Table I (A) gives the complete figures for thirty-three fish, all males, stripped in the Selwyn in June, 1915. The scales were collected by Dr. C. Morton Anderson, who kindly handed them over to me. It is interesting to note that these scales had been simply folded up in paper for nearly two years when I received them, and had not deteriorated during that time.
The second parcel of scales was taken by myself on the 17th June, 1917, and consists of scales from 140 fish, all being females except one, a particularly large male weighing 10 lb. The full figures are given in Table I (B).
The average growth-curves are shown in fig. 1. The curve for the 1915 fish is a broken line, and that for the 1917 fish a continuous line. On the same diagram are also shown the curves for 173 fish from Lake Mjosen, in Norway, plotted from figures given in Dahl's book. The broken line again is the curve for males, the continuous line for females. In each case the males continue vigorous growth for a longer period than the females, and eventually outstrip them. As the males were from fish taken in 1915 and the females from fish taken in 1917, I thought it desirable to test this apparent difference between the sexes further, and with that object collected scales from twenty-nine males and thirty-six females at the annual stripping this winter (1918). The full figures are given in Tables I (C) and I (D). The average growth-curves are shown in fig. 2. Again the males continue vigorous growth longer and attain a greater size than the females.
From the Acclimatization Society's records I have calculated the average length of the fish tagged each year since 1915. The figures in parentheses give the number of fish measured:—
1915—Males, 20.8 in. (100); females, 20.1 in. (98).
1916—Females, 19.3 in. (199).
1917—Females, 20.4 in. (140).
1918—Males, 22.6 in. (66); females, 21.5 in. (156).
In the years 1915 and 1918, when both sexes were tagged, the males averaged about 1 in. longer than the females. The average lengths of the samples from which I took scales are as follows:—
1915—Males, 21.3 in. (33).
1917—Females, 20.4 in. (140).
1918—Males, 22.5 in. (29); females, 21.7 in. (36).
These figures agree closely with the averages for the total fish measured, so the samples were in all probability fairly representative. The year 1918 was remarkable both for the number and large size of the spawning fish. The average ages [see Tables I (A) to I (F)] indicate that the males either have a shorter life, or cease to run up the river at an earlier age. This bears out the general belief that the spawning mortality is greater amongst the males.
A point to notice in these curves is that they are nearly straight lines for the first four years. This does not mean that each individual fish increases in length by approximately the same amount each year up to four years old. So far as my experience goes, growth of this character is almost unknown amongst trout in Canterbury, although such apparently is not the case in Norway. In Canterbury I have found that unless some outside influence is at work the rate of growth almost invariably starts to decrease quite appreciably in the third year, and this decrease is
Fig. 1.—Brown trout, ♀, 20 m., 4 years; Selwyn River, 17th June, 1917; B 48 [Table I (B)].
Fig. 2.—“Displacement” scale, from Rakaia River; taken from the same fish as fig. 2 of Plate IV.
Fig. 1.—Brown trout, ♀, 21½ in 6 years, Selwyn River, 17th June, 1917; B 105 [Table I (B)]. (First and second winter bands not well shown in photograph.)
Fig. 2.—Brown trout, ♀ 23 in., 6 years 4 months: Selwyn River, 28th October, 1917; B 136 [Table I (E)] (Taken from the same fish as fig. 1 of this plate. Note the new growth, corresponding to 1½ in. increase in length.)
Fig. 1.—Brown trout, ♀, 22½ in., 6 years; Selwyn River, 17th June, 1917; showing migration after second winter; B 107 [Table I (B)]
Fig. 2.—Brown trout, ♀ 22 in., 5½ years; Lake Ellesmere, 17th November, 1917; showing migration after fourth winter, and pronounced spawning-mark in fifth winter; B 151 [Table I (E)]
Fig. 1.—Brown trout, ♀, 26½ in., 8 years (nearly); Opihi River, 28th March, 1918; showing 4 years of poor growth in river followed by 4 years of vigorous growth subsequent to migration. Length at completion of each winter, 4¼ in., 7¼ in., 9 in., 10½ in, 16 in., 21½ in., 24¼ in., 26½ in
Fig. 2.—Brown trout, ♀, 25 in., 4½ years, Rakaia River, 12th January, 1918; second winter band divided; B 190 [Table III]. (From same fish as fig. 2 of Plate I.
more marked in the fourth year. A close examination of the figures in Tables I (A) to I (F) will show that in practically every case there is one year for each fish in which it has made more than normal growth; it may be the second, third, fourth, or fifth year, but in almost every case there is this break in the growth-curve. This sudden jump or break is generally attributed to migration to more favourable surroundings, and there is every reason to believe that this is the case with the Selwyn fish. Practically all the spawning in the Selwyn takes place in the shallow, shingly part. Except in the spawning season, fish of any considerable size are rare in this part of the river. The traps are set just about the junction of the shallow water and the deep, to catch the fish working up to the spawning-beds. Consequently every fish caught has come from the deep water. Probably every fish was hatched and spent its early youth in the shallow part of the river; therefore at some period it must have migrated to deep water. An examination of its scales will generally disclose when that migration took place The average curves, therefore, are really compounded of a number of different curves representing one-, two-, three-, four-, and possibly five-and six-year-old migrants. In fig. 3 are shown typical curves for a two-year-old and a four-year-old migrant. Plate III, figs. 1 and 2, show scales from these fish respectively, in which the period of better growth subsequent to migration is very distinctly shown. Whenever an average growth-curve closely approximates to a straight line for four or five years it is a fairly definite indication that the fish from that locality are migratory.
Table I (E) shows the figures for thirteen trout caught last summer with rod and line at the mouth of the Selwyn and other streams running into Lake Ellesmere. The average rate of growth is about the same as that of the 1915 males, or intermediate between those of the 1918 males and females.
In order to ascertain whether results in any way reliable could be obtained from smaller samples I calculated the average growth for the first, second, third, &c., twenty fish in Table I (B). Considering the very complex nature of the water, the agreement is quite satisfactory, and indicates that results of some value can be obtained from quite small samples.
Trout in the Selwyn, whatever the mode of growth, seem to have a more or less fixed limit of growth at about 23 in., which is rarely exceeded. Other waters also seem to show a maximum size-limit. It is curious, however, that this limit is occasionally considerably exceeded, and not necessarily by very old fish. These abnormally large fish, so far as I can ascertain, show no peculiarity of growth common to all, but their scales seem on the average unusually broad in proportion to their length, though I am at present unable to state this definitely. Whether the large size is determined by heredity or by unusually favourable environment I cannot say, though I am inclined to attribute it to the former. It is certainly a point worthy of further investigation. Particulars of five of these abnormally large fish are given in Table I (F).