In wild salmonid fish, individual behavioural traits have been suggested to be coupled with the timing of fry emergence form gravel spawning nests, in such a way that early emerging fish have shown to be more aggressive and to have a higher probability to become socially dominant than those fish emerging at a later stage. Besides aggression and dominance, other behavioural and metabolic traits such as boldness, metabolic rate or growth had also been coupled to emergence time. Altogether, early- and late-emerging fish have traits resembling those of proactive and reactive stress coping styles, respectively. Proactive fish are considered to be more resilient to stress. However, it is currently unclear if that coupling is maintained in farmed fish populations, which showed no consistent evidence of a clear relation between emergence time and stress coping style. In this study, fish were hatched and larvae were fractionated according their emergence time (Early fraction: first 20 % of fish to emerge; Intermediate fraction: mid 20 %; Late fraction: last 20 %). Several months later, the resilience against a mild stressor (30 min of high stocking density), along with the stress habituation ability was investigated in 5 g fish from the different fractions. Results showed that fish from different fractions played a similar neuroendocrine response to a novel stressor. Interestingly, the capacity of habituation to stress was however better in the fish from the early emergence fraction, which showed no cortisol response to the stressor after being exposed daily for 15 days to another mild acute stressor (1.5 min of air exposure). These results demonstrate that at least some behavioural differences related to emergence time exist in a domesticated trout population. The aquaculture related implications of these stress resilience differences are currently under study
