METHODS OF INVESTIGATION. 



25 



values thus obtained I endeavoured to calculate the length of the fish at 

 the time of migration as smolts, and with the following results : 



The values thus obtained of the length of these fish when they migrated 

 as smolts are exactly similar to what we know of the general length of 

 smolts when they migrate to the sea. They vary in length, as a rule, 

 between 9 cm. and 15 cm., and may occasionally reach 20 cm. The average 

 length, viz., 13 cm., is a little higher than that which I have found in 

 the Trondhjem district, but later investigations have shown that in the 

 south the average length of the smolt is rather higher than in the north. 



In the case of trout, as previously mentioned, I have examined a 

 number of individuals and have classified them, both by analysis of age 

 according to size, and by calculating their size on completion of each 

 winter-band by means of measurements taken from their scales. The 

 results of these comparative investigations have been tabulated by means 

 of growth-curves like the one shown below. 



The abscissa axis is divided into winters and months, with a line for 

 every second month. The ordinate axis is divided into centimetres indi- 

 cating the length of the fish. For the sake of comparison, both here and 

 subsequently in all similar curves and tables, . I have assumed that the 

 growth of a trout begins on April 1, as this date may be. taken as the 

 average date of hatching throughout the whole country. In the same way 

 January 1 is reckoned as mid- winter. 



In the diagram shown below, the dotted line has been drawn through 

 the points indicating the average length ascertained for each year-class by 



