The research inspiration of fish endocrinologists is usually derived from their interest in aquatic life. However, the chapter on fish research often improves the study of other vertebrates and meets their own needs. Fish life is marked by their aquatic environment, which requires numerous physiological adaptations apart from terrestrial life. The endocrine system is highly involved in maintaining homeostasis, particularly in fluid balance within the body. According to environmental regulatory mechanisms, certain hormones are primarily involved in physiology and evolved both functionally and molecularly throughout their life. The present review provides examples of the role played by hormones in general physiological processes. In the text, the discussion will center on reproductive hormones in fish and their origin and effects on gonadal development through the entire reproductive cycle. This emphasizes neuro-endocrine pathways that regulate the production and release of pituitary gonadotropins, specifically follicle-stimulating hormone (FSH) and luteinizing hormone (LH), in teleost fish. One of the primary objectives is to describe the various kinds of gonadotropin-releasing hormone (GnRH), its receptors, and their interaction in the pituitary and brain. This chapter examines the role of various factors at different stages of gonadal development and discusses the activation of the GnRH-FSH/LH axis during sexual maturation and spawning. The activity of the pituitary–gonad axis is correlated with undetermined environmental factors, suggesting their spawning strategy after disease outbreaks and the effects of different steroid hormones during artificial reproduction. In this way, studies conducted on fish often give more comprehensive perspectives on hormone function across the vertebrates.