198 The Life Story of the Fish 



fat, and he eventually becomes so buoyant that he seeks the 

 less dense fresh water in order to be able to handle himself 

 with more comfort. This fails to take into account the grilse 

 which mature and come into fresh water before they have 

 reached the stage of great obesity. 



Still another theory is that their keen sense of smell leads 

 salmon into those streams in which there are at the moment 

 young salmon. This is based on an experiment in which 

 salmon fry were placed in a stream which had once had a 

 salmon run, which had then been closed by a dam, and 

 which even after the dam had been out ten years had failed 

 to attract a run. That very summer adult salmon entered 

 the stream and spawned — brought in, according to the theory, 

 by the smell of the little fish. This sounds convincing, but 

 it is not conclusive: a group of lost fish may have strayed in. 

 Further similar experiments are needed to prove that coinci- 

 dence played no part. 



And the most recent theory, based on deep study of physi- 

 ology, osmosis, metabolism and chemistry, and set forth with 

 elaborate and convincing technical detail, maintains that the 

 carbon dioxide content of the water holds the true answer, 

 and that the salmon is absolutely obliged to follow a carbon 

 dioxide gradient which leads him unfailingly to spawning 

 grounds in fresh-water streams. 



Now, any or all of the reasons advanced in these various 

 hypotheses may have something to do with the salmon's 

 migration into fresh water. But the fact remains that none of 

 them tell us how the salmon finds its home stream. And the 

 fact remains that trout carry on the same migration in a minor 

 key, but may go upstream or down, into cooler or warmer 

 water, into water higher or lower in oxygen j and their sense 

 of smell can play no part in a downstream movement, and 

 their fat content can play no part in a movement which be- 

 gins and ends in fresh water. When they come to a gravel 



