LIFE HISTORY OF LAKE HERRING OF LAKE HURON 293 



ascertained that an annulus formed during a certain definite period of the year (Lea, 

 1911; Fraser, 1916 and 1917; Clark, 1925; Hodgson, 1925; and many others). (2) 

 In a fixed, homogeneous fish popuhition a dominant age group, as determined by 

 scales, persistently preponderates in all the representative samples of a series taken 

 in the same locality and in the same year but on difi^erent dates (Lea, 1910). (3) 

 An unusually abundant year class may persist in the commercial catches for two 

 or more years, and in each successive year the dominant group will show one addi- 

 tional annulus on their scales (Lea, 1910; Hjort, 1914). (4) Fluctuations in the 

 fisheries have been definitely correlated in several cases with the scarcitj' or abun- 

 dance of a particular j^ear class, as ascertained by the scales (Hjort, 1914; Jarvi, 

 1920; Storrow, 1922). (5) The interval between the periodic "big spawning runs" 

 in certain species of Pacific salmon coincides with the number of years (as determined 

 by the scales) required for. the attainment of sexual maturity by most of the indi- 

 viduals of a year class (McMurrich, 1912, p,5; Gilbert, 1914, p. 56). (6) Norwegian 

 herring recognized in the commercial catches of 1910 to 1915 by their abnormal 

 scales (the third summer zone was narrower than the fourth) each successive year 

 showed an additional annulus on their scales (Lea, 1919). (7) The well-known fact 

 that the lengths calculated from scales have nearly the same average value as the 

 corresponding average length of directly measured fish whose approximate age is 

 known points to the correctness of the age hypothesis. (See Gilbert, 1914, p. 64; 

 Thompson, 1917, p. 61.) (S) Life-history data acquired by the scale method have, 

 in many species, been found to agree with similar data obtained for these species by 

 other methods, such as the frequency curves first employed by Petersen (1891 and 

 1895; Thomson, 1904). (9) Immature fish are the young fish. The fact that the 

 scales of these fish show them to be young fish lends support to the accuracy of the 

 scale method (H. Thompson, 1924). (10) Mathematical tests based on the theory 

 of probabilities show that, on the average, the age method by scales is correct in 

 salmon, herring, and gadoids for at least the first two or three years of life (Lee, 1920). 

 So far as I know, the last statement is the only-one of the 10 evidences listed 

 above whose supporting data have been challenged. (See Sherriff, 1922.) The other 

 statements, though supported by accurate even though at times fragmentary data, 

 have not always been accepted as evidences in favor of the age hypothesis. For 

 example, evidences 3, 4, and 6 in the herring did not convince D'Arcy Thompson 

 (1914), but the prolonged dominance of one year class was used by him as an argu- 

 ment against the age hypothesis. (See p. 290.) 



DIRECT OR EXPERIMENTAL EVIDENCES 



Because of the general lack of emphasis on the direct evidences, many excellent 

 opportunities to obtain good experimental data were disregarded by investigators 

 engaged in studying the rates of growth of fish in the field and laboratory. In some 

 instances the experimenters made no reference at all to the number of growth zones 

 or annuli on the scales, but, fortunately accompanied their reports with clear photo- 

 micrographs of the scales; in others the number of annuli or growth zones was men- 

 tioned in a casual way only. In this section the papers are reviewed in chronological 

 order under two major subdivisions, the first to include the evidences obtained from 

 fish leared in artificial ponds of commercial institutions or of hatcheries and in 



