28 The Life Story of the Fish 



microscope, and find that it is now in its third year and has 

 never spawned. This is an essential condition j the method 

 cannot be applied to fish which have spawned because in them 

 the scale-resorption which we have already mentioned de- 

 stroys the relation between scale-length and fish-length. We 

 want to know how long this trout was at the end of its first 

 year. 



We measure, with a microscope micrometer or some other 

 device, the distance from the center of the scale to its front 

 edge. Let us suppose that this is 0.9 millimeter. Similarly, 

 we measure the distance from the center of the scale to the 

 front of the first annual check, which we will suppose in this 

 case to be 0.3 millimeter. Since the scale grows at the same 

 rate as the fish, the length which the fish had reached at the 

 end of its first year must be the same fraction of its present 

 length that the size which the scale had reached at the end 

 of the first year is of its present size. Now, the first annual 

 check we have just found to be 0.3/0.9, or %, of the present 

 scale-size. The fish at the same time had reached the same 

 fraction, %, of its present length, 12 inches. The fish was 

 therefore 4 inches long at the end of its first year. 



The only remaining question is, why does anyone want 

 to know these things? In general, to satisfy the curiosity of 

 that enormously inquisitive animal, the human being. More 

 particularly, because age and growth-rate are two of the 

 most important tools of the fisheries workers — and this not 

 very enlightening reply may be best elucidated by an 

 example. 



The main object of fisheries work, whether in the field of 

 commercial or of sport fishing, is to maintain the supply of 

 fish at a level sufficiently high to provide good fishing. One 

 of the methods employed is to determine at what size the 

 fish spawns, and to forbid the capture of fish under this size, 

 thus giving each individual a chance to reproduce itself at 



