Body Covering 27 



hand, food is scarce, and temperatures are so low that the 

 fish has not much appetite. He feeds little, and grows slowly, 

 if at all. The scale grows slowly, if at all, the growth-rings 

 are close together, and a dark band is formed, called the 

 annual check. Count the number of annual checks, and you 

 have the fish's age (Plate I). 



Furthermore, when a fish such as a salmon migrates from 

 fresh water, where food is comparatively scarce, to salt 

 water, where food is enormously abundant, he grows much 

 faster. The spacing between the .summer bands increases 

 greatly, and the salt-water years stand out in conspicuous 

 contrast to the preceding fresh-water years (Plate II). 



Still further, when a fish like the trout is preparing to 

 spawn, it gives up eating for a while. It ceases to grow, and 

 lives upon its accumulated fat. During that period the edge 

 of the scale is slightly resorbed, leaving an unmistakable 

 wavy line to mark the experience forever. And in the salmon, 

 which fights its way upstream from the ocean to the spawning- 

 bed, covering hundreds of miles and going for weeks or 

 months without eating, the drain on the reserves causes deep 

 resorption of the scales, and the heavy "spawning-mark" 

 which cuts across the old rings and divides them from the 

 new stands out like a black eye. This refers, needless to say, 

 to the salmon of the Atlantic Coast. Pacific salmon die on 

 spawning. 



As for determining the length of the fish at various times 

 in its life from its scales, while the methods actually used 

 are complicated by subsidiary factors, the underlying prin- 

 ciple is simplicity itself. It is merely a matter of the ratio 

 between the present length of the scale and its length at any 

 previous annual check, in proportion to the ratio between the 

 present length of the fish and its length at the time of that 

 same annual check. Suppose we have a trout which measures 

 twelve inches. We take a few scales, examine them under the 



