Context dependent differences in growth of two rainbow trout (Oncorhynchus mykiss) lines selected for divergent stress responsiveness

The relative growth of two lines of rainbow trout (Oncorhynchus mykiss) selectively bred for divergent plasma cortisol responsiveness to a confinement stressor (high-responders, HR; low-responders, LR) was evaluated over two generations (F1, F2), both when reared separately and when reared in co-culture. There was no significant difference in growth between the lines when reared separately. However, when reared in co-culture, the LR line significantly out-performed the HR line in both the F1 and F2 generations. It is likely that these results are related to the divergent behavioural attributes that characterise the HR and LR lines, rather than any differences in physiology. The growth disparity between the lines in co-culture may be linked to the greater degree of competitiveness/aggressiveness exhibited by fish of the LR line relative to those of the HR line, or to another as yet unidentified divergent behavioural trait. The implications of these results for the exploitation of the LR line in aquaculture are discussed.