PREDICTABILITY OF INDIVIDUAL GROWTH-RATES IN TILAPIA

Growth curves of 1370 individually tagged fingerlings (initial body we ight 1.8-14.6 g; age 78-119 days) of seven strains of Oreochromis nilo ticus: Egypt, Ghana, Senegal, 'Israel', 'Singapore', 'Taiwan' and 'Tha iland', reared communally in fertilized earthen ponds, were followed b y sampling every 3 weeks during a 90-day production cycle. Predictabil ity of growth rates was determined by serial correlation of individual growth performance and also by following different size groups of ind ividuals. Positive correlations were observed between size at stocking and size at harvest. The magnitude of correlations, however, decrease d towards the end of the production cycle. Predictability of final bod y weight was highest when body weights of males and females were great er than 33 g and 25 g, respectively. Onset of sexual maturation seemed to reduce predictability. Divergence of growth rates between sexes se t in early in the life cycle, before sexing based on external morpholo gy became possible. Sexing of individuals by size-grading had low succ ess at body weights less than 15 g. A significant result of this study is that growth performance was not affected by initial size (and age) differences.