PREDICTABILITY OF INDIVIDUAL GROWTH-RATES IN TILAPIA

Growth curves of 1370 individually tagged fingerlings (initial body weight 1.8-14.6 g; age 78-119 days) of seven strains of Oreochromis niloticus: Egypt, Ghana, Senegal, 'Israel', 'Singapore', 'Taiwan' and 'Thailand', reared communally in fertilized earthen ponds, were followed by sampling every 3 weeks during a 90-day production cycle. Predictability of growth rates was determined by serial correlation of individual growth performance and also by following different size groups of individuals. Positive correlations were observed between size at stocking and size at harvest. The magnitude of correlations, however, decreased towards the end of the production cycle. Predictability of final body weight was highest when body weights of males and females were greater than 33 g and 25 g, respectively. Onset of sexual maturation seemed to reduce predictability. Divergence of growth rates between sexes set in early in the life cycle, before sexing based on external morphology became possible. Sexing of individuals by size-grading had low success 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.