Determination of Fertilization Rate for Optimum Pond Productivity and Fish Growth in Inland Saline Groundwater Ponds: Monoculture of Grey Mullet and Milkfish

The effects of four rates of application of organic fertilizer (cow-dung) along with supplementary feed- ing were studied on water quality and pond productivity in terms of plankton production and fish biomass in inland saline groundwater ponds. Two experiments were conducted. In experiment 1, ponds were fertilized (at 5000, 7500, 10000 and 12500 kgha -1 •y -1 ) and stocked (at 6000 ha -1 ) with Mugil cephalus, while in experiment II, Chanos chanos was stocked (at 6000 ha -1 ) and ponds were fertilized at 2500, 5000, 7500 and 10000 kgha-1•y -1 . The growout period for both the experiments was 100 days. Irrespective of the species stocked, significantly (P<0.05) high growth performance of grey mullet and milkfish in terms of biomass and mean fish weight gain were observed in ponds fertilized at 7500 kgha -1 •y -1 of cow-dung. Significantly (P<0.05) high values of parameters indicative of productivity (viz. turbidity, alkalinity, net primary productivity-NPP, Chloro- phyll a concentration and plankton population) also coincided with the high fish growth. Nutrient release (o-PO4, NO3-N and total kjeldahl nitrogen) also remained significantly (P<0.05) high in ponds fertilized at 7500 kgha -1 •y -1 . At higher or lower rates of fertilization, the values of most of these parameters remained low. Application of stepwise multiple regression models showed a significant and positive correlation of nutrients, NPP and plankton population with fish growth. Statistically also fish weight gain showed a significant positive correlation with nutrients (viz. NO3-N (r=0.66, P<0.001; r=0.36, P<0.05), o-PO4 (r=0.70, P<0.001; r=0.27, P<0.05), alkalinity (r=0.45, P<0.001; r=0.50; P<0.001) and Kjeldahl-N (r=0.37, P<0.05; r=0.45, P<0.001)), net primary productivity (r=0.56, P<0.001, r=0.49, P<0.001) and plankton density (r=0.51, P<0.001; r=0.73, P<0.001) in both the experiments, respectively. A decline in fish biomass and pond productivity at higher rates of fertilization (10000 or 12500 kgha -1 •y -1 ) has been attributed to high NH4-N and BOD5 levels. Further, based on estimated production function relating pond productivity (fish production, dissolved oxygen concentration and net primary productivity) to fertilizer loading rate (dosages of cow-dung), optimum fertilizer loading rates were computed and found to be 7500 kgha -1 •y -1 . Therefore, a dosage of 7500 kgha -1 •y -1 of cow-dung along with supplementary feeding appears to be optimum for obtaining high productivity in inland saline water ponds with salinity levels ranging between 13.0-21.0 ppt.