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Volume 63, 1934
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Section VI.—Growth of the Scale in Relation to the Growth of the Fish.

Upon the validity of the proposition, that an annual increment in the length (or some other dimension) of the scale maintains a constant ratio with the corresponding annual increment in body length throughout the life of the fish, depends the accuracy of the scale method. It is essential that data should be available to indicate what correlation exists between the growth of the scale and that of the fish in length.

Van Oosten (1929) gives an excellent historical account of this subject.

For the purpose of the present investigation, 510 brown-trout scales were measured. The majority of these fish were obtained from the Tamiki River, near Dannevirke, during the winter of 1931.

From each fish a sample of scales was taken from the area (X) shown in Figure No. 1. Earlier in this paper the extent of variation exhibited in the scales from different regions of the body was

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shown. Considering the variation which occurs in the size of and in the number of circuli in the scales from this area of the body, it was considered that the average of five normal scales from this area would give sufficiently accurate results for the present purpose.

In the determinations of age and growth of the trout, we are concerned only with that part of the scale which is enclosed in the scale pocket—namely, the anterior radius. In the present paper only this measurement is dealt with.

The results are given in Fig. No. 6 and Table No. 9.

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Fig. No. 6.—To show the length of the anterior radius of the scale in relation to the length of the fish.

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Table No. 9.
Showing Mean Rate of Production of Circuli in the Scales of Brown Trout.
Length of Fish in Inches Average number of Circuli in Scale. Length of Fish in Inches Average number of Circuli in Scale.
9.0–9.9 52 19.0–19.9 86
10.0–10.9 51 20.0–20.9 94
11.0–11.9 59 21.0–21.9 94
12.0–12.9 56 22.0–22.9 92
13.0–13.9 63 23.0–23.9 100
14.0–14.9 68 24.0–24.9 99
15.0–15.9 73 25.0–25.9 101
16.0–16.9 73 26.0–26.9 104
17.0–17.9 77 27.0–27.9 123
18.0–18.9 81 28.0–28.9 111

Finally, there remains to examine the third, and last, assumption upon which the validity of the scale theory is based—namely, “that the scales remain constant in number and retain their identity throughout the life of the fish.” It is well known that the number of scales situated along the lateral line in any one species of fish is practically constant for the individuals of that species. That is, the number of scales does not increase as the fish increases in length.

We have also another line of evidence, in what is known as imperfect centred scales. Normally, a scale has a very small centre; if, however, a scale at some time or other is accidentally rubbed off, another scale will grow to take its place. In these new scales the centres are large; actually the size of the centre is proportional to the size of the scale which was displaced.

Such imperfect scales are frequently found, and are obviously of no use for age and growth determination.

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