Swimming in fish: integrating redox homeostasis and immune responses

Swimming is an essential behaviour in fish, underpinning crucial biological functions, such as locomotion, foraging, predator evasion and reproduction. In recent years, induced swimming has emerged as a non-invasive strategy to improve growth, stress resilience and overall welfare, particularly in aquaculture settings and conservation programmes aimed at restocking wild populations. While the physiological benefits of exercise, including modulation of oxidative stress and immune function, are well documented in mammals, comparable knowledge in fish remains limited and dispersed. This Review synthesises the current evidence regarding the impact of swimming activity on reactive oxygen species production, antioxidant defence mechanisms and immune responses across diverse fish species. Special emphasis is placed on the dual role of reactive oxygen species as both essential intracellular signalling molecules and mediators of oxidative damage, depending on exercise intensity and redox balance. The roles of key antioxidant enzymes and lipid peroxidation by-products are also discussed as biomarkers of oxidative stress. Additionally, the emerging concept of skeletal muscle as an endocrine organ capable of releasing myokines is explored, which may link exercise to systemic anti-inflammatory effects in fish, as supported by gene expression studies. The immune-modulatory effects of swimming on pro-inflammatory cytokines and humoral innate immune markers are discussed across induced-swimming protocols. Finally, we assess how these physiological responses can be leveraged in aquaculture and conservation programmes, emphasising species-specific responses, optimal exercise intensities and swimming modes. Understanding the redox-immune axis in exercised fish provides new practices to enhance welfare and disease resistance while informing sustainable aquaculture practices.