18 THE AGE AND GROWTH OF SALMON AND TROUT. 



is naturally quite out of the question to determine the age beyond a certain 

 number of years. 



When one examines the scale of a salmon one notices at once that on 

 migration to the richer feeding grounds of the sea a great change takes 

 place in the conditions of growth. The slow-growing scale of the migrating 

 smolt appears in the centre of the larger scale of the mature fish, like a 



small island of narrow growth - 

 rings surrounded by the wider 

 and boldly-marked rings which 

 are formed in salt water. We 

 can therefore easily distinguish 

 the boundary line on each scale 

 indicating the transition to other 

 conditions of nourishment. 



With trout, on the other 

 hand, which have spent all their 

 life under uniform conditions, 

 as, for instance, typical burn 

 trout, we find the poor growth 

 of the scales continued from the 

 early stages up to a considerable 



Other trout, however, whose 

 early life has been spent under 

 unfavourable food conditions and 

 which at a later stage of life may 

 have entered into more favour- 

 able environment, show on their 

 scales a sudden transition and a 

 consequent increase in the width 

 of the growth rings, and this is 

 similar in character to what we 

 have observed in the transition 

 stage of salmon. These characteristics we find on the scales of sea trout and 

 also of trout from a number of lakes, about which I will later give fuller par- 

 ticulars. No trout, however, even under the most favourable conditions, as 

 we shall see later, ever grows so fast as a salmon, and consequently the best 

 growth-zones found on trout scales are never as broad or as well marked as 

 those of salmon. Still, in the majority of cases the differences between 

 unfavourable and favourable environment are clearly indicated on the scales, 

 and therefore, as a rule, we can determine the period when the transition 

 from slow to more rapid growth took place. As an instance of this I would 



FIG. 21. Trout from Starnberg Bavaria. 

 Hatched in spring of 1908. 

 October 1909 (magnified). 



