Understanding the consequences of varying levels of stress on the fitness and behavior of spawning pink salmon

The role of corticosteroids in the final phases of maturation of Pacific salmon is poorly understood. Cortisol elevations during upriver migrations occur when fish are faced with challenging conditions and have been shown to correspond with a decrease in reproductive hormones, potentially impairing gonadal development. Theoretically, one might postulate that the stress response of semelparous salmon would be muted during this period such that they can maximize fitness. However, there is some suggestion that it may play a role in the maturation of oocytes in females. Fish on the spawning grounds are also entering their senescent phase where many physiological processes, such as hormone regulation, are altered as a result.  By using pink salmon (Oncorhynchus gorbuscha) as a model, we determined how varying degrees of stressors can affect fitness and behaviour on the spawning grounds. Specifically, we used cortisol and metyrapone (corticosteroid synthesis inhibitor) implants to experimentally determine how cortisol can influence reproductive hormones, behaviour, and success. It was determined that our most extreme stressor, a cortisol implant, had a negative influence on reproductive success and behaviour.  We discuss our findings in the context of understanding the role of cortisol during the final phases of maturation in semelparous Pacific salmon.