METHODS OF INVESTIGATION. 7 



rapid growth which then ensued is clearly depicted in broad and strongly 

 marked growth rings formed during the first summer in salt water. 

 Surrounding these is a band of narrow branching rings corresponding to 

 the winter spent in the aquarium, during which period the fish took only 

 very little food. Finally we find a wide belt of broad, well-marked rings, 

 corresponding to the growth during the second summer in the aquarium. 

 It is evident, therefore, that the rings on the scale correspond exactly with 

 the conditions under which 

 I knew the fish had lived 

 from the moment when it 

 first came into my hands. 



That the growth of the 

 fish referred to above and 

 also of the other small 

 salmon reared at Trondh- 

 jem was considerably less, 

 than would have taken 

 place under natural condi- 

 tions is a fact of which 

 I have been aware for 

 some considerable time, 

 but more especially so 

 now that Mr. Johnston's 

 investigations have been 

 published. 



To put the case briefly, 

 he discovered that grilse 

 from 45 cm. and upwards 

 in length and weighing a 

 kilogram or more had 

 spent only one winter in the sea, whereas larger salmon had spent two or 

 three, or in exceptional cases four winters in salt water, and also that the 

 annual winter bands were as a rule clearly shown. In addition to the 

 investigations which he had carried on for several years, he obtained further 

 confirmation of his conclusions from the well-known marking experiments 

 which were carried out on the Eiver Tay in Scotland. Several thousand 

 smolts were marked when migrating in the spring of 1905 by means of fine 

 silver wire placed in the dorsal fin.* 



A number of these fish were recaptured, and on examination of their 

 scales he was able to establish the fact that the structure and markings 



FlG. 7. Grilse 29'5 cm. unripe. Eeared in a Salt 

 Water Aquarium from June, 1900, to October 18, 1901 

 (magnified;. 



Of. W. L. Calderwood, " The Life of the Salmon," Chapter 11. 



