Coupling between stress coping style and time of emergence from spawning nests in salmonid fishes: evidence from selected rainbow trout strains (Oncorhynchus mykiss).

Correlations between behavioral and physiological traits, often referred to as stress coping styles, have been demonstrated in numerous animal groups. Such trait variations often cluster in two contrasting styles, with animals characterized as either proactive or reactive. In natural populations of salmonid fishes, emergence from spawning nests, when fry establish a territory and shifts from exogenous to endogenous feeding, is a crucial niche shift with a high selection pressure. The timing of this event is correlated to behavioral and physiological traits such as aggression, boldness/shyness, dominance, and metabolic rate; resembling those of proactive and reactive stress coping styles. In farmed fish populations, however the relation between emergence and stress coping styles seems to be absent, an effect which has been related to lack of selection pressure during emergence. In the present study two rainbow trout strains genetically selected as LR (low-responsive) and HR (high-responsive) trout, characterized with proactive (LR) and reactive (HR) stress coping traits, was used to further investigate the relationship between the time of emergence and stress coping style in salmonid fishes. For this task LR and HR larvae were hatched in mixed batches, and thirty individuals from the earliest and latest 25% of emerging larvae were randomly collected. Thereafter, a line specific genetic marker was used to distinguish the proportion of LR and HR occurring in early and late fractions. The result demonstrates a higher proportion of LR fry in the early fraction in comparison to the HR fry, which emerged at a higher proportion during the late period. Early emerging individuals had larger yolk reserves at emergence, lending further support to a relationship between emergence times, yolk reserves at emergence and stress coping styles in salmonids. Smaller larval bodies in early compared to late emerging individuals suggest that this difference in yolk size reflects differences in developmental stages at emergence. These data suggests that a genetic link between emergence time and stress coping style persists in captive salmonid fishes.