Endocrine control of smoltification in anadromous salmonids.

The parr-smolt transformation (smoltification) of juvenile anadromous salmonids involves a morphological, physiological and behavioural metamorphosis of the fish from a freshwater-adapted form to a salt-water-adapted form. Several endocrine glands are activated during the period of smoltification, including pituitary, thyroid, and interrenal tissues. The pituitary-thyroid axis appears to be the endocrine system most directly involved in controlling smoltification. A plasma thyroid hormone surge occurs during smoltification which appears to influence various tissues and other endocrine systems, and to induce the well-documented developmental changes associated with smoltification. The pituitary-interrenal axis has been implicated in several smoltification-related events, including development of hypo-osmotic regulatory ability. A plasma cortisol surge closely follows the thyroid hormone surge during smoltification, but in contrast to anuran metamorphosis, the peaks do not coincide. Despite recent attention, the role of the corticosteroids in development of hypo-osmotic regulatory ability remains unclear. The other endocrine tissues of the salmonids appear to be acting trophically with the thyroid hormones, or to have little involvement in the control of smoltification.