Cortisol and immune characteristics in rainbow trout (<i>Oncorhynchus mykiss</i>) selected for high or low tolerance to stress

Abstract

The influence of exposure to stressors on cortisol and the non‐specific immune traits lysozyme and serum haemolytic activity were examined in second generation rainbow trout ( Oncorhynchus mykiss ) selected for either high or low serum cortisol level following a confinement stress. Lysozyme and serum haemolytic activity were also assessed, together with levels of specific antibodies against Aeromonas salmonicida A‐layer, Vibrio salmonicida O‐antigen and Vibrio anguillarum O‐antigen, following injection of vaccines against these pathogens. Significant differences in mean cortisol levels between the two selection lines were observed, but in only one of two stress experiments was the ‘high‐stress’ line found to have the higher cortisol level; in the other experiment the ‘high‐stress’ line had significantly lower cortisol levels than the ‘low‐stress’ line. Lysozyme levels were in four of four assessments higher in the high‐stress line than in the low‐stress line, whereas components of serum haemolytic activity tended to be lower in the high‐stress line than in the low‐stress line. Levels of specific antibodies against all three bacterial pathogens were elevated following the injection of the vaccines. Only antibody production against A. salmonicida A‐layer was significantly different between the two lines, the higher production of antibody being in the high‐stress line.