Expression of Family Differences through Within‐Lot Competition in Juvenile Rainbow Trout Oncorhynchus mykiss

Effects of family differences on survival, length, weight, growth, and pyloric caeca number for juvenile rainbow trout Oncorhynchus mykiss were analyzed in binary associations of families (sib‐groups), using a genetic variant with “golden” phenotype as a marker. In such an association, the performance of a family i mixed with family j depends on its own ability (P 1 ), on the influence of the associated family (A J ), and on possible effects specific to the ij association. Experimental results showed that specific effects were negligible, and that the A J influence was significant on body length and weight (after 3 mo of age), and opposite to the P J value of the same family. Growth showed the same feature, but short of significance. The genetic marker caused some detrimental pleiotropic effects on length, weight, and growth, and some limited interactions with the familial factors studied. Results suggest that, when practicing selection, aquaculturists should prevent early size differences from becoming enhanced by competition, and should consider periodic size grading and multi‐step selection. Results also suggest that family interactions should be accounted for when analyzing mixed‐family genetic evaluation data.