THE AGE AND GKOWTH OF SALMON 

 AND TKOUT IN NOEWAY. 



CHAPTER I. 

 METHODS OF INVESTIGATION. 



1. THE SCALES OF SALMON. 



THE use of Salmon Scales as a means to the study of the age and other 

 biological conditions of the fish is due, so far as I can ascertain, to Mr. 

 H. W. Johnston. In three papers published in the Annual Reports of the 

 Scottish Fishery Board for 1904, 1906, and 1907, he has succeeded in 

 establishing the fact that periodic growth takes place in the scales of salmon, 

 and that groups of rings are formed in each year, and that it is possible by 

 counting the number of these to ascertain the age of nearly every individual, 

 and to form an opinion regarding its life history. In the first article he 

 relied mainly on scales taken from fish caught at various seasons of the 

 year, and he succeeded in demonstrating by the study of these scales that 

 various growth zones in the scales were formed in different periods of the 

 year. In his later papers he was able to throw additional light on the 

 subject from the investigation of scales of marked fish, and of fish whose 

 .age was known. Mr. Johnston further made it clear that the scales of 

 mature fish, which had returned to the river from the sea, were affected to 

 a most remarkable extent by their sojourn in fresh water. When these fish 

 returned to the sea after spawning and new growth was developed, the 

 consequences of their stay in the river were shown by a permanent and 

 ineffaceable scar on each scale, which Mr. Johnston termed the " Spawning 

 Mark." His investigations have been supplemented and popularised by 

 other English writers, notably Mr. Calderwood and Mr. Hutton.* 



In view of what had already been definitely proved, I decided to restrict 

 my investigations towards ascertaining whether the same conditions applied 

 also to Norwegian salmon, and also if possible to supply methods of 

 investigation which might lend themselves to statistical analysis. 



* The latter author lias also recently made a special study of salmon from the Eiver 

 Wye in England. 



8.T. 1 



