GENETIC TESTING OF COMMON CARP IN CAGES .3. COMPARISON OF THE EFFECT OF GRADING AND MULTIPLE NURSING ON GROWTH

The effects of initial weight differences between groups of common car p, generated by grading and multiple nursing, on their weight gains we re compared. Grading is carried out in the commercial culture of some salmonids and eels. Multiple nursing is an experimental technique, per formed to remove the bias in observed weight gains of different geneti c groups in communal testing. In this investigation, multiple nursing and grading, carried out sequentially on a number of genetic groups of common carp, enabled dividing each group into four samples. Their gro wth in cages was compared in conditions of communal and separate testi ng. The association between differences in mean initial weight and dif ferences in mean weight gain were estimated by a factor (b + a/d), whe re b = the linear coefficient of regression of weight gain differences on initial weight differences, a = the y-intercept of this regression line, and d = difference in initial weight between the larger and sma ller samples. The results indicate the existence of a genetic componen t, differentiating graded weight classes and absent between multiply n ursed samples. They also indicate the effect of competition among test groups in communal testing, influencing their weight gains. The facto rs (b + a/d) were monitored by applying them to adjust observed weight gains. This study indicates that only multiple nursing generates true correction factors for adjusting observed weight gains in genetic tes ting programs.