A STATISTICAL POWER ANALYSIS OF THE INTERNAL REFERENCE TECHNIQUE FOR COMPARING GROWTH AND GROWTH DEPENSATION OF TILAPIA STRAINS

Experiments were conducted to compare the growth and growth compensati on of three strains of juvenile Oreochromis niloticus. Ten full sib fa milies (10 replicates) per strain were split and grown under crowded a nd uncrowded conditions for 3 weeks (the treatment). Both treatments w ere then grown an additional 12 weeks under less crowded conditions (t he compensation). Standard length measurements were made at the end of crowding and the end of compensation. Each replicate included a size- matched control population of a fourth (red) reference strain. ANCOVA with the reference strain used as a cofactor revealed significant stra in effects on specific growth throughout the experiment. The reference strain removed most of the random among-replicate error variance as s hown by an increase of r(2) from 0.06 to 0.91 when it was included in the statistical models. If the reference fish had not been used, appro ximately 450 replicate families would have been needed to achieve the sensitivity of the present experiment (a difference of 7% among strain s significant at P=0.05). We conclude that the CLSU strain grows signi ficantly more slowly than the Israel and NIFI strains under the experi mental conditions, that the crowding effect was essentially eliminated after 12 weeks of compensation, and that the reference strain greatly improved the resolution of the strain-testing experiment. (C) 1996 Th e Fisheries Society of the British Isles