A comparison of plasma glucose and plasma cortisol as selection markers for high and low stress-responsiveness in female rainbow trout

Two groups of female rainbow trout displaying consistently divergent plasma cortisol responses to a 3 h period of confinement were identified following five separate confinement episodes at monthly intervals. High-responding (HRC) and low-responding individuals (LRC) continued to display divergent cortisol responses to confinement up to 21 months after the start of the study (342 ± 34 ng ml-1 and 208 ± 21 ng ml-1 respectively at the final sample; p<0.01). HRC fish were significantly larger than LRC fish throughout the study period (533 ± 13 g and 422 ± 10 g respectively overall; p<0.001), although significant differences in specific growth rate (SGR) were apparent only at the start of the study. Individual fish were also selected from the same population on the basis of their plasma glucose levels following confinement (HRG: 189 ± 6; LRG 121 ± 3 mg dl-1; p<0.001). However, the two selection traits (cortisol and glucose) identified separate subsets of the experimental population. HRG fish were also significantly larger than LRG fish although this difference was not so pronounced as for the cortisol-selected fish. There was no reciprocal relationship between body weight and stress responsiveness; fish selected from the population on the basis of high or low body weight displayed no divergence in either cortisol or glucose responses to confinement. Differences in size and SGR may indicate that HR fish adapted more rapidly to changes in environmental and social factors at the start of the study than LR fish did.