130 THE LIFE OF THE SALMON 
are in January and February from 5° to 10° colder 
than the sea. In March and April the conditions 
become more unsettled, and sudden and rather 
violent fluctuations are common (melting snow, frost, 
rain floods). A variation of 8° in the twenty-four 
hours is not uncommon. On an average, however, 
the river water is 4° colder than the sea. The 
Thurso is another well-known early river. Tempera- 
tures for the complete year 1886 were taken at 
three points, at the mouth, half-way up, and at the 
head, together with sea temperatures for the same 
period.* In January and February the river mean 
is 6° to 9° colder than the sea, and not till April do 
the river and sea temperatures equalise. Readings 
from the Dee show the same result, as also do 
readings from the Tay and Shin, although from 
these rivers the series of temperatures is not so long 
as from the Helmsdale and Brora. A series of 
valuable readings for ten years taken in the Ugie 
bear out, however, a precisely similar result.t Lest, 
however, it should be imagined that, with the 
greater fluctuation in river than in sea, periods 
occur, even in the early months of January and 
February, when river temperature equalises with 
sea temperature, it may be said that, the daily 
readings show that the river water is constantly 
colder than the sea. In those east coast rivers, 
therefore, the early running fish leave a cold sea for 
a considerably colder river in their migration to 
fresh water. 
* Jour. Scot. Meteor. Soc., Lil. No. iv., 1886. 
+ Nineteenth Annual Report, Fishery Board for Scotland, II. 
p. 72. 
