1. Seasonal variations in the growth rate, food reserves and activity of the thyroid gland of hatchery-reared brown trout have been investigated.

2. Two peaks of maximum growth rate were found, in spring and autumn. A marked depression of rate occurred during midsummer and winter.

3. Fat which was laid down along the mesenteries and pyloric caecae was found to be the main food reserve. Glycogen was found in small quantities in the liver and muscle. The composition of the muscle and liver was constant except for an autumnal fall in the female liver glycogen level. The fat reserves reached a peak of 23% by weight of the gut wall during July then fell to 5% in autumn.

4. Maturation of the gonads commenced in females in June and in males in July and was completed during October. The protein content of the ovary increased by 16%, and of the testis by 10%. The fat content of the ovary increased by 3%.

5. A new method is described for the determination of thyroid activity in fish using radioactive iodine. Peak thyroid activity was found to occur in midsummer.

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