THE ASSOCIATION BETWEEN INITIAL WEIGHT AND GROWTH IN TILAPIAS DURING COMMUNAL TESTING .1. POND TESTING

The association between variation in mean initial weight and mean weig ht gain was investigated in two tests involving year-old tilapia hybri ds tested in communal ponds. An understanding of this association is r equired to determine whether observed mean weight gains of different g enetic groups are biased by variation in their mean initial weight. Va riation in mean initial weight was generated during first year testing by stocking random samples of fry into a number of ponds receiving di fferent nutrient treatments. This resulted in samples differing in the ir mean weight as a result of environmental differences during their f irst year tests, equivalent to the 'multiple nursing technique' previo usly described. Two tests, carried out on year-old tilapias, generated inconsistent results. The mean difference in weight gain between the multiply nursed samples was zero in the first test. In the second one, mean weight gain of the small multiply nursed samples was significant ly greater than that of the large multiply nursed ones. These results do not indicate whether observed mean weight gains are biased by varia tion in mean initial weights, nor do they justify amending mean weight gains according to variation in mean initial weight.