The Occurrence of Increased Activity of the Thyroid Gland in Rainbow Trout at the Time of Transformation from Parr to Silvery Smolt

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Abstract
A study was made of the nature of the nature of the color transformation and associated changes in the secretory activity of the thyroid gland which young rainbow trout in certain Michigan streams exhibit before migrating into one of the Great Lakes. This change in color from the relatively dark parr to the silvery smolt was due principally to the deposition of a greatly increased amt. of guanine on the internal surface of the scales, laid down as a dense mat of crystals. Contraction and disintegration of the dermal melanophores constituted an important part of the transformation process, but this change was observed only in the ventral surface and the operculum. Examination of the thyroid glands of the rainbow trout smolts revealed markedly increased functional activity as compared with the parr thyroids.