[fraser] PACIFIC SALMON 165 



hours, so that the scale becomes set thus, after which the cover may- 

 be fastened permanently with a small strip of lantern slide binding on 

 each side. This keeps the slide flat and keeps out the dust. 



As the method of age determination was the same as that common- 

 ly used when the ridges on the scale are more or less concentric, as 

 they are in the salmon, and as it has been considered in other papers 

 and described in many instances, it is not necessary to treat it further 

 here, but as the method of growth calculation used differs in some 

 respects from those that have been described, it may be worth while 

 to give a description of the method. 



As in the case of scales previously reported upon, the increase in 

 length of the fish after the nucleus of the scale was formed until the 

 fish was caught has been divided in the same proportion as the radius 

 of the scale is divided by the points where the winter checks give way 

 to the more rapid growth of spring. To get this increase, the total 

 length of the fish (the length as here considered does not include the 

 caudal fin rays) is diminished by the length of the fry when the first 

 ring appeared around the nucleus. The latter length varies slightly 

 in different fish of the same species but as it is evidently impossible 

 to get that length for each individual, an average of several were taken 

 and no individual varies greatly from the average. 



The method of measuring and calculating growth rate described 

 by Lea^ has been extensively used, but I have not found it possible to 

 work it rapidly with any reasonable degree of accuracy, hence, an- 

 other method, which has given better satisfaction, has been adopted. 



Using a pad of paper, a straight line is drawn near and parallel to 

 the left hand margin, from top to bottom. By means of the camera 

 lucida, a portion of this line is made to coincide in position with the 

 long radius of the image of the scale. The points where the inner- 

 most ring, the outer limit of each of the winter checks and the margin 

 of the scale, meet the radius, are marked on the line. From the first 

 of these points as a point of origin, a straight line is drawn, making 

 any acute angle with the line on the paper. From the same point, 

 using any scale desired (half a millimeter to the inch has generally 

 been used), a length corresponding to the growth in length of the fish 

 during the life of the scale, is measured. Using a parallel rule, the 

 measured part of the oblique line is divided in the same proportions as 

 the portion of the vertical line that corresponds to the radius of the 

 scale. The growth in each year is then read off according to scale. 

 A diagram will show how the parallel rule is used. 



^ Publications de Circonstance, No. 53, 1910, p. 37. 



