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, III. No. iv., 1886. 



t Nineteenth Annual Report, Fishery Board for Scotland, II. 

 p. 72. 



