

FIGURE 17.— Scale taken from steelhead trout caught in Gulf 

 of Alaska. The four arrows pointing towards the center in- 

 dicate fresh-water year marks (annuli). The fish went to sea 

 in its fourth spring. The shorter arrow indicates the winter 

 it spent in the ocean — the fish was 5 years old, going into 

 its sixth year. 



FIGURE 18.— Experimental operation of semiautomatic ma- 

 chine for reading fish scales. Image of scale under microscope 

 (right) is fed to computer (center). The computer analyzes 

 the light and dark areas of the scale and results are printed 

 on teletype (left). 



The scales of fish reveal details of the past much as growth 

 rings tell the story of a tree (fig. 17). Data on scales, 

 measurements of body structure, and biochemical "blood 

 typing" were analyzed by computers to demonstrate the 

 intermingling of North American and Asian stocks of salmon. 



Each spring, scales of sockeye salmon caught by re- 

 search vessels are rushed back to Seattle where technicians 



15 



