Selection response of cortisol and lysozyme in rainbow trout and correlation to growth

Abstract

Progeny groups of rainbow trout ( Oncorhynchus mykiss Walbaum) selected for high or low post-stress levels of plasma cortisol, or similarly for high or low post-stress lysozyme activity, have been tested for their response to the selection. In four of the four stress exposures, individuals from the line selected for high cortisol responsiveness displayed significantly higher levels of post-stress cortisol than individuals of the low responding line. Phenotypic correlations of cortisol response between samplings, irrespective of line, were highly significant. The realised heritability of cortisol was 0.50, which is very similar to the estimated h 2 based on the parental generation. Only in the last two of the four stress experiments did the high lysozyme selected line exhibit significantly higher lysozyme activity than the low lysozyme line. The timing of vaccination may cause this, since the vaccine is known to affect lysozyme activity. Breeding values of parents were based on vaccinated fish only. The phenotypic correlations between samplings of lysozyme response were weaker than for cortisol, though still significant. The realised heritability of lysozyme was 0.32, which is also in agreement with the previously estimated h 2. The phenotypic correlations between cortisol and lysozyme in individual samplings were in cases of significance negative. There is qualified support for better growth performance in the low cortisol responding line as compared to the high responding line. The data are not conclusive as to establishing whether selection for altered post-stress lysozyme activity affects growth. In conclusion, the present data confirm that the progeny inherit stress-related traits identified in the parents; the response to selection for both cortisol and lysozyme is encouraging. The practical implications or gain of selecting for either trait under aquacultural conditions is still being resolved.