THE SALMON AND WATER TEMPERATURE 139 
Reference in this connection may be made to one 
other river, the Helmsdale, which, since it differs in 
physical features from the rivers already referred to, 
may stand as a further example of how this concep- 
tion of salmon ascent as affected by temperature 
works out in Scotland. Here there is no large loch 
in the course of the main ascent. Two streams come 
from two lochs at practically the extreme head waters, 
and unite to form the main river, which has then a 
course of about 20 miles to the sea. About half-way 
down this main river, however, there isa fall. What 
we find here is that the early fish enter freely from 
the sea and slowly make their way upwards, but do 
not ascend the fall. All the spring fishing is between 
the fall and the sea. When, however, the wintry con- 
ditions of water have passed, the ascent of the fall 
is freely made, as in the upper waters previously 
referred to, so that summer angling is carried on in 
the entire length of the river. Falls such as we have 
been referring to are therefore not serious obstruc- 
tions to fish when the water is comparatively warm, 
but are total obstructions when the water is cold. 
Such a statement, I must again remind the reader, 
applies to Scotland ; it would, I believe, be entirely 
erroneous in a country like Norway, where all sal- 
mon, although late, ascend obstacles in the cold 
water which comes from melted snow. Salmon passes 
on such Scottish falls, to be of any use, must be of 
extremely easy gradient, so that fish may readily 
swim them, and at the same time they should con- 
tain a large body of water. 
We have now to consider the part water tempera- 
