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 followin g five separate confinement episodes at monthly intervals. High-responding (HRC) and low-responding individuals (LRC) continued to display divergent c ortisol responses to confinement up to 21 months after the start of the stu dy (342 +/- 34 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 and 422 +/- 10 g, respectively, overall; p < 0.00 1), although significant differences in specific growth rate (SCR) were app arent only at the start of the study. Individual fish were also selected fr om the same population on the basis of their plasma glucose levels followin g confinement (high-responding glucose (HRG): 189 +/- 6; low-responding glu cose (LRG) 121 +/- 3 mg dl(-1); p < 0.001). However, the two selection trai ts (cortisol and glucose) identified separate subsets of the experimental p opulation. HRG fish were also significantly larger than LRG fish although t his difference was not so pronounced as for the cortisol-selected fish. The re was no reciprocal relationship between body weight and stress responsive ness; fish selected from the population on the basis of high or low body we ight displayed no divergence in either cortisol or glucose responses to con finement. Differences in size and SGR may indicate that HR fish adapted mor e rapidly to changes in environmental and social factors at the start of th e study than LR fish did. (C) 1999 Elsevier Science B.V. All rights reserve d.