Effect of initial age variation on production of Florida red tilapia fry under intensive, brackishwater tank culture

Abstract. Growth and survival were compared among uniform- and mixed-age groups of Florida red tilapia fry during sex reversal in brackishwater (12ppt) tanks, under commercial-scale hatchery conditions. Three treatment groups of post-yolksac (i.e. free-swimming) stage fry of different age compositions were compared: (1) 100% fry collected at the free-swimming stage (FSF); (2) 100% fry obtained through artificial incubation of eggs and non-swimming sacfry (ENS); and (3) a mixed group consisting of 50% of each type (FSF+ENS). Fry were stocked into 530–1 cylindroconical tanks at a density of seven fish/l (3700/tank) and reared on an androgen-treated diet for 30 days. While specific growth rates (range = 17·1–17·8% body weight/day) did not differ (P > 0·05) among treatments, significant (P < 0.05) differences were observed for survival and feed conversion ratio (FCR). ENS showed highest survival (76·8%) and lowest FCR (1·14), FSF showed intermediate survival (59·2%) and FCR(l·33), while the mixed-age group (FSF+ENS) showed lowest survival (38·5%) and highest FCR (1·70). Under all treatments, a majority of fry losses were attributed to cannibalism. A highly significant ( P < 0·001) negative regression between survival and coefficient of variation of initial weights (CV = SD/) indicated that cannibalism was reduced by minimizing age/size variation at stocking. The CV of initial weight may be a useful parameter for predicting fry survival under intensive tank culture.