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Volume 63, 1934
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Section V.—Formation of Winter and Summer Zones in the Scales.

Information on the points to which we now turn is of the greatest value, and should, as Thompson (1904) says, be properly worked out for each species of fish from which it is desired to obtain data by scale examination.

It is quite evident that the nature of the winter bands, and the time of the year during which they are formed, varies in individual fish from the same locality, and to a much greater extent in individuals from different localities. The present material does not permit of an investigation being made into the variability of the time of formation of the winter band in fish from different localities.

An interesting phenomenon is indicated by the present study, namely, that the North Island fish show relatively wide winter bands on their scales, while those of the South Island, and especially in Southland, show relatively narrow bands.

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The data obtained by measuring the increasing distance and number of circuli of the peripheral band on the scale of fish taken each month during the course of a year are given in the following tables (Nos. 6 and 7, and Fig. 4).

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Fig. No. 4.—Showing formation of summer and winter bands.

Fish with two completed bands of narrow circuli on their scales were taken, and the data given relate to the formation of the third band.

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Table No. 6.
Time of year during which the Bands of Broad and Narrow Circuli are laid down in the Scale.
Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec.
Broad Circuli (Summer Band).
Peripheral band—
No. of circuli in peripheral Band 19.00 18.00 19.25 18.20 4.00 12.95 14.50 18.14
Width of peripheral band 6.91 9.50 14.25 9.00 3.00 7.16 7.77 11.90
Narrow Circuli (Winter Band).
No. of circuli 4.50 4.75 6.75 7.50 8.21 14.13 15.75 16.66
Length (in micrometer divisions) 2.67 2.34 2.87 6.11 4.11 7.50 8.30 8.83

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Table No. 7.
Number of Fish per cent. of each Monthly Sample Showing Broad or Narrow Peripheral Band.
Jan. Feb. Mar. April. May. June. July. Aug. Sept. Oct. Nov. Dec.
Broad 100.0 67.0 22.0 20.0 40.0 100.0 100.0 100.0
Narrow 33.0 78.0 80.0 100.0 100.0 100.0 100.0 60.0
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The information obtained from the above tables may be summarised as follows:—

(1)

The band of narrow circuli is laid down in the scale generally during the autumn and winter months of the year.
(2)

The bands of broad circuli are formed during the spring and summer months.
(3)

The bands of narrow circuli may begin to form as early as February, or, on the other hand, summer growth (broad circuli) may persist until April. The majority, however, show the beginning of a winter band on their scales by March.
(4)

Early in September the summer band begins to develop, and by the end of October the majority show summer growth to be taking place.

In some cases the winter check is formed by only two, three, or four circuli. In such cases there are grounds for believing either:—

(1)

That growth, at any rate of the scale, has for a period, probably June and July, completely ceased, resuming to produce summer circuli towards the end of August or early in September. This would be expected where winter conditions are exceptionally severe. Or
(2)

That summer growth has persisted well into the autumn, and very early growth occurred in the following spring.

It is probable that the first condition may occur in the southern portion of New Zealand, while the second condition may be the rule in the more equable conditions prevailing in the northern districts. However, the data upon this point are of too meagre a nature to justify definite conclusions. The significance of this nevertheless should not be lost sight of in future investigations.

Fig. No. 4, plotted from Table No. 6, shows the rate of production of circuli during the year.

We may conclude that the terms summer and winter bands can be applied to the bands of broad and narrow circuli respectively as observed on the scales of brown trout in New Zealand. As a corollary to this we may state that only one band of narrow circuli and one band of relatively wide circuli are laid down during the year. Thus by the enumeration of these alternate bands of wide and narrow circuli the age of the fish in years may be ascertained.

It is interesting to correlate the time of formation of the summer and winter zones in the scales with the mean monthly temperature of the water in which the fish have lived.

At the present time the necessary data on water temperature are not available for a complete treatment of this subject.

Such a study should be undertaken in a particular locality from which scale and temperature data could be collected side by side over a period of several years.

