Reduced stress response in juvenile Chinook Salmon reared with structure

Abstract

Understanding how rearing environments influence the stress response is relevant to many fish management practices. This is particularly relevant in aquaculture, where it is important to be able to minimize the effects of stress to enhance animal wellbeing. The stress response is also interesting from a basic science perspective, where it is important to understand how environmental differences can influence phenotypic traits and the physiological reaction to stressors. We tested the hypothesis that rearing fish in a more complex environment would reduce their stress response when presented with stressors. Specifically, we tested for the effect of structure in rearing tanks on the stress response of juvenile Chinook Salmon (Oncorhynchus tshawytscha) exposed to a handling and crowding stressor, something like hatchery fish might experience during transportation to new locations. For both rearing treatments (with and without structure), we compared fish that were subjected to the stressor to fish sampled from the respective rearing tank as the control. For the stressed treatment, we subjected fish to net capture followed by three hours in lowered water levels in a novel tank and then moved to a second novel holding tank. This process was designed to mimic typical stressors fish may experience at a hatchery. We analyzed plasma collected before the start of the stressor, during the stressor, and throughout 20 h post-stressor for cortisol in both stressed and unstressed fish. For both rearing treatments, fish that experienced the stressor had elevated plasma cortisol compared to control fish. However, the cortisol levels of fish in the stressed treatment reared with structure were significantly less than fish reared without structure. Additionally, plasma cortisol levels in structure-reared fish returned to baseline by 20 h post-stress while fish reared without structure had not fully recovered. Our findings suggest that increasing complexity of the rearing environment might be advantageous as a way of minimizing effects of handling-type stressors.