THE SALMON 43 



where the fish were, and what size they were at 

 different seasons, Dr. Dahl has established a number 

 of facts concerning the growth and migration of 

 Salmon and Trout. 



We may next mention the marking experiments 

 which have been carried out by the Fishery Boards 

 in Scotland and Ireland under the respective direction 

 of Messrs. W. L. Calderwood and E. W. L. Holt. 

 On the Tay smolts were marked by a silver wire 

 fastened near the base of the front of the dorsal fin, 

 and were recaptured on their return from the sea. 

 Kelts and clean fish also were marked in the same 

 way, only with a numbered silver label attached 

 to the wire, and on recapture the difference in size 

 and weight were noted. By these methods it has 

 been established that Salmon nearly always keep 

 to their own river, that they do not return as grilse 

 in the same season as they go to the sea as smolts, 

 that some are annual spawners and others are not, 

 and many other facts which have already been 

 detailed. 



The third means of investigating the life of the 

 Salmon is due -to the discovery by Mr. H. W. 

 Johnston of the fact that every Salmon carries its 

 own history plainly written for those who are able 

 to read it ; this gentleman has made a detailed 

 study of the structure of the scales as shown by 

 the microscope, and has shown that these reveal the 

 age and life-story of the fish. The number of 

 scales is constant throughout life, and consequently 

 as the fish grow the scales increase in size ; the 

 new tissue is added in the form of a series of 

 concentric ridges, having somewhat the same 

 appearance under the microscope as the rings of 

 4 