In Table No. 6 are given the mean monthly temperatures for four localities, corresponding roughly in latitude with the localities from which the present scale material was obtained. These temperatures are shown graphically in Graph No. 2.

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Table No. 8
Mean, Maximum, and Minimum Monthly Temperature from the Various Localities.
Locality. Jan. Feb. Mar. April. May. June. July. Aug. Sept. Oct. Nov. Dec. Mean Annual Range.
Feilding (N.I.)—
Min. 15.0 16.0 11.0 9.0 6.5 5.5 5.0 15.0 13.0 15.0
Max. 20.0 22.0 16.0 14.0 10.0 9.0 8.0 16.5 18.0 24.0 4.8° C.
Mean. 17.7 18.7 13.9 11.2 8.0 7.2 6.7 16.0 16.1 18.3
Canterbury—
Avon R. (S.I.):
Min. 13.0 12.0 13.0 11.0 10.5 8.0 8.5 9.0 8.5 11.0 12.0 13.0
Max. 16.5 15.5 14.5 14.0 12.5 11.5 10.5 11.0 13.0 14.0 15.0 16.5 2.9° C.
Mean. 14.5 14.2 13.8 13.0 11.5 9.9 9.7 10.2 11.2 12.3 13.6 14.5
Canterbury—
Selwyn R. (S.L.):
Min. 9.5 6.0 4.5
Max. 11.0 9.0 7.5
Mean. 9.8 7.6 6.7
Otago—
Orona R. (S.L.):
Min. 8.5 9.0 7.0 1.0 1.0 1.0 1.5 4.5 5.5 8.0
Max. 15.5 12.5 12.0 5.3° C. 5.3° C. 7.5 3.5 4.5 7.5 9.0 10.0 17.5 5.3° C.
Mean. 11.1 10.6 9.9 3.2 1.5 3.0 4.7 6.7 8.2 10.7
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Fig. No. 5.

Bearing in mind the meagre nature of the data upon which the above table is based, attention may be drawn to the following apparently significant points:—

(1)

During the spring, summer, and autumn months the temperature at Feilding in the North Island is significantly higher than in Canterbury or Otago.
(2)

The Otago temperatures are considerably lower throughout the year than the Feilding and Canterbury records.
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(3)

The increase in temperature during the spring is more rapid in Feilding than in Otago and Canterbury.

Bhatia (1932) has investigated the effects of temperature on the width of circuli in the scales of rainbow trout (Salmo irideus). He found that, irrespective of high or low temperatures, abundant nutrition produced broad circuli, and deficient nutrition produced invariably narrow circuli. He states: “The temperature seems to have no effect, except that at low temperatures the fish are less inclined to feed than at higher temperatures, owing, probably, to the lower rate of metabolism. In Nature this fact, coupled with the scarcity of food in winter, must, therefore, be regarded as the causative factor for the production of narrow ‘winter’ rings.

Percival (1932) has shown, however, that there is abundant food available for trout in New Zealand throughout the year.

In four collections made during July, 1930, in the lower Selwyn River, he found on an average 607 organisms per sq. foot.

It would also appear from the work of Percival and Whitehead (1930), on the River Wharfe (England), that there does not occur in English streams a very significant decrease in abundance of aquatic organisms during the winter months.

With regard to the opinion that trout feed little, or not at all, at low temperatures, Percival (1932, pp. 18) makes the following interesting statement: “That this is not so throughout the Dominion is evidenced by the fact that freshly spawned female fish taken in early August, from Lakes Taylor and Sheppard, were gorged with Corneocyclas (a small bivalve mollusc), Potamopyrgus, various caddis larvae, chiefly Pycnocentria, Gobiomorphus, and vegetable fragments. A male and female, newly spawned, were gorged with freshly deposited trout-eggs. The water temperature was 7 ¼° C. (45° F.). The indications were that, in the lakes under consideration, fish commenced to feed voraciously as soon as they had recovered from the act of spawning.

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