Yaeyama Station, National Center for Stock Enhancement (NCSE), Ishigaki, Okinawa 907-0451, Japan
Retention rate, feeding condition and predation of hatchery-reared juveniles of the coral trout Plectropomus leopardus released on a coral reef in Urasoko Bay, Ishigaki Island were investigated to evaluate the effectiveness of acclimation of juveniles to the natural environment before release. Two experiments were conducted and 2000 juveniles [mean total length (TL), 137 mm] and 900 juveniles (mean TL, 79 mm) were stocked in a net cage (400 m2) which was set on the release site in the first and second trials, respectively. After acclimation for 7 days in the net cage, acclimated juveniles (test group) were released with the control groups of 2000 juveniles (mean TL, 142 mm) in the first trial and 900 juveniles (mean TL, 86 mm) in the second trial. Test and control groups were distinguished by different color tags. The second release experiment could not be conducted satisfactorily due to a typhoon. Judging from the feeding incidence, changes of condition factor and visceral weight index of juveniles after release in the first trial, it was considered that juveniles of the test group were more acclimated to the natural environment than those of the control group. Furthermore, juveniles of the test group tended to be less vulnerable to predation by piscivorous animals than those of the control group due to the improvement of behavior of lurking in the coral rubble.