128 THE LIFE OF THE SALMON 



do the salmon in the warmer Atlantic. The theory 

 sounds plausible, but in accepting it one has to pre- 

 sume that the salmon prefers warm water to cold. 

 The more crude idea that early rivers are those 

 which flow out of large lochs is an offshoot of the 



o 



same theory, and may be dismissed at once by merely 

 noticing how many early rivers do not come out of 

 large lochs, and that practically all the late rivers 

 of the West Highlands do. A third idea which has 

 been developed at considerable length, more especi- 

 ally in magazine articles, is that the ascent from sea 

 to river takes place when the temperatures of the 

 salt and fresh water approximate. We have there- 

 fore the physiologist stating that the condition of 

 nutrition determines the " return " from the sea, 

 and the physicist stating that the influence of 

 temperature alone determines the same movement. 

 I have already referred to the question of nutrition, 

 and have shown that in my view it is not a complete 

 explanation of the habit of ascent from sea water. 

 In examining the theories as to temperature it is re- 

 markable how much of theory and how little of fact 

 is to be found. Only in arguments as to the ap- 

 proximation of sea and river temperatures do we find 

 a genuine attempt to deduce from thermometric 

 readings. When such readings are taken in sufficient 

 numbers to show the real conditions which usually 

 obtain, it becomes at once evident that the idea that 

 salmon are drawn, as it were, from a cold sea by and 

 to a warm river must be given up. The facts show 

 that at the time when early fish are running all our 

 rivers are much colder than the sea on either coast. 



