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

Abstract

A statistical study of the relation of the food of young salmon during their first 2 yrs. of life in fresh water to the fauna was carried out in the river systems of Eden and Thurso in northern England and Scotland, 1935-1938. The relative availability of various spp. in the river fauna as food is detd. by their abundance, size, activity, conspicuousness, degree of exposure, presence of a hard outer covering, and by the behavior of the fish. An availability factor for each sp. was worked out as the ratio of the av. % which the animal makes up in the stomach contents and its av. % in the fauna. These availability factors were found in consistent agreement in several streams where detd., especially in the case of the more abundant spp. When a fish begins feeding after a period of comparative rest, it first chooses its food at random from that which is available, but when it gets 10 or more animals in its stomach at least 80% of the fish exercise some selection in the choice of further food. In the lst-yr. fish, this selection was mostly confined to the 2 most numerous groups of food animals, and in the 2d-yr. fish to the 5 most numerous groups. A few groups of animals were rarely selected although fairly abundant in the available fauna. Methods for evaluation of river faunas as food supplies for young salmon were worked out to include both there relative availability and this selection factor. Examination of the quantity of food taken at the water''s surface showed that more surface food was taken in the Thurso than in the Eden river system, and that surface feeding is more prevalent from Aug. to Oct. than at other times.