22 THE AGE AND GKOWTH OF SALMON AND TEOUT. 



and when once acquired are, except in the case of accident, retained 

 throughout life. 



It is therefore evident that the actual number of scales on a fish is 

 constant, and practically remains the same however old the fish may be. 

 As an example I may mention the fact that the number of scales on salmon 

 or trout on any particular portion of the body (as, for instance, on a slanting 

 line drawn from the posterior edge of the adipose fin to the lateral line) is 

 always found to vary within fixed limits in the case of every single trout or 

 salmon which may be examined irrespective of age. 



It therefore follows that the scale increases in size with the growth of the 

 fish, and that this increase is proportionate to the increased size of the fish. 



Seeing therefore that the growth of the scale is proportionate to the 

 growth of the fish, it should be possible to calculate how long a given fish 

 was on the completion of each winter of its existence by taking measure- 

 ments of the various growth-zones shown on the scales. Such measurements 

 would have to be made on the anterior portion of the scale which grows in 

 a regular manner and where the periods of winter and summer can be 

 differentiated. The measurement should be taken first from the centre of 

 growth to the anterior edge of the scale, then from the centre to the last 

 completed winter -band, then from the centre to the winter before, and so 

 , on. On comparing the measurements thus obtained in relation to the total 

 length of the fish when caught it should be possible to calculate the length 

 it had on the completion of each winter-band. 



I give below a description of the method which I have adopted, and 

 which is practically the same as the one that has gradually been adopted 

 at the Fishery Directors' Laboratory for similar investigations of the 

 scales of other fish, principally herrings and sprats. The method and 

 apparatus have been described in detail by Mr. E. Lea in the " Publications 

 de Circonstances du Conseil International pour 1'Exploration de la Mer." 

 It is therefore unnecessary for me to give a full description, and I merely 

 give the main outlines of the method of investigation adopted. 



By means of the Abbe drawing apparatus an image of the scale is 

 thrown on the table, the long axis of the scale being kept parallel with the 

 right-hand side of the microscope, and care being taken that the image is 

 always in the same line of orientation. A strip of thin cardboard about 

 8 cm. broad and of suitable length is then placed with its bottom left-hand 

 corner on the centre of the projected image, and so that the left edge lies 

 along the long axis of the scale. On the edge of this strip the position of 

 each winter-band and finally the anterior edge of the scale are marked 

 with pencil. (See the sketch in Fig. 26.) 



The exact amount of magnification is of no importance as the measure- 

 ments obtained are merely relative. 



