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 
