110 DESTRUCTION OF AQUATIC ANIMALS. 
destructive to fish which live or spawn in fresh water. Mill- 
dams impede their migrations, if they do not absolutely prevent 
them, the sawdust from lumber mills clogs their gills, and the 
thousand deleterious mineral substances, discharged into rivers 
from metallurgical, chemical, and manufacturing establish- 
ments, poison them by shoals.* 
We have little evidence that any fish employed as human 
food has naturally multiplied in modern times, while all the 
more valuable tribes have been immensely reduced in numbers. 
This reduction 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 comparatively rare in many waters where 
they formerly abounded. The result is, that man has greatly 
reduced the numbers of all larger marine animals, and conse- 
quently indirectly favored the multiplication of the smaller 
aquatic organisms which entered into their nutriment. This 
change in the relations of the organic and inorganic matter of 
the sea must have exercised an influence on the latter. What 
that influence has been we cannot say, still less can we predict 
‘what it will be hereafter; but its action is not for that reason 
the less certain. 
Our author states that, in 1796, a terrible inundation was produced in the 
Indalself, which rises in the Storsj6 in Jemtland, by drawing off into it the 
waters 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 soil are still continually falling into the water and 
destroying its purity.—Resa genom Sverge, ii., p. 51. 
* The mineral water discharged from a colliery on the river Doon in Scot- 
land 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- 
soned by the juices thus extracted from the plant.—DoRroTEA, Sommario 
della storia del? Alieutiea, pp. 64, 65. 
+ Among the unexpected results of human action, the destruction or multi- 
plication of fish, as well as of other animals, is a not unfrequent occurrence. 
