32 THE AGE AND GROWTH OF SALMON AND TROUT. 



there are about as many ripe as unripe. Among the three -winters fish all 

 the males are ripe. 



In the Orkla some of the two-winters fish are ripe, and among the 

 three-winters fish there are as many unripe as ripe. 



In Finmarken these conditions are postponed until a year later. With 

 reference to the age at migration, the only conclusion we can draw from the 

 table are as to the maximum age up to which the parr remain in the river. 

 In order to find out the age of migration it would be necessary to catch the 

 smolts when migrating, and to study their age. Here one immediately 

 meets the difficulty, that in certain cases it is a question whether the fish 

 has or has not the migratory colouring. In my opinion it is therefore a 

 safer method to obtain one's information by examining the central portion 

 of the scales of larger fish, and this is the method I have adopted. 



With reference to the time of the year when migration takes place, my 

 previous investigations, both with salmon and sea trout, pointed to the fact 

 that the main period of migration is during the spring, when the rivers are 

 flooded with melting snow. At the same time during the summer one could 

 find fish with the migratory dress (i.e., with more or less silvery coating), 

 especially in the lower reaches of the river. I concluded, therefore, that 

 migration also took place during the course of the summer. 



Further investigations showed that " silveriness " is not an absolute 

 proof that migration will immediately take place, for I have found amongst 

 other things that male fish with well developed milt, caught during the summer, 

 can be fairly silvery in colour. The assumption of the silvery coating can 

 thus apparently begin a long time before migration. 



In the Trondhjem district the largest migration took place in June. 

 Further south May seems to be the most important month. 



In the Oselv in 1909 I fished with a net to catch smolts for marking. 

 For this purpose I chose a suitable place in the rirer about 100 metres long. 

 The net was sufficiently long to sweep the whole breadth of the small river, and 

 this place was fished for a considerable time almost every day. The smolts 

 were migrating continuously, and at the height of the run no falling off 

 was noticed in their numbers, even though we made cast after cast in the 

 same water, and school after school came down through the same reach. 

 The netting began on May 10, when 'jit was noticed that there was a 

 considerable number of bright and large fish migrating, but towards the 

 end of the month the catch fell to a minimum in spite of all attempts that 

 were made. The numbers caught were as follows : 



May 10 and 11 ... 613 fish. 

 13 15 ... 327 

 18 19 ... 211 

 21 , 26 64 



