380 INFLUENCES OF CULTIVATION 



smaller organisms of the waters, and have in some cases 

 seriously checked the movements of even the large migra- 

 tory fishes. This effect was clearly shown in America where 

 the erection of mills and their dams on the 'Connecticut 

 River greatly diminished the number of migrating Salmon, 

 though in this case a compensating increase of other fishes 

 was the result, for the Striped Bass on which the Salmon 

 fed, multiplied as their destroyers disappeared. But no 

 such compensation has occurred in Scottish rivers, for our 

 migrating Salmon seldom feed to any extent on their sojourn 

 in fresh waters, and the erection of cruives, weirs and dykes, 

 which impede or check migration, has definitely, and in 

 some rivers seriously, reduced the quantity and value of the 

 fish fauna. 



Mills themselves are in some cases exceedingly de- 

 structive to fishes, as where smolts enter a lade in their 

 descent towards the sea and so pass over the water-wheel 

 or through the turbines. In the case of the river Don in 

 Aberdeenshire evidence was given before the Royal Com- 

 mission on Salmon Fisheries (1902) showing that during the 

 period of smolt migration seawards, when, owing to the 

 lowness of the river, the whole of its water passed through 

 the mills, great numbers " cartloads " a witness said of 

 these young Salmon, compelled to pass through the turbines, 

 were cast out, bruised and dead, in the water beneath. 



But this actual destruction is less common and less sig- 

 nificant than a simple interference with the upward move- 

 ments of migrating Salmon, by the erection of mill-dams 

 and weirs. The importance of such interference lies partly 

 in its actual reduction of numbers, but more in that it cuts 

 off suitable spawning beds in the upper reaches from several 

 or from all the mature immigrants from the sea, and in so 

 doing reduces the potential stock of future years. In Scot- 

 land alone it has been estimated that as many as 50 lochs 

 and some 360 miles of river, suitable for the existence of 

 Salmon, are rendered useless partly by impassable waterfalls, 

 but mainly through the interference of man in creating 

 impediments to migration by the erection of obstructions 

 and contamination of the waters. 



The tendency of such obstacles as dam-dykes, cruives 

 or caulds must be so clear that examples of their actual 



