This chapter of the article "Antioxidant defenses in fish: Biotic and abiotic factors" by Rosa M. Martínez-Álvarez, Amalia E. Morales, and Ana Sanz, explores the influence of various biotic and abiotic factors on the antioxidant defenses of fish. The authors highlight that while oxygen is essential for aerobic life, it also poses significant oxidative stress due to its reactive nature. Fish, like other aerobic organisms, have evolved effective antioxidant defenses to counteract this stress. These defenses include both primary antioxidants (antioxidant substances like vitamins C and E, uric acid, glutathione, and carotenoids) and secondary antioxidants (antioxidant enzymes such as catalase, superoxide dismutase, glutathione peroxidase, and glutathione reductase). The chapter discusses how these antioxidant defenses are influenced by factors such as age, phylogenetic position, feeding behavior, environmental conditions, and the presence of xenobiotics and parasites. It also reviews the historical context of the discovery of oxidative stress and its role in various diseases and age-related dysfunctions. The authors conclude that understanding these antioxidant defenses can provide valuable insights for improving fish farming and artificial production.This chapter of the article "Antioxidant defenses in fish: Biotic and abiotic factors" by Rosa M. Martínez-Álvarez, Amalia E. Morales, and Ana Sanz, explores the influence of various biotic and abiotic factors on the antioxidant defenses of fish. The authors highlight that while oxygen is essential for aerobic life, it also poses significant oxidative stress due to its reactive nature. Fish, like other aerobic organisms, have evolved effective antioxidant defenses to counteract this stress. These defenses include both primary antioxidants (antioxidant substances like vitamins C and E, uric acid, glutathione, and carotenoids) and secondary antioxidants (antioxidant enzymes such as catalase, superoxide dismutase, glutathione peroxidase, and glutathione reductase). The chapter discusses how these antioxidant defenses are influenced by factors such as age, phylogenetic position, feeding behavior, environmental conditions, and the presence of xenobiotics and parasites. It also reviews the historical context of the discovery of oxidative stress and its role in various diseases and age-related dysfunctions. The authors conclude that understanding these antioxidant defenses can provide valuable insights for improving fish farming and artificial production.