World Aquaculture 9 50:50 percent males:females was obtained from the control group (Chakraborty et.al. 2007). Selection of the Site A sewage-fed pond at Jhagrashisa in East Kolkata Wetland (coordinates 22˚25’-22˚40’N latitude and 88˚20’- 88˚35’E longitude) was selected for the experiment. Transportation and Release of Fry in Cages The treated and control fry were transported to the field using oxygen packs. The oxygen packs containing the tilapia fry were at first floated in the pond water for about 30 minutes for temperature acclimatization after which the fry were released into the cages. The size of the cages was 1m x 1m x 1m. Fry were released in the cages in the early morning. Treated (0.76±0.016g) and control (0.38±0.028g) fry were stocked in separate cages at a density of 25 fry/cage in three replicates. Collection of Data Body weight (BW) of fish from both the groups was measured every four weeks from December 2005 to May 2006. Four water quality parameters, dissolved oxygen, free CO2, temperature and pH were measured following standard methods (APHA 1998) at the time of fish sampling. The mean weight gain per month for both the groups was also calculated at the end of the culture period. Statistical Analysis The results are presented as mean ± standard deviation. Fish growth rates were subjected to one-way analysis of variance (ANOVA) to test the effect of hormone treatment on growth parameters. Results and Discussion A linear increase in the mean body weight of the fish with respect to time throughout the culture period was observed for both the treated and control groups (Figure 1). This indicates that the environmental conditions for fish growth were favorable in the culture pond as previously mentioned by Mohammad (2006). After one month of rearing in the laboratory, the hormone treated groups attained better weight (0.76±0.016g) compared to the control groups (0.38±0.028g). Moreover, at the end of the culture period, the hormone treated monosex tilapia population obtained a significantly higher mean body weight than the control mixed-sex population (Figure 1). In addition, hormone treated fish showed significantly higher weight gains throughout the culture period (Figure 2). This result is in agreement with other similar studies (Guerrero and Guerreo 1975, Shelton et.al. 1978, Hanson et.al.1983, Muhaya 1985, Jae-Yoon Jo et.al.1988, Pandian and Varadaraj 1988). The increased growth performance of the androgen-treated sex reversed tilapia may be attributed to the phenomenon of muscular hypertrophy by increased muscle protein synthesis through testosterone treatment (Bhasin et. al. 2001). The total production obtained in five months of culture was 3.70 kg/m3 for the treated group and 2.41 kg/m3 in the controls. The production obtained was significantly higher in the treated group than in the control group. Thus, cage culture of hormone treated, monosex tilapia in a waste fed wetland Fig. 1. Comparative body weight (g) of control and hormone treated tilapia population during cage culture. Fig. 2. Comparative account of mean monthly weight gain (g) for control and hormone treated tilapia population during cage culture. was more beneficial than culture of mixed-sex tilapia reared under using the same culture method and conditions. From Figure 2 it is clear that for the androgen-treated group of fish, the mean monthly weight gain gradually increased with culture duration and was highest during the fourth month. Comparatively lower mean weight gain was obtained during the fifth month than the fourth month. This may be a result of a decrease in the residual effect of testosterone on somatic growth of the fish. On the other hand, for the control group, the monthly mean weight gain was highest in the third month of the culture period. Culture of that group during an additional two months again showed comparatively lower mean weight gain than that in the third month. Tilapia are reported to sexually mature at about three months (Chapman 1992). Hence, the reduction in weight gain after three months in the control tilapia population may have been a result of the utilization of most of the food energy for reproductive purposes. So it can be concluded that cage culture of hormone treated monosex tilapia for four months and mixed-sex control tilapia for three months in a waste fed ecosystem is optimum to get high production with a minimum culture period.
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