Structure and growth of scales of yellow perch of Green Bay

The appearance of the scales of yellow perch differs with the location on the fish's body. Comparison of scales of Green Bay perch taken above and below the lateral line reveals the former to have more sharply defined circuli and to exhibit fewer false annuli and less of the shading that impedes accurate assessment of age.

Estimates of age from modes in length-frequency distributions and from scale readings were the same for age-groups O, I, and II. Older age groups could not be detected in length distributions, but the similarity of all annuli makes it acceptable to conclude that the annulus is in fact a year-mark at ages beyond the II group.

Scales of yellow perch may exhibit new growth in the latter part of May whereas others may not have started growing even in late July. The time of annulus formation varies with calendar year, with age (new growth starts earlier in the younger fish), and possibly with locality. Ages usually can be read from scales collected during the period of annulus formation, but occasionally difficulties are encountered.

For larger yellow perch the relation between fish length and the radius of key scales was described by a straight line with a 2-inch intercept on the length axis when the scales were taken above the lateral line and by a straight line through the origin when the scales came from below the lateral line. Lengths calculated from both relations were inaccurate for small fish. Tables of corrections are given, based on the body-scale curves for small perch.

The body-scale relations (determined from the same key scale) were similar for Lake Erie and Saginaw Bay perch but both differed sharply from the relation in the Green Bay populations.

Good procedure requires high consistency in the field as to the point on the fish's body from which scale samples are taken and also a careful determination of the body-scale relation in each population studied.