The Stress and Stress Mitigation Effects of Exercise: Cardiovascular, Metabolic, and Skeletal Muscle Adjustments

1. Introduction 2. Physiological Demands of Swimming Exercise and the Stress Continuum 2.1. Introduction 2.2. Neuroendocrine Aspects of Stress and Exercise 2.3. Energy Metabolism During Stress and Exercise 2.4. Cardiovascular and Respiratory Adjustments to Stress and Exercise 2.5. Limits of Swimming Exercise and Stress 3. Physiological Adaptations to Swimming and Relevance to Stress 3.1. Introduction 3.2. Effects of Swim Training on the Cardiovascular System 3.3. Effects of Swim Training on Feeding and Energy Metabolism 3.4. Effects of Swim Training on Skeletal Muscle Growth 3.5. Effects of Swim Training on Stress: Behavior, Health, and Welfare 3.6. Summary, Future Perspectives, and Key Unknowns Fish use swimming as their mode of locomotion and many species swim constantly to engage in feeding, migratory, reproductive, and predator avoidance behaviors. The vastly diverse lifestyles among fish species in different aquatic environments (eg, pelagic, benthic, anadromous) are reflected by extremely different capacities for swimming activity. Swimming, by way of increased activity of skeletal muscle and the cardiorespiratory systems, demands an increased production of metabolic energy to maintain homeostasis during and after exercise. Irrespective of swimming activity, a consistent feature of a stress response is the stimulation of the cardiovascular system and oxygen transfer and uptake to tissues. Fish must prioritize oxygen delivery in response to elevated metabolic states during stress and/or physical exercise. The provision and use of energy are fundamental determinants of physiological performance and swimming exercise can be viewed as both a potential physiological stressor and a stress-reducing mechanism. Swimming is generally classified as either burst or sustained according to intensity and duration. Burst swimming can impose significant stress upon many physiological systems, causing disturbances in metabolic, acid–base, osmotic, and electrolyte balance. In contrast, sustained swimming for extended periods does not lead to significant changes in circulating cortisol and catecholamines and yet can induce positive physiological responses and improved resistance to subsequent stressors. Specifically, sustained swimming in active species increases growth, improves aerobic performance, reduces cortisol levels, promotes schooling, and reduces aggressive interactions. In this light, restrictions in the natural swimming behavior imposed by aquaculture or a research setting may deprive fish of the physiologically beneficial effects of swimming and, consequently, may be stressful to fish. In this chapter we approach the topic of swimming as a potential stressor as well as a stress-reducing behavior that contributes to homeostasis and diverse phenotypic adaptations.