Divided Winter Band in Brown Trout Scales from the Rakaia River.
[Read before Canterbury Branch, October 2, 1946; received by Editor, October 8, 1946; issued separately, September, 1947.]
In a previous paper (Trans. N.Z. Inst., 51, 1919, 42–67) I recorded a peculiarity shown by some scales of trout from the Rakaia River and illustrated one such scale (Pl. 4, Fig. 2). What appear to be the first two winter bands are clearly marked and, immediately outside, is a narrow band of widely spaced circuli (2 or 3) followed by a narrow but definite check and then normal wide spacing. The difficulty is to interpret the narrow check. Two explanations seemed possible.
The second winter band was complete and the two or three wide circuli between it and the check represent spring growth in the third year. Then some misfortune, such as temporary confinement in a land-locked pool, caused the narrow check. In this case the third spring-summer growth is interrupted by a temporary fast.
The second winter band was not completed when some stimulus, such as migration to the sea, caused a sudden rapid growth. In this case the narrow check really forms part of the second winter band, which is divided by a short period of rapid growth.
In the absence of evidence as to when the check was formed it was impossible to choose between these two alternatives, though I expressed a preference for the second.
I have recently received scales from a small trout (11 in.) caught in the Rakaia on February 12, 1945. One such scale is shown (Pl. 46. Fig. 1). For practical purposes it is identical with the middle portion of the scale previously illustrated and reproduced (Pl. 46, Fig. 2) and shows that the few wide-spaced circuli surrounding the second winter band were laid down in mid-summer. It is quite impossible that the fish is nearly three years old and that these few wide circuli represent the spring-summer growth of the third year, which would certainly be four or five times as great. The wide circuli represent rapid growth, but the actual growth is small. It must have been of short duration. The most probable explanation seems to be that they are the result of a temporary stimulus in February after the second winter band had started formation. I have shown (Ibid.) that in Canterbury the terms winter and summer bands are really misnomers, as maximum growth occurs in early spring and “winter” stagnation normally begins about Christmas. The fish was very bright and silvery and shed its scales freely, as is usual with sea-run trout. Scales from a similar trout of the same length caught in the Rakaia River on March 7, 1945, show the same characteristic and are practically identical.