40 THE SALMON FISHERIES. 
great manufacturing and mining centres are so seriously 
affected by pollutions that the fish are often deterred from 
entering them till a heavy flood, or the cessation from work 
on Sundays, happily reduces the volume or the virulence 
of the polluting matter. But at best their movements are 
regulated, not by the promptings of nature, but by the 
accidents of art. 
Land drainage again, by materially altering the conditions 
under which the rainfall finds its way to the rivers, has not 
been without its effect on the salmon fisheries; the rivers, 
instead of presenting a volume of water of fairly uniform 
average depth throughout the year, are affected by every 
shower: they are high in flood to-day, to fall to a lower 
level than ever to-morrow, and the movements of the fish 
are consequently rendered uncertain. 
It needs no argument to show that fish cannot live in 
polluted water : the evil which was threatened by weirs has 
been intensified and consummated by pollutions, and these 
two causes, singly or in combination, have done more to ruin 
our salmon fisheries than the most incessant over-fishing. 
And while the injury inflicted on the salmon fisheries by 
weirs and pollutions is far more serious than that caused 
in any other way, it is also immensely more difficult to 
mitigate. The mining and manufacturing industries of the 
country are of vastly greater value than the salmon-fishing 
industry. The annual value of the exports of British and 
Irish manufactures and produce is two hundred and fifty 
millions sterling. This enormous total takes no cognizance 
of home consumption, and against it the salmon fisheries 
of the three kingdoms can only show a gross estimated 
production of three-quarters of a million a year, towards 
which England and Wales contribute the modest sum of 
£150,000. But no more than a quarter of a century ago, 
