Abstract
Hormones are important signaling molecules produced and secreted in the endocrine system that show interesting close similarities between humans and salmon. They are transported to target organs where they bind to their receptors and control physiological regulation and behavioral activity to maintain homeostasis via feedback mechanisms. Various hormones control freshwater and seawater adaptations to maintain water and salt balances. The juvenile imprinting migration and adult homing migration of salmon are mainly controlled by the brain (thyrotropin-releasing hormone)-pituitary (thyrotropin)-thyroid (thyroid hormones) axis and the brain (gonadotropin-releasing hormone)-pituitary (gonadotropin)-gonad (steroid hormones) axis, respectively. This chapter describes hormone species and actions, hormonal control of freshwater and seawater adaptations, and hormonal changes during juvenile imprinting migration and adult homing migration in salmon.