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The Number of Circuli.
From the point of view of scale reading the character of and the constancy in the number of circuli in relation to the length of the anterior radius of the scale are of the greatest importance. There is little correlation between the number of circuli and the size of the scale in an individual fish. On the other hand, there is generally a close connection between the growth of the anterior radius and the production of circuli. This correlation will be shown later.
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| Anterior | A | B | C | D | E | F | Aver. | Posterior | |
| 1 | 44.0 | 52.0 | 46.0 | 44.0 | 63.0 | 31.5 | 46.8 | ||
| 2 | 50.0 | 56.0 | 48.0 | 60.0 | 45.0 | 50.0 | 51.5 | ||
| 3 | 54.0 | 56.7 | 47.0 | 47.0 | 51.0 | 49.0 | 50.8 | ||
| Lateral Line. | |||||||||
| 4 | 46.0 | 57.0 | 49.0 | 61.0 | 56.0 | 51.0 | 53.3 | ||
| 5 | 44.0 | 41.0 | 51.0 | 57.0 | 66.0 | 49.0 | 51.3 | ||
| 6 | 31.0 | 30.0 | 46.0 | 54.0 | 58.0 | 48.0 | 44.5 | ||
| Aver. | 44.8 | 48.8 | 47.8 | 53.8 | 56.5 | 46.4 |
There is apparently an increase in the number of circuli per scale from the head to a point a little past the adipose fin, beyond which the number of circuli decreases.
Those scales above the lateral line and anterior to the dorsal fin possess a larger number of circuli than those below the lateral line.
The area of the body, where the scales possess the largest average number of circuli, is that from the posterior insertion of the dorsal fin to midway between the adipose and base of caudal fin, both above
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and below the lateral line. The scales from this area are generally large and most suitable for age determinations. There is apparently no significant difference in a dorso-ventral direction.
In one specimen of Salmo salar which Esdaile examined the number of annuli (circuli) varied from 76 to 121. She shows that this variation is not due to any one particular peronidium (annulus), but to variations in each of the peronidia (annuli). This variation she states is caused to some extent by definite local differences. “That is to say, speaking generally, the number of annuli (circuli) in each peronidium (annulus) 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.”
The present data from brown trout bear out fairly well the conclusions that Esdaile came to in the case of the salmon (S. salar).
The results of the present investigation may be summarised as follows:—
| (1) |
The scales from different regions of the body in the case of brown trout (S. trutta) vary greatly in length and in the number of circuli they contain. |
| (2) |
This variation is not entirely due to casual variations in the scales, but is caused by definite local differences. |
| (3) |
It would appear that these differences in the scales are to a more or less degree correlated with the time of scale formation in the young fish. As has been previously shown, the scales first appear along the lateral line, immediately below and slightly anterior to the adipose fin. The scales spread dorsally and ventrally from this area, but the rate at which they spread is slower below the lateral line than above. At the same time they spread anteriorly and posteriorly, ultimately covering the whole surface of the body. In the case of a fry 4.5 cm. long, fully developed scales with one to three circuli were found in the areas C 2, 3, 4, and 5; D 1, 2, 3, 4, and 5; and E 1, 2, 3, 4, and 5; while only minute scale platelets were forming on the remaining areas of the body. |
| (4) |
The number of circuli laid down in each scale increases from the head to the adipose fin and then decreases towards the tail. No significant increase or decrease was observed on the dorsal and ventral sides of the lateral line. This agrees with Esdaile's findings. |
| (5) |
A similar distributional variation is shown to occur in the length of the posterior and anterior radii of the scales. |
| (6) |
The average number of circuli laid down in each scale anterior to the dorsal fin is greater above than below the lateral line; posteriorly to and slightly beyond the adipose fin the reverse is indicated, namely, the ventral scales possess on an average a greater number of circuli. This was not observed by Esdaile in the case of the Atlantic salmon, for |
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she states: “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 (annuli) than the scales from the ventral side.”
Fig. No. 3.—Graph showing variations in number of circuli on the anterior radii of the scales from different regions of the body.