INTEODUCTION. vii 



Since that time opinions as to the causes in the variation in these 

 species have chiefly followed two directions. According to one theory a 

 number of local races exist, more or less widely distributed, and their 

 peculiarities are based on the ordinary laws of selection and heredity. The 

 other theory lays greater stress on the question of food supply. The 

 assertion has been put forward that peculiarities in shape and colour may 

 easily be explained as being merely due to environment. The variation in 

 size, which is undoubtedly the most striking and characteristic dissimilarity 

 is explained as being due to different conditions of food supply. Others 

 again are of opinion that these variations are due to both race and 

 environment. 



For a fuller understanding of the subject it is absolutely necessary 

 that one should be able to determine the age and rate of growth of the 

 different fishes in their natural state in different localities, and it was for 

 this reason that I considered it advisable to investigate the matter fully with 

 a view to the classification of salmon and trout according to their age. 



The works published since 1899 by Hoffbauer, Eeibisch, Heincke, and 

 Stuart Thompson seemed to show that a classification of this kind was 

 feasible in the case of carp, plaice, and several of the gadidae. In some 

 cases by means of the scales and in others by investigating the bones it 

 was demonstrated that there existed periodic forms of growth in summer 

 and in winter, and that this difference was indicated by annual growth- 

 rings in these parts, similar to the rings found in trees. 



Investigations into this question were inaugurated in 1904 under the 

 supervision of Dr. Hjort at the Norwegian Fishery Institute, and Dr. Damas, 

 and I devoted special attention to the age of the gadidae, first by studying 

 their otolithB, and afterwards by examining their scales, which were found 

 to lend themselves admirably to the purpose. Subsequently we extended 

 these researches to many other species of fish, amongst others both sprats 

 and herrings, and the results obtained have been most satisfactory. A 

 summarised account of the history and literature of this work has been 

 published by me in the " Internationale Eevue der Gesammten Hydrobio- 

 logie and Hydrographie," Bd. 11, 1909, under the title of " The Assessment 

 of Age and Growth in Fish." 



The success attending our efforts led me in 1905 to extend my investi- 

 gations to the scales of trout and salmon which showed a distinct annual ring 

 formation. I was induced to proceed further with my investigations by 

 the publication in the same year of the first published work on salmon 

 scales, and which appeared as a contribution from Mr. H. W. Johnston in 

 the " 23rd Annual Keport of the Fishery Board for Scotland." 



From the University, to which I applied for support in carrying out 

 systematic investigations, I received a grant during the years 1906 and 



