2.1 Study area and study period The study was conducted in the pond complex of Bangladesh Fisheries Research Institute, Brackish water Station, Paikgacha Upazilla (22º35.3'N 89º20.2'E), Khulna district, Bangladesh. Nursery pond management of green back mullet was conducted for a period of 3 months (January-March) with three trials (each with 27 days of culture) in 2015 in nine experimental ponds. 2.2 Methodology Study on the efficacy of different fertilizers on the growth and survival of green back mullet, Chelon subviridis in nursery phase was carried out in nine nursery ponds of 60 m2 each. Management techniques of nursery ponds are as follows.
2.2.1 Pre-stocking management Pond preparation: The ponds were prepared by sun drying followed by 6 cm bottom soil removal Boom and dyke repair: Dyke was repaired carefully before stocking the fry in nursery pond. Liming: Liming of soil was done with CaO and dolomite (3:1) @ 250 kg/ha by means of method throwing diluted lime. Filling of pond: Nursery ponds of 60 m2 were filled with tidal brackish water up to 100 cm directly from river through screening using pumps. Removal of unwanted fish or other aquatic animal: Water of the ponds was treated with rotenone as powder forms @ 1.5 ppm used to kill unwanted fishes and removal of predatory and unwanted species of animal. Liming: The pond was treated with dolomite @ 20ppm to increase the buffer capacity of the ponds. Most common method of application was identified as diluted lime and applies it by throwing. Fertilization: After 5 days, particular pond was fertilized with respective fertilizers and dose following the experimental design.
Control of aquatic insects: After Five days of fertilization, dipterex @ 1 ppm was spread all over the water surface followed by netting for mixing uniformly to kill aquatic crustaceans and insects. dipterex was a popular liquid insecticide in study areas and found in local markets. It was very effective to remove “Haspoka” and “Argulus (Fish lice)”.
2.2.2. Stocking Management 2.2.2.1 Fry collection: Fry was collected from the natural source of adjacent Shibsha river during full moon from local fry collectors. 1.1 cm and 0.3 g small fish were transported mainly by rickshaw in a polythene bag with oxygen supply. 2.2.2.2 Water toxicity test: Twenty four hours after spreading of dipterex, toxicity of water was tested by releasing some fries in a drum full of nursery pond water for 24 hrs before stocking the fry in the pond. 2.2.2.3 Natural food testing: For testing natural foods in ponds transparency was measured. 2.2.2.4 Conditioning of fry: Conditioning of fry was done for 30 min in the mixture of pond water and water used in transportation @ 1:3, 1:1, 2:3. 2.2.2.5 Releasing: Twenty four hours after spreading of dipterex, 1.1 cm and 0.3 g small fry were released in 60 m2 pond. The fry was stocked in the early morning (around 6:30 am). fry of C. subviridis was stocked uniformly @ 200 Nos/m2 in all ponds Releasing method of fry in the nursery pond was simple. The polythene bag containing fish fry was immersed in the pond water and after 20 minutes, bag was opened to release the fry.
2.2.3. Post-stocking Management 2.2.3.1 Pond monitoring: Growth and well-being of the fries Effect of plankton production on production of fish was checked at weekly interval. Physico-chemical parameters of water viz., temperature, salinity, transparency, pH, dissolved oxygen (DO) and alkalinity was determined following standard methods and plankton samples was analyzed at four days interval. After 27 days of nursing, all fries were harvested first by repeated netting, followed by draining the ponds and growth and survival of fingerlings was estimated. 2.2.3.2 Fertilization: Subsequent to stocking of fry, the ponds were fertilized regularly at weekly interval with one fourth of the initial dose (Table 1) of fertilizer. 2.2.3.3 Feeding: From the second day of stocking, fries were fed twice daily with a mixture of finely powdered commercial shrimp nursery feed (35% protein). Feed was supplied @ 6 kg/million of fry and feed was increased to 10 kg from the 6th day of stocking. Subsequently, feed was increased by 5 kg/million hatchlings in every five days. 2.2.3.4 Control of aquatic weeds: Traditional methods were followed (manual and using simple device like bamboo stick, straw made twisted rope, netting etc.) to control aquatic weed. 2.2.3.5 Harvesting: After 27 days of rearing (with three trials), the fish seeds/fingerlings were harvested. Harvesting was done through one special type of net made by nylon which is locally called “Mosari jal” for Harvesting and performed in the early morning (around 6:00 am).
2.2.3.Statistical analysis T-test, ANOVA and correlation of limnological aspects, growth performance and fish production of the ponds under three treatments were calculated using SPSS package programme.