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Esdaile (1912) has shown in the case of the Atlantic salmon (S. salar) that scales from different parts of the body vary in length and number of circuli laid down. An intensive and systematic examination of the scales from various regions of the body is, as Esdaile points out, an essential preliminary to accurate work. She says: “How can comparisons be rightly made of scales from different fish when the extent of variation on one fish is not known?” Esdaile's work still remains the most complete study that has been carried out on this subject, and for this reason her results are given here in full. Her data were obtained from the examination of three individual salmon, viz.:—
| (1) |
101b. fish taken from the Wye in August, 1910. |
| (2) |
171b. fish taken from the Wye in February, 1911. |
| (3) |
261b. fish taken from the Wye in March, 1910. |
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Esdaile came to the following conclusions:—
| “(1) |
The number of annuli in each peronidium* increases from the head to the adipose fin, on both dorsal and ventral sides of the lateral line, and then decreases again towards the tail.” |
| “(2) |
A great variation in the number of annuli (circuli) and in lengths of the scales taken from different parts of the same fish is clearly indicated. This was found on each of the three fish, but the results obtained seemed to be in no way correlated.” |
| “(3) |
In a comparison of scales taken from positions at corresponding distances from the head on both the dorsal and ventral sides of the lateral line, it is seen that, as a general rule, the scales on the dorsal side have fewer annuli in each peronidium (annulus) than the scales from the ventral side.” |
| “(4) |
I find it impossible to recognise any distinction between different types of annuli (circuli).” |
It was thought advisable, for the present purpose, to carry out an intensive examination of the scales of three specimens of brown trout, each specimen, if possible, to be exactly the same length and weight. Three suitable fish were obtained in June, 1931, from the Hinds River, Ashburton County. Each of these fish measured exactly 12 in. in length and 15 oz. in weight. The length of the fish was taken from the tip of the snout to the end of the middle ray of the caudal fin.
Scales were taken from the right side of the body in transverse rows as shown in Fig. 1. The letters indicate the transverse rows from the anterior (A) to the posterior (F) ends of the fish. The numbers indicate the longitudinal rows from the dorsal (1) to the ventral (6) regions. Each of the areas from which scales were taken is therefore designated by its position on the transverse and longitudinal lines. Thus A 1 denotes that area (approximately ⅓ of an inch square) on the transverse row A and the longitudinal row 1. In each of the areas three scales are included, and the figures in the following tables are the average measurements or counts generally of three scales. It was found in some cases that all the three scales were imperfect, and in some cases a blank appears in the corresponding tables. It was found that when the three sets of tables corresponding to each of the three fish were combined, relatively few blanks remained.
Esdaile's tables show very significant variations in the length and number of circuli in the scales taken from practically identical positions (within the same square inch) in the different specimens. This variation is due, in the main, to the large difference in the size of the fish from which she obtained her material. Even by taking corresponding scales from fish of identical length and weight, individual variations will occur in the scales. Esdaile found that the number of circuli and the size of the scales were correlated
[Footnote] * Esdaile uses the terms annulus and peronidium. More recent usage replaces her annulus by circulus and her peronidium by annulus. This is indicated when quoting Esdaile by enclosing in parentheses the notation used in the present paper.
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either separately or together with the age of the fish from which they were taken.
The notation used to designate the various scale measurements is shown diagramatically in Fig. 1. In the following account each of the characters examined will be considered separately.
Fig. No. 2.—Graphs showing variation in length of anterior (A) and posterior (B) radii of the scales from different regions of the body.