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 tvN'o 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 is a 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 ujDper 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- 



