Genetic testing of common carp in cages. 3. Comparison of the effect of grading and multiple nursing on growth

Abstract The effects of initial weight differences between groups of common carp, generated by grading and multiple nursing, on their weight gains were compared. Grading is carried out in the commercial culture of some salmonids and eels. Multiple nursing is an experimental technique, performed to remove the bias in observed weight gains of different genetic 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 growth in cages was compared in conditions of communal and separate testing. The association between differences in mean initial weight and differences in mean weight gain were estimated by a factor ( b + a d ), where 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 smaller samples. The results indicate the existence of a genetic component, differentiating graded weight classes and absent between multiply nursed samples. They also indicate the effect of competition among test groups in communal testing, influencing their weight gains. The factors ( 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 testing programs.