Antioxidant defenses in fish are influenced by both biotic and abiotic factors. Oxygen, essential for aerobic life, is inherently dangerous due to reactive oxygen species (ROS) it generates. Fish, like all aerobic organisms, have antioxidant defenses to counteract oxidative stress. This review examines how factors such as age, phylogenetic position, feeding behavior, environmental conditions, oxygen levels, temperature, and xenobiotics affect antioxidant defenses in fish. Understanding antioxidant activity in fish provides insights into fish physiology, benefiting fish farming and artificial production. Oxygen, in its molecular form (O₂), is a free bi-radical. Its reduction leads to ROS, including superoxide radical (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radical (OH). Oxidative stress occurs when ROS production exceeds their removal, causing damage to proteins, DNA, and cell membranes. Evolution has developed antioxidant strategies, including antioxidant substances like vitamin C, vitamin E, uric acid, glutathione, and carotenoids, as well as enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX). Fish, like other aerobic organisms, have antioxidant defenses, with enzymes and lower-molecular-weight antioxidants detected in many species. Studies show that antioxidant enzyme activity correlates with phylogenetic position, with more ancestral species having less activity. For example, dogfish (Squalus acanthias), a primitive elasmobranch, has low CAT and SOD activity, but high levels of urea, glutathione, and vitamin K may compensate for this. These findings highlight the diversity of antioxidant defenses in fish and their adaptation to different evolutionary positions. Understanding these defenses is crucial for managing fish health and improving aquaculture practices.Antioxidant defenses in fish are influenced by both biotic and abiotic factors. Oxygen, essential for aerobic life, is inherently dangerous due to reactive oxygen species (ROS) it generates. Fish, like all aerobic organisms, have antioxidant defenses to counteract oxidative stress. This review examines how factors such as age, phylogenetic position, feeding behavior, environmental conditions, oxygen levels, temperature, and xenobiotics affect antioxidant defenses in fish. Understanding antioxidant activity in fish provides insights into fish physiology, benefiting fish farming and artificial production. Oxygen, in its molecular form (O₂), is a free bi-radical. Its reduction leads to ROS, including superoxide radical (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radical (OH). Oxidative stress occurs when ROS production exceeds their removal, causing damage to proteins, DNA, and cell membranes. Evolution has developed antioxidant strategies, including antioxidant substances like vitamin C, vitamin E, uric acid, glutathione, and carotenoids, as well as enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX). Fish, like other aerobic organisms, have antioxidant defenses, with enzymes and lower-molecular-weight antioxidants detected in many species. Studies show that antioxidant enzyme activity correlates with phylogenetic position, with more ancestral species having less activity. For example, dogfish (Squalus acanthias), a primitive elasmobranch, has low CAT and SOD activity, but high levels of urea, glutathione, and vitamin K may compensate for this. These findings highlight the diversity of antioxidant defenses in fish and their adaptation to different evolutionary positions. Understanding these defenses is crucial for managing fish health and improving aquaculture practices.