INFLUENCE OF NILE TILAPIA (OREOCHROMIS-NILOTICUS) STOCKING DENSITY INCAGES ON THEIR GROWTH AND YIELD IN CAGES AND IN PONDS CONTAINING THE CAGES

An experiment was conducted for 90 days at the Asian Institute of Tech nology in Thailand to investigate the appropriate stocking density of large Nile tilapia placed in cages in earthen ponds where small Nile t ilapia were stocked in open water to utilize the wastes derived from t he cages. Large male tilapia (141 +/- 11.1-152 +/- 2.1 g) were stocked at 30, 40, 50, 60, and 70 fish m(-3) in 4-m(3) net cages. One cage wa s suspended in each of 15 earthen ponds, and three replicates were use d for each density, Small male tilapia (54 +/- 2.3-57 +/- 1.2 g) were stocked at 2 fish m(-3) in open water of all ponds. Caged tilapia were fed twice daily at 3%, 2.5%, and 2% body weight day(-1) during the fi rst, second, and third month, respectively, with commercial floating p ellets containing 30% crude protein. Water quality was analyzed biweek ly. Stocking densities of caged tilapia had significant (P < 0.05) eff ects on the survival, growth, and food conversion ratio of caged tilap ia, and on the growth of open-pond tilapia. The survival of caged tila pia decreased from 91.4% +/- 5.0 to 57.2% +/- 8.1 with increased stock ing densities from 30 to 70 fish m(-3), while survival of pond tilapia was higher than 90.0% in all treatments. The average treatment mean w eights of tilapia harvested from cages ranged from 509 +/- 26.0 to 565 +/- 13.9 g. The growth of pond tilapia was quite slow, with daily wei ght gain increasing from 0.30 +/- 0.02 to 0.47 +/- 0.08 g per fish day (-1), is response to increased feed inputs to caged tilapia. The combi ned net yield of both caged and open-pond tilapia was highest in the t reatment with 50 fish m(-3). Water quality analyses indicated that the wastes from caged tilapia were insufficient to generate abundant natu ral food for the growth of open-pond tilapia.