142 THE LIFE OF THE SALMON 



has a temperature a few degrees lower than the Tay 

 water, a good stock of spring fish may not be ex- 

 pected in Glen Lyon at the opening of the season ; 

 but, other things being equal, it is found by actual 

 thermometric readings that the order in which fish 

 enter the Tay tributaries, in cases where free access 

 is at all times possible, is precisely the order of 

 descending temperature. To state the case in more 

 general terms, it may be said that the tributaries 

 referred to are not entered by early fish till their 

 temperatures have become approximated to the 

 temperature of the main river near the influx of 

 the tributaries. 



It might further be noticed that the tendency of 

 the early salmon in making his steady ascent of such 

 a river as the Tay seems to be to pass by tributaries 

 which enter the lower reaches, and especially to pass 

 by tributaries of small size. I know of no instance 

 in Scotland of early fish entering tributaries near to 

 the mouth of the main river, as autumn fish un- 

 doubtedly do. In the Spey, for instance, the tribu- 

 taries of Fiddoch and Avon are frequently crowded 

 with late running spawners, but they never contain 

 spring fish. In the Tay the tributaries Almond and 

 Isla, both large enough to hold spring fish, are passed 

 by, but there may be other reasons, since the former, 

 as we have seen, is a cold river, and moreover is too 

 much obstructed to make the ascent of spring fish 

 possible, while the latter, for which I have no tem- 

 perature records, is provided with a poor mouth. 



In other cases which might be cited there is 

 some uncertainty in determining whether a river 



