Effect of Stocking Size and Nutrient Inputs on Productivity of Oreochromis Shiranus in Ponds

A study to investigate the effects of three different stocking sizes (5, 10, 20 g) and two isonitrogenous input regimes (maize bran × urea and napier grass × urea) on the production of Oreochromis shiranus was conducted between June and November 2000 at the Malawi National Aquaculture Center. Six treatments (three stocking sizes × two input regimes), each in triplicate, were used in the study. Inputs were applied to ponds stocked with fish at the three stocking sizes such that each input regime supplied 20 kg N ha-1 wk-1. Fish were stocked at 2 fish m-2 and sampling (mean weight of 100 fish) was conducted biweekly. Water quality parameters (dissolved oxygen, pH, electrical conductivity, and Secchi disk visibility) were measured weekly, and total ammonia nitrogen and chlorophyll a were measured biweekly. The experiment was conducted over a period of 150 days. The two isonitrogenous input regimes did not significantly affect fish net yield and growth rate. There were significant differences (P < 0.05) in fish growth rate and net yield between treatments. The highest fish growth rates and production (net yield) were achieved in ponds when fish were stocked at 5 g and either input regime was used, while ponds stocked with 20-g fingerlings and supplied with either napier grass × urea or maize bran × urea had the lowest net mean yield. There were significant differences (P < 0.05) in gross margins between treatments, with treatments where fish were stocked at 5 g and napier grass × urea were applied giving higher gross margins than the rest of the treatments. Mean fish survival rate was not significantly different between treatments. Results from this study suggest that stocking Oreochromis shiranus at 5 g results in higher fish production and gross margins compared to stocking larger fish. The results further show that under conditions where inorganic fertilization is used, substituting napier grass for maize bran increases profitability without affecting overall fish yield. NINETEENTH ANNUAL TECHNICAL REPORT 162 The objectives of this study were to: 1) Determine the effects of different fish stocking sizes on O. shiranus productivity; 2) Evaluate the effect of two different isonitrogenous input regimes on O. shiranus productivity and profitability; and 3) Recommend, based on objective 2 above, stocking strategies that optimize fish productivity and input regimes on fish yield. METHODS AND MATERIALS Eighteen randomly selected 200-m2 ponds located at the Malawi National Aquaculture Center in Zomba district were used for this study. Three fingerling sizes (5, 10, and 20 g) and two isonitrogenous input regimes (urea × maize bran and urea × napier [Pennisetum purpureum] grass) served as treatments in this study (Table 1). The total Kjeldahl nitrogen contents of the input materials used in these treatments are shown in Table 2. Each treatment was replicated three times. Fish were stocked at a rate of 2 fish m-2. The experiment ran for 150 days (22 June to 24 November 2000). Water was added weekly to replace losses due to evaporation and seepage. Water temperature was taken daily, while dissolved oxygen (DO), pH, electrical conductivity, and Secchi disk visibility were measured weekly. Total ammonia nitrogen and chlorophyll a were analyzed every fortnight using standard methods (APHA, 1989). Ponds were harvested by seining followed by complete draining to remove any fish that remained in the pond sediments. Data were analyzed using Statistical Analysis Package for Scientists (SAS Institute, Inc., 1988). Specific fish growth rate (g d-1) and extrapolated gross and net yield (kg ha-1 yr-1) were calculated for each replicate pond. Gross margins (Upton, 1987) were calculated using input cost estimates for fingerlings, maize bran, urea fertilizer, and gross revenue from fish sales. Treatment means (± 1 standard deviation) were considered to be significantly different at an alpha level of 0.05. Means that were significantly different were separated using the Duncan’s Multiple Range Test (Montgomery, 1997). Gross margins were calculated using the following formula (Upton, 1987): GM = TR – (Fc + Mc + FGc) where GM = gross margin (Malawian Kwacha ha-1 yr-1); TR = total revenue (MK ha-1 yr-1) from fish sales; Fc = cost of urea fertilizer (MK ha -1 yr-1); Mc = cost of maize bran (MK ha -1 yr-1); and FGc = cost of fingerlings (MK ha -1 yr-1). The cost of labor required for napier grass application was not included in the analysis because farmers use their own and family labor for this operation.