METHODS OF INVESTIGATION. 



17 



It is therefore quite clear that the summer-zones and winter-bands on 

 the scales of trout are respectively formed in the corresponding seasons, 

 which affect their growth, and that these zones indicate annual epochs in 

 the life of the fish. If, therefore, we count the number of these zones, we 

 can ascertain the age of the fish. A more lengthy explanation is, I think, 

 unnecessary, but I would 

 merely refer my readers to 

 the more detailed account 

 previously given .in connec- 

 tion with salmon. 



As the trout scale is, 

 relatively to the fish's size, 

 smaller than that of the 

 salmon, and as the majority 

 of trout are small fish, it is 

 evident that in most cases 

 their scales are very small. 

 This renders it difficult at 

 first to count the number 

 of winter-bands, especially 

 when we are dealing with 

 small fish of considerable 

 age. However, a certain 

 amount of practice enables 

 one to do this accurately. 

 An Abbe drawing attach- 

 ment is of very great assist- 

 ance, for with it one can 

 project the enlarged image 

 of the scale on to the table 

 by the side of the micro- 

 scope. If we place a strip 

 of thin cardboard with its 



left bottom corner in the centre of the image, we can mark on its edge 

 with a pencil the exact situation of each winter-band, and we shall find 

 that by this method counting is both easier and more accurate. Most of my 

 countings have been done in this way. 



In the case of fish which have attained a relatively great age, it may 

 happen that the determination of the bands near the edge of the scales may 

 be exceedingly difficult. Frequently in such cases the edges of the scales 

 have become thickened, and may be seen to contain several bands, without, 

 however, one's being able to ascertain their number. In cases like these it 

 S.T. 2 



FIG. 20. Trout 21-5 cm. Biological Station, Bergen. 

 Kept three winters after hatching. Killed March 1 , 

 1907 (magnified). 



