| The adaptive glucocorticoid (GC) stress response exhibits substantial inter-individual variation that is thought to mediate behaviour and fitness but results remain inconclusive and sometimes contradicting. However, intra-individual variation in GC titres remains relatively unexplored and there exist few examples of links with direct fitness measures. I explored these issues in wild fish by measuring cortisol, the primary GC in fish, following exposure to experimental standardized stressors. In bluegill sunfish ( Lepomis machrochirus) stress-induced cortisol is repeatable overall and infra-individual variation is influenced by individual size and condition. In pink salmon (Oncohrynchus gorbuscha), fish dying pre-spawn exhibited elevated stress-induced cortisol relative to spawning fish and variability in baseline cortisol defined behavioural traits. Significant repeatability of stress-induced cortisol validates the use of this metric as an individual trait in wild fish. Furthermore, the observed differences in reproductive success associated with stress-induced cortisol in a semelparous species confirm a relationship between GCs and fitness. |
