DECREASE OF ANIMAL LIFE 381 



effects need scarcely be cited. Most hinder migration in 

 some or in every state of the river, and by causing an 

 unusual crowding of Salmon in the pools immediately below, 

 offer easy opportunity for the spread of contagious disease 

 or for the raids of poachers. Thus below the dam-dykes of 

 Mugiemoss Paper Mill on the Aberdeenshire Don, as many 

 as a thousand Salmon have been gathered in a comparatively 

 small pool, all awaiting the flood which would give them 

 opportunity to ascend ; and it was customary for the water- 

 bailiffs in mid-autumn to cart several hundreds of Salmon 

 from this pool to an open stretch farther up the river. Even 

 where a dyke does not form a complete barrier to the passage 

 of Salmon it differentiates against the most valuable indi- 

 viduals: it is stated, that, before the making of a fish-pass in 

 1900, the Dupplin Cruive Dyke on the Earn, while allowing 

 the passage of young and specially vigorous Salmon, formed 

 an obstruction impassable to Salmon weighted with spawn. 

 Even the alteration of the river-bed, the creation of 

 shallows above and below a dyke, may lead to the discom- 

 fiture of migrants. There appeared in the Scotsman of i3th 

 March 1918 an account of a remarkable run of Salmon on a 

 Border river, the Tweed. But a weir at Melrose interfered 

 with the consummation of an event which ought to have 

 stocked the upper reaches with unusual wealth of breeding 

 fish. 



" The fish ascended the cauld in large numbers," wrote the correspondent, 

 " and in the shallow water on either side it was a matter of no difficulty to 

 seize some of them as they made the passage. The spectacle of so many 

 fish passing to the upper waters led to a general relaxation of ordinary 

 conditions. On one of the days of the week-end, men, women and boys 

 could be seen in the water, standing up to the knees, and armed with gaffs. 

 The operations of those actively engaged were watched by large crowds on 

 the banks. The natural instinct for capture, aided by the food stringency, 

 became so prevalent that an unprecedented spectacle was witnessed on 

 the Sunday. Many who had been attending the morning service found the 

 spectacle of one particular hole, which had practically become a moving 

 mass of fish, too much for ordinary restraint. The quantity of salmon taken 

 at this point is understood to have been extraordinary... Two of the 

 captured fish weighed 50 Ib. and 48 Ib. respectively." 



The accompanying map (Map VI), where are indicated by 

 shading the river-basins to which migration is interfered with, 

 but not necessarily prevented by artificial obstructions, gives 

 some idea of the extent of this influence upon the migratory 



