30 THE SALMON RIVERS OF SCOTLAND 



a smaller normal water-level and of more sudden and extreme 

 fluctuations, is more clearly understood. With regard to 

 salmon disease it may be explained that while it only became 

 epidemic as late as 1878 and 1879, it was present in the river 

 long before that time. In the Severn, salmon disease was 

 epidemic about the year 1856, and the trouble seems to have 

 been recognised and to have become more or less well known 

 as early as 1836. 



Saprolegnia of course exists in a sporadic form in ordinary 

 water and is found to a greater or less degree in pure waters. 

 The tissue attacked by the vegetable fungus has previously 

 been affected by the action of Bacillus salmonis pestis, and 

 unless the bacillus has been at work the fungus does not grow 

 upon the fish. It is therefore the life of the bacillus which 

 has to be studied, and which Mr. Hume Paterson, the discoverer 

 of the bacillus, studied very profitably in Tweed district. 

 The presence of disease in fish inhabiting pure water is never, 

 it seems to me, so marked as in water where the fish have been 

 subjected to the health impoverishing action of pollution. 

 Between 1880 and 1884 the Tweed police removed from the 

 water 37,969 diseased fish. In 1907 the number was 4,426. 

 Certain pollutions are no doubt much more suitable for the 

 cultivation of the bacillus than others ; this aspect of the 

 question is being worked out. That salmon disease can occur, 

 however, in water believed to be perfectly pure is believed to 

 have been shown by a most marked epidemic which broke 

 out amongst coarse fish in ponds supplied with spring water at 

 Ightham in Kent. 1 As in the case of grouse disease, the state 

 of nutrition and health of the fish has probably much to do 

 with it. That fish enter from the sea in some seasons in a 

 poorly nourished condition seems sufficiently clear. In 1908 

 the grilse were in many cases remarkably thin and poor. The 

 influence of polluted water on such fish is likely to be much 

 more deleterious than on strong well-nourished fish. 



In addition to this there is the statement made by those 

 who have much more Tweed experience than I, that the ova 

 of spring fish frequently dies before spawning time ; that 

 spring fish caught dropping back in summer have the ovaries 

 white and dead, and the silvery hue of the fish has turned to a 

 1 Official Report, by Buckland, Walpole & Young, 1880. 



