384 INFLUENCES OF CULTIVATION 



the breaking-down process entails the using up of the life- 

 giving oxygen stored in the waters. Not only does a dimi- 

 nution of the quantity of dissolved oxygen in river waterto one 

 half of its normal (6 ccm. per litre) seriously affect Salmonoids, 

 but the influence of deoxygenation is possibly much more 

 important if, as has recently been suggested, the excess of 

 oxygen as between sea-water and river-water is one of the 

 determining factors in the movements of migratory fishes 

 from one to the other. If this be so, it is easy to see how a 

 river once fertile in Salmon may altogether lose its attraction 

 owing to the accumulation of human life along its banks. 

 Before the days when London sewage reached its present 

 extraordinary proportions, the Thames was well stocked 

 with fishes in its lower reaches, Salmon and Sea-Trout moved 

 yearly up and down the river, and Shad, which abounded 

 there also, have left memories of their long-past presence in 

 the names of Shad-Thames and Shadwell. The Lower 

 Thames is now destitute of fishes. Harmful effects such as 

 those which follow upon the decomposition of sewage result 

 likewise, but to a less marked extent, from the disruption of 

 the organic content of the effluents of breweries and dis- 

 tilleries, tanneries and paper-mills. 



Scottish rivers afford examples of all these types of con- 

 tamination (Map VII). During the week-ends, the Don in 

 Aberdeenshire runs milky white under the Brig o' Bal- 

 gownie to the sea, because of the mills along its banks. 

 Distilleries pour into the Spey and its tributaries enormous 

 quantities of organic waste which cannot fail to affect the 

 inhabitants of its waters. In 1850 there were eleven dis- 

 tilleries on the river using 2270 bushels of malt a week, 

 but in 1900 the number had increased to twenty-seven, and 

 the weekly consumpt of malt to 50,000 bushels. There is 

 clear evidence that this pollution drove mature fish from 

 many spawning beds in the Fiddich and in the main river. 

 The waters of Gala and the Teviot, tributaries of the 

 Tweed, are "blae" with the products of wool mills. Dead 

 fish found in the Nith are said to have been killed by coal 

 washings from the collieries on the Ayrshire borders. Dis- 

 charges from collieries and ammonia works on the Doon 

 have on several occasions poisoned many fish, and in 1870 

 extirpated practically the whole fish population, how much 



