The Growth of Fish

Abstract

ABSTRACT Certain scales of Salmonoid fish exhibit a series of concentric ridges or circuli, the number of which increases with the size of the whole fish. Each year of normal growth is recorded on the scale of a salmon by a concentric band which is more or less sharply divided into two regions, (i) in which the concentric ridges are relatively wide apart, (ii) in which the ridges are closer together. It is tolerably certain that the first region is formed whilst the fish is in the sea, and the second region whilst the fish is in estuarine or fresh water ; roughly speaking the two regions mark the summer and winter periods respectively. The factor or factors responsible for the difference between winter and summer circuli are not fully understood. There are two possibilities. The variation in width may be dependent on an inherent rhythm upon which external factors may exert little or no control ; or the variation may be due to variations in the external environment of the fish. In view of the high economic importance of the facts, curiously little experimental data are available for a solution of the problem.