Supplemental Feeding for Semi-intensive Culture of Red Tilapia in Brackishwater Ponds

An experiment was conducted at the Asian Institute of Technology, Thailand, to investigate effects of feeding regimes on growth of sex-reversed Thai red tilapia (Oreochromis sp.). There were five different supplemental feeding regimes: 0, 25, 50, 75, and 100% of satiation. Red tilapia fingerlings (33.2 to 33.4 g size) were stocked at 62.5 fish m-3 in fifteen 0.8-m3 net cages suspended in a 200-m2 earthen pond and cultured for 90 days. The pond was maintained at 10‰ salinity and fertilized weekly at rates of 4 kg N and 1 kg P ha-1 d-1. Growth performance of red tilapia was significantly better in the feeding treatments than in the non-feeding treatment. Red tilapia growth and average feeding rate increased, but the Feed Conversion Ratio (FCR) and net economic return decreased with increasing percentages of satiation feeding levels from 25 to 100%. Considering low FCR, good growth and yield performance, high economic return, and potential for growing to greater size, 50% satiation feeding was the most efficient feeding rate. NINETEENTH ANNUAL TECHNICAL REPORT 98 Co., Ltd., Bangkok, Thailand) consumed by red tilapia from 0800 to 0900 h and 1400 to 1500 h every Saturday. The tested percentages of satiation feeding were used to calculate the amount of feed to apply for respective treatments from Sunday through Friday. Sex-reversed fingerlings of Thai red tilapia (33.2 to 33.4 g size) were purchased from a local farm and acclimated to 10‰ salinity in acclimation tanks by raising salinity level 5‰ every two days. The fingerlings were then stocked at 62.5 fish m-3 in all cages on 30 March 2001. During the experiment average weights of red tilapia were determined biweekly by bulk weighing 50% of the initial stock in each cage. Fish were randomly sampled by dip net. Red tilapia in every cage were harvested, counted, and bulk weighed on 28 June 2001 after 90 days of culture. Daily weight gain (g fish-1 d-1), yield (kg m-3), and extrapolated yield (kg m-3 yr-1) were calculated. The pond was fertilized weekly with urea and triple superphosphate (TSP) at 4 kg N and 1 kg P ha-1 d-1. Initial pond fertilization took place two weeks prior to stocking fish. Salinity was initially regulated by trucking hypersaline water (150‰) to AIT and diluting it to 10‰. Salinity was maintained at 10‰ and monitored weekly. Water depths in the pond and cages were maintained at 1.0 and 0.8 m, respectively, throughout the experiment by adding water at about 10‰ salinity weekly to replace evaporation and seepage losses. All cages were aerated for six hours daily from 0200 to 0800 h using one airstone in each cage. Integrated water samples were collected from the entire water column near the two ends and the center of the pond and also from each cage biweekly at approximately 0900 h. Analyses were done for pH, alkalinity, total ammonium nitrogen (TAN), nitrite-nitrogen (nitrite-N), nitrate-nitrogen (nitrate-N), total Kjeldahl nitrogen (TKN), soluble reactive phosphorus (SRP), total phosphorus (TP), chlorophyll a, total suspended solids (TSS), and total volatile solids (TVS) using standard methods (Parsons et al., 1984; APHA et al., 1985; Egna et al., 1987). At the time of collecting water samples, Secchi disk visibility was measured using a Secchi disk, while temperature and dissolved oxygen (DO) were measured with a YSI model 54 oxygen meter (Yellow Springs Instruments, Yellow Springs, Ohio, USA). Diel measurements for temperature, DO, and pH were conducted in each pond at 0600, 0900, 1600, 1800, 2200, and 0600 h once a month. Data were analyzed statistically using analysis of variance, paired-sample t-test, and linear regression (Steele and Torrie, 1980) with SPSS (version 7.0) statistical software package (SPSS Inc., Chicago, Illinois, USA). Differences were considered significant at an alpha of 0.05. Statistical analyses for survival rates (%) were performed on data after arcsine transformation. Mean values of survival rates in this text are listed in normal scale followed by their confidence limits. All other means are given with ± 1 standard error (SE). A partial budget analysis was conducted to determine economic returns of red tilapia in the different treatments (Shang, 1990). The analysis was based on farm-gate prices in Thailand for harvested red tilapia and current local market prices for all other items expressed in US dollars (US$1 = 40 baht). Farmgate prices of red tilapia were $0.50 and $0.75 kg-1 for the sizes 100 to 200 g and 200 to 300 g, respectively. Market prices of sexreversed red tilapia fingerlings ($0.0125 piece-1), electricity ($0.05 kWh-1), pelleted floating feed ($0.4875 kg-1), urea ($0.1875 kg-1), and TSP ($0.3125 kg-1) were applied to the analysis. The calculation for cost of working capital was based on an annual interest rate of 8%.