Indicators of reproductive performance in rainbow trout Oncorhynchus mykiss (Walbaum) selected for high and low responsiveness to stress

A number of indicators of reproductive performance were assessed in rainbow trout divergently selected on the basis of their plasma cortisol response to a standardized stressor. For both male and female rainbow trout Oncorhyn chus mykiss (Walbaum), body weight was significantly related to stress resp onsiveness. High-responding (HR) females were significantly heavier than lo w-responding (LR) females overall, and HR males were significantly heavier than LR males overall. There was no significant difference in the specific growth rate (SGR) of HR and LR female fish during the experimental period, whereas in contrast the mean SGR of HR males was significantly greater than that of LR males. The divergent confinement-induced levels of blood cortis ol in HR and LR groups were stable for more than 1 year after selection. Th ere was no significant difference in the extent to which confinement stress reduced blood oestradiol-17 beta levels in HR and LR females, despite a la rge difference in relative blood cortisol levels. The onset and rate of ovu lation was similar within the HR and LR groups. Differences in fecundity, r elative fecundity and egg volume and weight were wholly attributable to the different mean body weights of the HR and LR fish. There was no difference between the sperm counts of HR and LR males. There was significantly highe r mortality among eggs derived from HR parents overall between fertilizatio n and hatch, but maximum mean mortality did not exceed 12%. Mean time to ey eing in the HR groups was 224 degree days, and in the LR groups was 244 deg ree days. Hatching commenced at 344 and 347 degree days in the HR and LR gr oups, respectively, and was complete within 412 and 416 degree days respect ively. Overall, selection for stress responsiveness was associated with no significant benefits or costs in reproductive performance.