Moav, R., Genetics Department, The Hebrew University, Jerusalem, Israel
Wohlfarth, G.W., Fish and Agriculture Research Station, Dor, Israel

Ten experiments were carried out during the period 1961-69. In each experiment a pair of genetically different groups of carp was tested simultaneously for differences in growth rate in mixed ponds as well as in separate ponds. The aim of the experiments was to estimate the genetic correlation of relative growth rate under inter-group competition in mixed ponds and in the absence of intergroup competition in separate ponds. The estimate of the genetic correlation coefficient was found to be 1.0, and that of the regression coefficient of growth rate in mixed ponds on growth rate in separate ponds was 2-0. The intergroup genetic variance in mixed ponds was four times higher than that of the variance in separate ponds. The higher variance in mixed ponds was partitioned into a component dependent on growth rate in separate ponds and a second independent component. The high genetic correlation means that the dependent component could account for all the amplification of the variance in the mixed ponds and that, apparently, in carp there are no specific genetic factors affecting growth rate under competition. The practical implications of this result for carp breeding is that selection for the improvement of yield capacity can be performed more effectively in mixed ponds than in separate ponds. A theoretical model for analysis of competition is presented and discussed. © 1974 The Genetical Society of Great Britain.