THE ASSOCIATION BETWEEN INITIAL WEIGHT AND GROWTH IN TILAPIAS DURING COMMUNAL TESTING .2. CAGE TESTING

Culture of tilapias in cages, both freshwater and marine, has been stu died mainly during the last decade. Genetic testing is required to com pare different species and stocks to isolate the more productive ones. This involves investigating adequate testing techniques, including th e possible bias resulting from variation in mean initial weight among the genetic groups tested. In the investigation here described, differ ent genetic groups were stocked communally into a series of net cages. Stocking each cage with two random samples of one of the groups previ ously multiply nursed to two different mean initial weights, enabled i nvestigating the influence of variation in mean initial weight on mean weight gain. The results of this test were inconsistent. In some cage s samples with the higher mean initial weight showed the higher weight gain, whereas the opposite was true in other cages. These results do not justify transforming mean observed weight gains according to varia tion in their mean initial weight. Thence growth of different genetic groups of tilapias in communal cages may be compared by their observed (untransformed) weight gains.