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 
different 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 
