Effects of Different Stocking Densities on Growth, Serum Steroid Hormone Concentrations, Gonadosomatic Index and Sex Reversal in the Rice Field Eel, Monopterus albus (Zuiew)

The effects of stocking density on various growth parameters and sex reversal in the rice field eel (Monopterus albus) were investigated by evaluating steroid hormone, the gonadosomatic index (GSI) and sex ratio. Fish (mean initial weight of 9.88 ± 0.70 g) were stocked to densities of 4, 20, 36, 52, 68, 84 or 100 fish/m2 in cages (2 m × 1 m × 1.5 m) in a pond, with five replicate cages for each density. Fish were fed for 420 days on a formulated isocaloric diet containing 40.22% crude protein and 12.86 MJ/kg. Serum E2 and T concentrations were determined by radioimmunoassays. There were no significant differences in final bodyweight (FBW) and daily weight gain (DWG) for fish in the groups containing less than 52 fish/m2, while FBW and DWG were significant lower in groups at 68 fish/m2 or more. Specific growth rate (SGR) differed significantly between the 84 and 100 fish/m2 groups compared with the 4, 20 and 36 fish/m2 groups. There was no significant difference in SGR in groups containing less than 84 fish/m2. There were no significant difference between the 4 and 20 fish/m2 groups in net yield (NY), but these groups did differ significantly from the other groups. The final condition factors of fish stocked at 84 or 100 fish/m2 were significantly lower than fish stocked at the other densities. E2 and T hormone concentrations declined as stocking density increased, while GSI values increased initially but then reduced with increasing stocking density. Female ratio and survival decreased with increasing stocking density. The proportion of male fish was significantly greater in the three highest stocking density groups. Therefore, high stocking density may promote sex change from female to male in M. albus.