Optimizing fertilizer dose for rearing stage production of carps under polyculture

Using seven different input doses (105, 211, 422, 844, 1689, 3378 and 6757 g tank(-1) (4.5 m3) week(-1) of mixed fertilizers (poultry droppings [PD], cattle manure [CM], single super phosphate [SSP] and urea [U]) with a fixed CNP ratio of 88.6:7.5:1 and a stocking density of 35,500 fish ha(-1), yield of carps under polyculture (catla, bata and common carp) in a rearing stage was evaluated. Advanced fry of catla (1.2 +/- 0.04 g), bata (0.99 +/- 0.05 g) and common carp (1.3 +/- 0.06 g) were introduced at the rate of 16 fish tank(-1) (catla 4/bata 6/common carp 6) or equivalent to 35,500 fish ha(-1) and reared for 120 days. Samples of water were collected and monitored for primary productivity, planktons and water quality parameters (temperature, pH, dissolved oxygen, chemical oxygen demand, organic-C, orthophosphate (OP), soluble reactive phosphate, ammonium-N, nitrite-N and nitrate-N) and sediment total phosphate at weekly or biweekly intervals. The fish yield tended to rise with increase in fertilizer dose showing maximum yield at 844 g tank(-1) week(-1) (D96: 96,000 kg ha(-1) year(-1)) and decline thereafter. Total fish mortality was encountered when the dose was between 3378 and 6757 g tank(-1) week(-1). Using ecological and economical sustainability as the criteria, optimum dose of fertilizer for fish yield was found to be at 422 g tank(-1) week(-1) (D48: 48,000 kg ha(-1)). This was evident from the cost-benefit analysis (CBA) showing maximum net revenue at 422 g tank(-1) week(-1) (D48: 48,000 kg ha(-1)) and total revenue at 844 g tank(-1) week(-1) (D96: 96,000 kg ha(-1)). Critical appraisal of the data indicated four nutrient levels of fertilizer dose responses; poor growth of both fish and algae under first nutrient level (105-422 g tank(-1) week(-1)); their accelerated growth under second nutrient level (>422 to 844 g tank(-1) week(-1)); retarded fish growth but continued algal growth under third nutrient level (>844 to 1689 g tank(-1) week(-1)) and poor growth of algae coupled with total fish mortality at fourth nutrient level (>1689 to 6757 g tank(-1) week(-1)).