DECREASE OF ANIMAL LIFE 385 



of the lesser life can only be imagined from the point of 

 discharge to the sea, a distance of some 1 5 miles. On the 

 Tay, the poisonous effluents of bleaching works have, 

 according to Mr P. D. Malloch, at times left the lades quite 

 white with dead smolts. The most general contamination, 

 however, is probably due to the untreated sewage of towns. 

 There are few large rivers free from this nuisance, and 

 although in many its effects pass unobserved, in others its 

 influence is forced upon notice : in the Forth in 1899, more 

 than 400 dead Salmon were taken out below Stirling, and 

 many more must have been carried away by the tide a 

 destruction attributed to excess of sewage during a period 

 of low water in the river. Again on rivers such as the Clyde 

 and the Irvine, pollution has practically put an end to the 

 migration of Salmon. 



Probably in all such cases the extermination is pro- 

 gressively selective, only creatures of special sensitiveness 

 or habit succumbing in the first instance: it is on record that 

 the influx of lead pollution to Coniston Lake destroyed the 

 Char, although the Trout were able to survive. 



The ultimate effect of pollution upon the fauna of a river 

 depends to some extent upon the character of the river 

 itself, especially at those parts where the noxious discharges 

 enter. A swift river speedily dilutes the poisons added to 

 it, and absorbs from the air fresh oxygen to replace that 

 stolen from it by decomposing organic matter. The reverse 

 is the case in a slow river, where a stagnant pool into which 

 a poisonous effluent discharges may become a death trap to 

 any fish entering it, and a barrier to migration upwards or 

 downwards. The interference is even greater, from the 

 point of view of fish migration, when the flow of a polluted 

 river is held up for a long distance by the tide, for a barrier 

 of pollution at the mouth may be sufficient to render value- 

 less the whole course of the river, no matter how suitable 

 its upper reaches in themselves may be. The accompany- 

 ing map, following the Report of the Salmon Fisheries 

 Commission of 1902, indicates in a general way the catch- 

 ment areas of Scottish rivers and their tributaries where 

 Salmon migration is interfered with by pollution. No infor- 

 mation is available as to the effects of river contamination 

 upon the invertebrate fauna, although this aspect is equally 



R. 25 



