Principles of Genetics and Breeding

11.1.4 Correlated traits

• The phenotypes of organisms comprise thousands of characters and many of these characters are interlinked in one way or another. Therefore, there is a danger that if we select for one particular trait which is of value in aquaculture, we might inadvertently be selecting for another trait which could be deleterious. For example, fast growth rate in salmon could be correlated with age at maturity such that, beyond a certain point, the advantages of fast growth will be outweighed by the tendency of these fish to mature earlier than wanted by the fish farmer. Fortunately, where such tests have been carried out in fish, high correlations have been found between feed conversion efficiency and growth rate and low correlations between growth rate, and survival.
• One unwanted correlation, however, is between body weight and fat content in salmonids which means that it is not easy to artificially select for increased growth rate without also increasing the body fat content.
• In channel catfish (Ictalurus punctatus) the time of maturation is dependent on age rather than size. This can create difficulties as selected fast growing fish in some strains reach such a large size before becoming mature that they are too big to be accommodated in conventional spawning refuge chambers. However, one useful genetic correlation in this species is that between the traits of body density and dress out percentage (the proportion of the fish’s weight remaining after gutting). Normally, dress out percentage can only be assessed after slaughter but, because of the strong genetic correlation of this trait with body density, measurements of body density on living fish can be used to estimate dress out percentage.