ha and with urea and TSP at the rate of 50 kg/ha of each. About one-month-old fingerlings of normal GIFT tilapia (0. niloticus) with mean initial weight of 3.60±0.18 g were collected from Freshwater Station, Bangladesh Fisheries Research Institute, Mymensingh and the fingerlings of monosex GIFT (0. niloticus) with mean initial weight of 3.62±0.20 g were collected from Reliance Aqua Farm, Trisal, Mymensingh. These fingerlings were brought to Field Laboratory Complex, Bangladesh Agricultural University, Mymensingh using plastic polythene bags.
Different types of feed used in this study namely Saudi Bangla tilapia feed, rice bran and mustard oilcake were collected from Mymensingh local market. The fingerlings were stocked after 7 days of pond fertilization. Each pond was stocked with 250 fingerlings at the rate of 125/decimal.
Fertilization of the ponds was done weekly only in T3 with cowdung, urea and TSP. Cowdung was applied as organic manure at the rate of 3 kg/decimal and urea and TSP were applied as inorganic fertilizer at the rate of 80 g and 40 g/decimal respectively. Both the organic and inorganic fertilizers were applied separately after dissolving them in water in a bucket and then spread over the pond surface manually. Fish were fed twice daily at the rate of 10% of body weight at the beginning. The feeding rate was gradually reduced to 5%, 4% and 3% of the body weight for the last three months respectively. The feeding ration was adjusted during the fortnightly sampling of fish. The total amount of feed was divided into two equal rations for using at 10:00 and 17:00 hrs daily. Saudi Bangla tilapia feed was dispersed by hand broadcasting over the surface of water in a particular place. In case of mixed feed, 2kg of mustard oilcake was soaked into 5 liter tape water for 12 hrs and then 2kg of rice bran was mixed together and the chime made into small balls which were used in the ponds.
A fortnightly sampling of about 10% of stocked fish from each experimental pond was caught by using a seine net. The weight of sampled fish was recorded by using an electronic balance (Model EF-1-3k). The sample weight was used to adjust the feeding rate for the next fortnight.
The water temperature, dissolved oxygen, pH and transparency in the experimental ponds were monitored fortnightly between 10.00 to 11.00 hrs on the previous day of sampling. The water temperature of each experimental pond was recorded using a Celsius thermometer at a depth of approximately 10-12 cm from the surface. The dissolved oxygen (DO), pH and transparency of water were determined by DO meter (YSI, Model-58, USA), pH meter (Model-445, UK) and a Secchidisc respectively.
The proximate composition of diets was analyzed by following the AOAC (2000) standard method. Fish weight gain (g), specific growth rate (SGR %/day), food conversion ratio (FCR) and survival (%) were calculated according to CasteII and Tiews (1980).