Cortisol profiles in sockeye salmon: Sample bias and baseline values at migration, maturation, spawning, and senescence

Accurate and controlled methods to measure physiological stress are crucial to effectively monitor and assess the health of wildlife populations and evaluate resilience to external stressors. Glucocorticoids, particularly cortisol, are frequently used to measure stress in fish. While measurements of cortisol concentrations provide a powerful indicator of physiological stress, there are important considerations in accurately measuring and interpreting results. We assessed methods to capture and sample wild populations of salmonids and evaluated potential biases from sampling disturbance. We present results of a stress series and suggest approaches to mitigate bias associated with sampling disturbance. Studies on physiological stress in salmonids often focus on particular life stages (e.g. outward migration to marine waters, return migration to freshwater systems), or processes (e.g. fisheries interactions, spawning success), characterized by dramatic physiological challenges related to the developmental stage of the fish and the external environment. Such pressures influence baseline cortisol levels and complicate efforts to interpret the effects of additional external stressors. We present a profile for naturally occurring shifts in cortisol levels at migration, reproductive maturation, spawning, and senescence. This profile provides a crucial baseline for use as reference in evaluating physiological stress in Pacific salmon during crucial life stages. Our findings provide guidance for sampling wild salmonids and highlight the need for caution in interpreting cortisol in the context of physical challenges and physiological developments relevant to their complex life history.