1ENDOCRINE CONTROL OF SMOLTING: AN OVERVIEW

Environmental entrainment of the parr-to-smolt transformation is mediated by the endocrine system. Smolting is associated with changes in plasma levels of a constellation of hormones, including insulin, growth hormone (GH), insulin-like growth factor-I (IGF), thyroid hormones, and cortisol, among others (Fig. 1). Plasma insulin is often the first to become elevated in spring-smolting fish. Plasma GH, IGF, thyroxine and cortisol become elevated almost simultaneously during spring concomitant with a decline in plasma insulin. GH may indirectly be responsible for elevated blood cortisol since GH has been shown to enhance cortisol production by interrenal tissue in response to adrenocorticotropin (ACTH). Increased IGF production may be due to direct stimulation by GH. Enhanced IGF production may also derive from elevated insulin and thyroid hormones. Functions of the hormones in controlling smolt physiology are becoming established. GH not only regulates growth and behavior, but also influences seawater tolerance in cooperation with cortisol and IGF (see Th. Bjornsson, this symposium). Thyroid hormones clearly promote body silvering, among other actions, and also synergize in actions of other hormones