Semi-intensive Culture of Red Tilapia in Brackishwater Ponds

An experiment was conducted at the Asian Institute of Technology, Thailand, to investigate effects of fertilization rates and salinity levels on the growth of sex-reversed Thai red tilapia (Oreochromis sp.). The experiment was designed to test two fertilization rates (28 kg nitrogen and 7 kg phosphorus ha-1 wk-1, N:P = 4:1; and 14 kg N and 7 kg P ha-1 wk-1, N:P = 2:1) and three salinity levels (10, 20, and 30‰). An additional treatment using optimized fertilization rates (28 kg N and 7 kg P ha-1 wk-1, N:P = 4:1) in freshwater ponds served as a control. Red tilapia fingerlings (20.2 to 23.7 g size) were stocked at 2.4 fish m-2 in 5-m2 cement tanks with soil bottoms. They were cultured for 160 days. Growth performance of red tilapia was better in brackish water than in fresh water. Growth of red tilapia in brackish water was inversely related to the salinity levels (r = –0.63, P < 0.05), decreasing significantly with increasing salinity. Best growth performance was achieved in the treatment with N:P ratio of 4:1 at 10‰ salinity. The highest net economic return was achieved in the treatment with N:P ratio of 2:1 at 10‰ salinity, and all treatments had positive returns. Preliminary trials using a single species of marine phytoplankton showed that growth of red tilapia fed with Chaetoceros sp. and Thalassiosira sp. was significantly better than those fed with Tetraselmis sp. and Chlorella sp., and the former two resulted in a significantly higher protein utilization efficiency than the latter two. The prey ingestion rate of red tilapia for Chaetoceros sp. and Thalassiosira sp. was significantly higher than that for Tetraselmis sp. and Chlorella sp. NINETEENTH ANNUAL TECHNICAL REPORT 90 tilapia and to investigate the nutritional value and prey consumption of specific marine phytoplankton as food organisms for Thai red tilapia. METHODS AND MATERIALS This experiment was carried out at the Asian Institute of Technology (AIT), Thailand, from June through November 2000 to determine an appropriate fertilization regime and salinity level for culture of Thai red tilapia in brackish water. The experiments were conducted in a randomized complete block design with a 2 × 3 factorial arrangement to test effects of two fertilization regimes (28 kg N and 7 kg P ha-1 wk-1, N:P = 4:1; and 14 kg N and 7 kg P ha-1 wk-1, N:P = 2:1) and three salinity levels (10, 20, and 30‰) on growth of Thai red tilapia. There were six combinations (treatments) and an additional treatment using the PD/A CRSP standard fertilization regime (28 kg N and 7 kg P ha-1 wk-1, N:P = 4:1) in fresh water (0‰) that served as a control. Three replicates were used per treatment. The experiment was conducted in twenty-one 5-m2 (2 × 2.5 m) cement tanks filled with 10 cm of soil on the bottom. The tanks were grouped into three blocks, and treatments were allocated randomly to tanks in each block. Thai strain red tilapia was used in the experiment. Sexreversed all-male Thai red tilapia fingerlings (20.2 to 23.7 g size) were purchased from a local hatchery and acclimated to appropriate salinity levels in acclimation tanks by raising the salinity level 5‰ every two days until the target salinity was reached. The acclimated Thai red tilapia fingerlings were stocked at 2.4 fish m-2 in all experimental tanks on 8 June 2000. During the experiment 50% of the initial tilapia stock was seined, counted, and weighed en masse biweekly for each tank. All fish were harvested on 15 November 2000, after 160 days of culture. Daily weight gain (g fish-1 d-1), yield (kg pond-1), and extrapolated yield (kg ha-1 yr-1) were calculated. All tanks were fertilized weekly with urea and triple superphosphate (TSP) to achieve the treatment dosage. Initial pond fertilization took place one week prior to fish stocking. Sodium bicarbonate was applied at 250 kg ha-1 in the third week. Salinity was regulated by trucking hypersaline water (150‰) to AIT and diluting to the appropriate concentrations. Salinity levels in all tanks were monitored weekly. Water depth in all tanks was maintained at 1 m throughout the experiment by adding water of appropriate salinity levels weekly to replace evaporation and seepage losses. All tanks were aerated for 24 hours daily using one airstone in each tank. Water samples integrated from the entire water column were taken biweekly near the center of each tank at approximately 0900 h for analysis of pH, alkalinity, total ammonium nitrogen (TAN), nitrite-nitrogen, nitrate-nitrogen, total Kjeldahl nitrogen (TKN), soluble reactive phosphorus (SRP), total phosphorus (TP), chlorophyll a, total suspended solids (TSS), total volatile solids (TVS), and plankton composition (Parsons et al., 1984; APHA, 1985; Egna et al., 1989). At the time of collecting water samples, Secchi disk visibility was measured using a Secchi disk, while temperature and dissolved oxygen (DO) were measured at the time of collecting water samples 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, 1000, 1400, 1600, 1800, and 0600 h once a month. Preliminary trials were also carried out to investigate the nutritional value and prey consumption of specific marine phytoplankton species, Tetraselmis sp., Chlorella sp., Thalassiosira sp., and Chaetoceros sp., for Thai red tilapia. The first trial on nutritional value was conducted over 15 days in twelve glass jars of 15-l volume with three replicates for each species. Each jar was filled with 15‰ salinity water and continuously aerated using an airstone throughout the experimental period. Thai red tilapia (1.42 to 1.61 g size) were stocked at 0.4 fish l-1 and fed twice daily with the same quantity (dry matter, DM) of each phytoplankton species, which were cultured in the AIT laboratory and were approximately three to four days old. Red tilapia growth was determined by bulk-weight at the beginning and end of the culture period. Water in jars was changed