Notes on the Double Rings Appearing on the Scales of “Ayu”, Plecoglossus altivelis T_EMMINCK et S_CHLEGEL, in its First Year

Abstract

Double rings can often recognized on the scales of “Ayu”, Plecoglossus altivelis T_EMMINCK et S_CHLEGEL, under a year. Inner ring (Fig. 1:I) appears in March, when the fish becomes about 6cm in length, while outer one (Fig. 1:II), in July when the body-length ranges from 11 to 15cm (Fig. 4, 6). Appearance of these rings can be attributed to the change in food taken by the fish from animals to plants, for the inner ring, and to the sexual maturity of the fish, for the outer ring.
Outer ring has two structural types A-type detectable by the intersection of pairs of excentrically formed circuli and B-type, by the narrowed width between successive concentric circuli (Fig. 1: A, B). Either of the two types appears with different relative frequency in the different parts of the fish-body (Fig. 2). The said frequency calculated with the scales taken either from every or from a certain definite part of the body shows local difference, i. e. it has seemingly close relation to the forest zones through which the home river flows. A-type as well as the total of A and B types appears frequently in the fish from tropical or subtropi-cal forest regions, while B-type alone has the larger frequency with flush from temperate forest region (Fig. 3).