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Section I.—Introduction.
Since the works of Hoffbauer were published in 1899, many investigators have found it possible to determine, presumably with a fair degree of accuracy, the age of many species of fish by an examination of their scales. The practice has become an important branch of fishery research, especially during the past fifteen years, in all countries that are carrying out extensive research into their freshwater and marine fisheries.
The soundness of the scale method depends upon the validity of certain fundamental assumptions, which may be stated as follows:
| (1) |
That the scales remain constant in number, and retain their identity throughout the life of the fish. |
| (2) |
That corresponding to a given increment in the length of the fish there is a corresponding increment in the scales, and that this relative rate of growth of the scales to that taking place in the fish remains constant from the time when the first circulus is laid down throughout the life of the fish. |
| (3) |
That the bands of narrow and broad circuli are formed at approximately the same time every year (or that some other discoverable relationship exists between their formation and increment of time). |
Incidentally other questions are raised, but the validity of the scale method of computation is not affected by them. They are given by Van Oosten (1929) as follows:—
| (1) |
“Do the annuli represent periods of retarded or arrested growth of the scales?” |
| (2) |
“Is the growth of the fish in length retarded or arrested at the time of formation of the annuli?” |
| (3) |
“What factors are responsible for the arrest of or retardation of growth in the fish and scales?” |
As the life-history of different species, and even different individuals of the same species, varies considerably in the more important details, under different conditions, it is obviously essential that the questions outlined above should be carefully investigated before we can be certain of the validity of the methods we are employing. We are not justified in assuming that that which occurs
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in one species will occur in another species, even under similar conditions, or for the same species under different conditions, as has previously been assumed in many cases. Before the application of the scale method for the elucidation of the age, growth, and life-history of a particular species is justified, certain preliminary investigations must be carried out. The study of the age and growth of fishes is of a statistical nature. We are not concerned actually with the age and growth of any particular individual. What does concern us is the average age and growth of certain restricted fish populations, or a particular section of such a population. It is of the greatest importance to give in quantitative expressions variations occurring in their average age and growth-rate over a fairly long period of time, tracing, as it were, certain definite sections of a fish population from birth to maturity, and noting any changes occurring, as measured from previous observations. As the methods are purely statistical in nature, the accuracy of the results depends upon the adequacy of the material upon which the results are based. These methods tend to assume the form of vital statistics.
Data are presented, in the present paper, to test the validity of the scale method, as applied to brown trout in New Zealand.