Studies on the Biology of the Early Stages of the Salmon (Salmo salar)

Abstract

The growth of young salmon in 3 tributaries of the Thurso River in northern Scotland between Aug., 1937, and Oct., 1938, is compared with a similar study in the river Eden, published previously. Growth of the salmon occurs only between Apr. and Sept. and is slower during the latter than during the early part of the summer. The amt. of food in the stomach of salmon is greatest during the early part of the summer, when growth is rapid, and then falls steadily to the low value that is maintained over winter. Seasonal changes in the condition factor,. (104 X W)/L3, correlate positively with changes in the growth rate. Feeding and growth in the salmon depend on the water temp. surpassing a threshold of 7[degree]C. The time growth begins in the spring is correlated in various rivers with the date of the rise of temp. to this threshold. Total annual increment of growth in the Thurso River System is < in the Eden. The ratio of the amt. of growth from Aug. to Oct. to the amt. from June to Aug. is the same in the 2 rivers for the 1st year of life of the fish but varies widely during the 2d. In both yrs. the rate of growth in late summer is relatively higher in the Eden than in the Thurso System. Max. temps. and seasonal variations in the food supply appear to modify only slightly the rate of growth. The size of the fish population bears the same positive relation to the food supply as does the rate of growth in early summer. Comparison between the 2d-yr. increment of growth and the date of rise of temp. to 7[degree] throughout the natural range of the salmon shows that while this factor may affect the amount of growth in restricted areas, as the British Isles, some other factor may be of greater significance in determining its magnitude in different parts of the range.