108 DESTRUCTION OF AQUATIC ANIMALS. 



support so great a change of circumstances.* Industrial opera- 

 tions are not less destructive to fisli which hve or spawn in fresh 

 water. Mill-dams impede their migrations, if they do not abso- 

 lutely prevent them ; the saw-dust from lumber-mills clogs their 

 giUs ; and the thousand deleterious mineral substances, discharged 

 into rivers from metallurgical, chemical and manufacturing es- 

 tabhshments, poison them by shoals.f The Scottish journals, a 

 few years since, stated that, in an inundation of the river Dee, 

 many factories had been destroyed, and a large quantity of chem- 

 ical substances, employed for dyeing and other purposes, were 

 swept into the stream. The effect of this was such as absolutely 

 to exterminate all the fish in the river for the distance of eight or 

 ten miles. The London Times also made important statements 

 respecting the injury to the sahnon-fishery in the Tweed, in 1874, 

 by the discharge of factory refuse into the river. 



We have httle evidence that any fish employed as human food 

 has naturally multiplied in modern times, while all the more valu- 

 able tribes have been immensely reduced in numbers. This re- 

 duction must have affected the more voracious species not used as 

 food by man, and accordingly the shark, and other fish of similar 

 habits, even when not objects of systematic pursuit, are now com- 

 paratively rare in many waters where they formerly abounded. 



* A fact mentioned by Schubert — and which, in its causes and many of its 

 results, corresponds almost precisely to those connected with the escape of Bar- 

 ton Pond in Vermont, so well known to geological students — is important as 

 showing that the diminution of the fish in rivers exposed to inundations is 

 chiefly to be ascribed to the mechanical action of the current, and not mainly, 

 as some have supposed, to changes of temperature occasioned by clearing. 

 Our author states that, in 1796, a terrible iaundation was produced in the In- 

 dalself , which rises in the Storsjo in Jemtland, by drawing off into it the wa- 

 ters of another lake near Ragunda. The flood destroyed houses and fields ; 

 much earth was swept into the channel, and the water made turbid and muddy ; 

 the salmon and the smaller fish forsook the river altogether, and never returned. 

 The banks of the river have never regained their former solidity, and portions 

 of their soU are still continually falling into the water and destroying its pu- 

 rity. — Resa genom Sverge, n., p. 51. 



f The mineral water discharged from a colUery on the river Doon, in Scotland, 

 discolored the stones in the bed of the river, and killed the fish for twenty 

 miles below. 



The fish of the streams in which hemp is macerated in Italy are often poi 

 Boned by the juices thus extracted from the plant. — Dorotea, Sommario della 

 ttoria delV Alieutica, pp. 64, 65. 



