THE SALMON AND WATER TEMPERATURE 133 



has therefore seemed to me that the natural deduc- 

 tion may be said broadly to be that temperature has 

 nothing to do with the seasonable character of a 

 river in Scotland, in so far as the actual entrance of 

 salmon from the sea is concerned. The primary 

 causal factor in the existence or absence of an early 

 run of fish is not the prevailing condition of tempera- 

 ture. Once a fish has entered the mouth of a river 

 fluctuations of temperature exercise a distinct in- 

 fluence, as we shall presently see, but in the making 

 of an early or of a late river temperature does not 

 appear to play a part. Large and deep lochs 

 naturally exercise their influence on the temperatures 

 of rivers which flow out of them — such lochs have 

 relatively high mean temperatures ; but apart from 

 such conditions, the temperature of a river follows 

 close, so to speak, upon the heels of the air tempera- 

 ture of the surrounding district ; and if a warm 

 river were the chief cause of early runs of fish the 

 small streams of the western isles should of all others 

 be early streams, and the rivers in the south-west of 

 Scotland should also be all early rivers. As observed 

 at the commencement of this chapter, however, 

 small streams are never early streams. A certain 

 volume of water and a good stock of fish are 

 essentials for the early run. Given these two condi- 

 tions, the time at which early fish will enter fresh 

 water may still be subject to some variation in 

 diflerent districts. On the east coast and in the 

 Pentland Firth salmon may draw to the shore rather 

 earlier than they do on the west coast, the habit in 

 the sea where circumstances determining the growth 



