Trout and Salmon 197 



tists always want to know more than they know now. The 

 unknown is the food on which they live. 



They have found that in some cases what has been called 

 instinct is in reality a definite reaction to definite conditions. 

 It is this kind of reason which they have tried to find for the 

 spawning migration of salmon. One theory is that the fish 

 seek lower temperatures, choosing always the stream with 

 colder water. This has the defect of not always harmonizing 

 with the facts, any more than the theory that they always 

 choose the stronger current. 



A second hypothesis is very complete. It holds in the 

 first place that, as spawning-time approaches, the salmon's 

 metabolism is accelerated, it needs more oxygen, and it there- 

 fore ascends those rivers in which oxygen is more abundant 

 than in the ocean. Theoretically the oxygen content in- 

 creases the higher up a stream you go, and this would lead 

 the salmon on up to the spawning-beds. Actually there are 

 often stretches where for a short space the oxygen content 

 remains stationary or even decreases. If, then, this theory 

 were true, the salmon would cease to advance at such a 

 point. The second part of the theory deals with the descent 

 of young salmon to the sea, which it accounts for by the 

 fact that when they change from their dark parr coloring to 

 their light smolt shade, they become exceedingly sensitive 

 to light, and seek the deeper ocean waters for protection 

 against it. This double-barreled theory should be the de- 

 light of those who like to explain everything on purely 

 mechanistic grounds. 



A very engaging hypothesis is the argument that a salmon 

 is practically forced to return to fresh water by the buoyancy 

 of the fat which he stores up. He comes down from the river 

 a thin little fish, well adapted to the heavier salt water he is 

 about to enter. For a year or two his food goes into growth. 

 Then, in the last year before he matures, he starts to put on 



