Pd/a Crsp Seventeenth Annual Technical Report

An experiment was conducted in twelve 200-m2 earthen ponds at the Asian Institute of Technology, Thailand, for 91 days from 11 September to 11 December 1998. The experiment was designed to determine the optimal rate of nitrogen fertilization in cool season, to determine which of the nitrogen fertilization rates evaluated to produce Nile tilapia had the greatest profitability, and to develop a full-cost enterprise budget for the fertilization level that resulted in greatest profitability. Treatment ponds were fertilized with TSP at a rate of 8 kg ha-1 wk-1, and with urea at 0, 10, 20, and 30 kg N ha-1 wk-1, respectively. Sex-reversed male Nile tilapia were stocked at 1,000 kg ha-1 at a size of 23.1 to 25.5 g in all ponds (4.1 fish m-2). Sodium bicarbonate was applied to all ponds weekly to attain and maintain the minimum alkalinity (75 mg l-1 as CaCO3) based on weekly measurement of alkalinity in pond water. The experiment showed that greater nitrogen inputs resulted in better growth performance of Nile tilapia. Growth in the treatment without N inputs declined after the first fish sampling, which was earlier than the decline (around day 70) in treatments with N inputs. During the entire culture period, the estimated fish biomass was highest in the treatment with 30 kg N ha-1 wk-1, intermediate in the treatments with 10 and 20 kg N ha-1 wk-1, and lowest in the treatment without N inputs. The highest gross yield of Nile tilapia was obtained in the treatment with 30 kg N ha-1 wk-1 (1,938 ± 257 kg ha-1, mean ± SE), intermediate in the treatments with 10 and 20 kg N ha-1 wk-1 (1,628 ± 190 and 1,755 ± 190 kg ha-1, respectively), and the lowest in the treatment without N inputs (818 ± 109 kg ha-1). The partial budget analysis indicated that the treatment with 30 kg N ha-1 wk-1 was most profitable. The full-cost enterprise budget showed that US$2.1 net return could be produced from a 200-m2 pond in this treatment during a three-month culture period. SEVENTEENTH ANNUAL TECHNICAL REPORT 44 3) Develop a full-cost enterprise budget for the fertilization level that results in greatest profitability. METHODS AND MATERIALS An experiment was conducted using a randomized complete block design in twelve 200-m2 ponds at the Asian Institute of Technology, Thailand. The experiment involved the culture of Nile tilapia using four nitrogen fertilization rates. All ponds were fertilized with triple superphosphate (TSP) at a rate of 8 kg ha-1 wk-1 and nitrogen (N), as urea, at 0, 10, 20 and 30 kg ha-1 wk-1. All treatments were in triplicate. Initial pond fertilization took place two weeks prior to stocking of fish. Sodium bicarbonate was added weekly to attain and maintain minimum alkalinity (75 mg l-1 as CaCO3) based on weekly measurement of alkalinity in pond water. Sex-reversed male Nile tilapia were stocked at 1,000 kg ha-1 at a size of 23.1 to 25.5 g in all ponds on 11 September 1998. The stocking density averaged 4.1 fish m-1 in all ponds. Water depth in all ponds was maintained at 1 m throughout the experiment by adding water weekly to replace evaporation and seepage losses. Most parameters of pond water quality were analyzed with column water samples taken from the center of each pond. Parameters of pond water quality, including total nitrogen (TN), total ammonium nitrogen (TAN), total phosphorus (TP), Secchi disk visibility, total alkalinity, and chlorophyll a, were analyzed at 1000 h during the second week (15 September 1998), midway (29 October 1998), and final week (8 December 1998) of the experiment using Standard Methods (APHA, 1980) modified by Egna et al. (1987). Dissolved oxygen (DO), pH, and temperature measurements were made on these three sampling dates at 0600, 1000, 1600, 1800, and 0600 h the following morning at 5-cm, 25-cm, 50-cm, and 75-cm depths in the water column. Total alkalinity and total hardness were determined weekly at 1000 h for calculating the amount of sodium bicarbonate required to maintain the minimum alkalinity defined as above. During the experiment approximately 10% of the initial stock was seined, counted, and weighed en masse biweekly for each pond. All fish were harvested on 11 December 1998 after 91 days of culture. Daily weight gain (g fish-1 d-1), yield (kg pond-1), and extrapolated yield (kg ha-1) were calculated. Fish biomass on the sampling dates was calculated by the measured mean fish weight from sampling and the estimated number of fish surviving. It was assumed that surviving fish number decreased linearly from the beginning to the end of the experiment. Data from the experiment were analyzed statistically by regression analysis and analysis of variance (ANOVA) using the SPSS 7.0 statistical software package. Differences were considered significant at an alpha level of 0.05. All means were given with ± 1 standard error (SE). A partial budget analysis was conducted to determine which fertilization rate yielded the greatest profitability, and a full-cost enterprise budget was developed for the fertilization rate that yielded the greatest profitability (Shang, 1990). The economic analyses were based on the current local market prices expressed in US dollar (US$1 = 40 baht) in Thailand. Prices of urea and TSP were $0.200 and $0.325 kg-1, respectively. Market value of Nile tilapia fingerlings around 30-g size was $1.50 kg-1. To simplify the analyses the prices for stocked and harvested fingerlings were fixed to US$1.50 kg-1. Total fixed cost in the full-cost enterprise budget was derived from a previous study (Engle and Skladany, 1992).