ON TEOUT. 



63 



On examining the scales the reason for this arrangement will be 

 evident. If we look at Fig. 23 (Plate VIII.), which represents the scale of 

 one of these trout, nine winters old, we are at once struck by the sudden 

 change in growth which occurred after the fourth winter. From the 

 beginning of growth up to that period development must have been 

 comparatively small, but in the following period of growth its development 

 has been extremely rapid. Similar conditions were shown by the scales of 

 all the fish in the sample, with perhaps one or two exceptions, as will be 

 seen from the figures in Table XXVIIc. 



This table contains the calculated lengths of every fish on the completion 

 of each winter-band (the calculations have been made in the manner 

 previously described by measuring the growth-zones of the scales). 



On looking through the series of figures for each individual it will be 

 seen that in every case the fish had, after a shorter or longer period of 

 adolescence with relatively poor growth, experienced a sudden change in 

 development. 



I have not investigated the younger stages of Mj0sen trout. Still, as 

 the fish spawn in the Laagen, and the young trout undoubtedly live for a 

 long time in this river before migrating to the lake, I have felt myself 

 justified in considering this alteration in the rate of growth as an indication 

 of the time at which analogous to salmon and sea trout they left the 

 river and repaired to the richer waters of the lake. It is, moreover, 

 immaterial whether we call the boundary line between the two periods of 

 growth, one of migration, or one of change of growth. The main point is 

 that we can sufficiently clearly distinguish this division and determine the 

 duration of the two periods. 



We shall now proceed to consider what the tables teach us. 



From Table XXVII. we obtain the following figures relating to the age 

 at which migration or change of growth takes place : 



Here we find that migration or change of growth does not take place in 

 every instance at the same age, which is precisely what we discovered in 

 the cane of salmon and sea trout. In some fish it begins early, in others 

 later, and so, too, the table shows us that the majority appear on the 

 whole to be older than salmon and sea trout at the time when this change 



