A genotype-environment interaction for growth rate in the common carp, growing in intensively manured ponds

The growth rate of three crossbred groups of the common carp was estimated in five different environments. These groups consisted of carp of the Chinese and the European race and an inter-racial Chinese × European crossbred. The environments differed in stocking rates of fish per unit area and in the nutrients supplied to the fish. By manipulating these management factors, mean growth of all the carp progenies stocked varied between 2 g per fish per day in the poorest environment (high stocking rate, poultry manure as the only nutrient input) to over 7 g per fish per day in the best environment (relatively low stocking rate, poultry manure plus high protein feed pellets). The mean growth of all the carps in a given environment is taken as an estimate of the quality of that environment. The fastest growth in poor, intermediate and good environments was shown by Chinese, Chinese × European and European carps respectively. Response curves were constructed by plotting, for each genotype, its growth in each of the tested environments. The differences in slopes and intercepts of the response curves of each genotype demonstrate the genotype-environment interaction. The coefficients of regression of growth on environmental mean were 0.56, 1.03 and 1.29 for the Chinese, Chinese × European and European carps respectively while their respective intercepts (on the Y-axis) were 0.96, 0.39 and −0.85. These results show that carp of the Chinese and European races are each adapted to the conditions in which they were domesticated and the potential importance of the inter-racial crossbred for intermediate conditions. Due to the differences in slopes and intercepts between the response curves they intersect at different places. This demonstrates that, for the trait growth rate the dominance relationships between the two races are a function of the environment. In a poor environment the Chinese is dominant over the European, in a good environment the European is dominant over the Chinese, while in intermediate environments the inter-racial crossbred is overdominant.