Hyperadrenocorticism in spawning migratory and nonmigratory rainbow trout (Salmo gairdnerii); comparison with Pacific salmon (genus Oncorhynchus).

Abstract A comparative study was made of the physiological and histological changes associated with sexual maturation and spawning in migratory (anadromous) and nonmigratory (lacustrine) rainbow trout. These changes in migratory trout (steel-head); hyperplasia of the adrenocortical tissue, elevated concentrations of 17-hydroxycorticosteroids (17-OHCS), hyperglycemia, depletion of lymphocytes from spleen and thymus, hypertrophy of the islets of langerhans, and degeneration of many of the internal organs, are characteristic of hyperadrenocorticism. Whereas the nonmigratory trout showed adrenal hyperplasia, 17-OHCS concentrations, and lymphocyte depletion as marked as in steelhead, hyperglycemia was absent and degenerative changes in the internal organs were slight. Comparison with similar physiological and histological alterations in Pacific salmon brought out the fact that the salmon, all of which die after spawning, show more pronounced changes than do the steelhead trout which suffer only a partial post-spawning mortality. The nonmigratory trout usually survive their first reproduction. Differences in food intake during sexual development in these three salmonids; complete starvation in salmon, limited and irregular feeding in steelhead, continued active alimentation in nonmigratory rainbow trout, are discussed, and their possible relationship to the catabolic effect of 17-OHCS and to post-spawning survival, is indicated.