Culture of Mixed-sex Nile Tilapia with Predatory Snakehead

An experiment was conducted in eighteen 200-m2 fertilized earthen ponds at the Asian Institute of Technology, Thailand, from March through October 2000. This experiment was designed to assess the efficiency of snakehead (Channa striata) in controlling recruitment of mixed-sex Nile tilapia (Oreochromis niloticus) in ponds and to assess growth and production characteristics of Nile tilapia in monoculture and polyculture with snakehead. There were six treatments: A) monoculture of sex-reversed allmale tilapia; B) monoculture of mixed-sex tilapia; C) polyculture of snakehead and mixed-sex tilapia at 1:80 ratio; D) polyculture of snakehead and mixed-sex tilapia at 1:40 ratio; E) polyculture of snakehead and mixed-sex tilapia at 1:20 ratio; F) polyculture of snakehead and mixed-sex tilapia at 1:10 ratio. Sex-reversed and mixed-sex Nile tilapia were stocked at 2 fish m-2 at sizes of 10.5 to 11.6 g and 7.2 to 8.1 g, respectively. Results show that snakehead were able to completely control Nile tilapia recruitment at all tested predator:stocked-prey ratios, and the best predator:stocked-prey ratio was 1:80. The addition of snakehead into Nile tilapia ponds did not result in significantly greater tilapia growth, but it significantly lowered total net and gross yields of adult plus recruited tilapia. Snakehead growth was density-dependent, decreasing significantly with increasing stocking densities. While snakehead biomass gain was not significantly different at stocking densities from 0.025 to 0.1 fish m-2, the gain was significantly lower at a stocking density of 0.2 fish m-2. The present experiment demonstrates that snakehead are able to control Nile tilapia recruitment completely and provide an alternative technique for Nile tilapia culture. NINETEENTH ANNUAL TECHNICAL REPORT 68 METHODS AND MATERIALS The experiment was conducted using a randomized complete block design in eighteen 200-m2 earthen ponds at the Asian Institute of Technology (AIT), Thailand. There were six treatments with triplicates, one in each block: A) monoculture of sex-reversed all-male tilapia; B) monoculture of mixed-sex tilapia; C) polyculture of snakehead and mixed-sex tilapia at 1:80 ratio; D) polyculture of snakehead and mixed-sex tilapia at 1:40 ratio; E) polyculture of snakehead and mixed-sex tilapia at 1:20 ratio; F) polyculture of snakehead and mixed-sex tilapia at 1:10 ratio. The Chitralada strain (Thai strain) of Nile tilapia was used in the present experiment. Nile tilapia fry were obtained from the AIT Hatchery, while snakehead fingerlings were purchased from a local market. Sex-reversed Nile tilapia (10.5 to 11.6 g size) and mixed-sex Nile tilapia (7.2 to 8.1 g size) were stocked at 2 fish m-2 in treatment A and treatments B through F, respectively, while snakehead (88.0 to 100.0 g size) were stocked at 0.025, 0.05, 0.1, and 0.2 fish m-2 in treatments C, D, E, and F, respectively, on 30 March 2000. During the experiment, approximately 10% of the initial Nile tilapia stock was seined, counted, and weighed en masse biweekly for each pond. All fish were harvested on 10 October 2000 after 194 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 ponds were dried for one month prior to the experiment to eliminate wild fish. Each pond dike was enclosed with a fine mesh net fence about 1 m tall, supported by bamboo sticks, with the lower end of the net buried in the dike soil to prevent entry of wild fish and movement of stocked snakehead from one pond to another. All ponds were fertilized with urea and triple superphosphate (TSP) at a rate of 28 kg nitrogen (N) and 7 kg phosphorus (P) ha-1 wk-1. Initial pond fertilization took place two weeks prior to fish stocking. Water depth in all ponds was maintained at 1 m throughout the experiment by adding water weekly to replace evaporation and seepage losses. Integrated water samples were taken biweekly from the entire water column near the center of each pond at about 1000 h for analyses 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), and total volatile solids (TVS) (APHA et al., 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 with a YSI model 54 oxygen meter (Yellow Springs Instruments, Yellow Springs, Ohio, Treatments Parameters A (Sex-reversed) B (Mixed-sex) C (1:80) D (1:40) E (1:20) F (1:10) STOCKING Density (fish m) 2 2 2 2 2 2 Total Number (fish pond) 400 400 400 400 400 400 Mean Weight (g fish) 9.0 ± 0.4 7.4 ± 0.1 7.7 ± 0.1 7.5 ± 0.3 7.6 ± 0.0 7.7 ± 0.3 Total Weight (kg pond) 3.62 ± 0.17 2.96 ± 0.05 3.08 ± 0.04 3.01 ± 0.12 3.02 ± 0.01 3.07 ± 0.13