THE SEA FISHERIES OF EASTERN NORTH AMERICA. 61 
D.—NUMBERS AND ABUNDANCE OF FISH. 
That fisk of many varieties have decreased greatly in abundance 
within the historic period in all parts of the world is well established, the 
reduction in some cases being truly enormous. This, however, applies 
only to certain varieties, especially of the anadromous fish, or those 
running up the rivers from the sea to spawn, and to the more inshore 
forms. The most indubitable cases of diminution are those of the shad, 
fresh-water herring, salmon, and striped bass. On the other hand, 
there is no reason to suppose that the cod, mackerel, bluefish, and the 
sea herring have been reduced essentially, if at all, in numbers, the stock 
of these fishes being from year to year about the same, and an apparent 
diminution in one region being balanced by a greater supply in another. 
In previous pages of this article, in illustrating another subject, I 
have referred to the difference in the numbers of shad and _ herring in 
the Potomac at the present time and in the past, an experience which 
is shared toa greater or less extent by all the rivers of the Atlantic 
coast. Many streams which formerly furnished a vast quantity of 
food, within easy reach, have now become entirely unproductive, 
so that itis only by a combination of measures of artificial propaga- 
tion in the rivers and judicious legislative enactments that anything 
like the earlier experience can ever be again realized. . 
The causes of this variation in abundance, so far as they can be de- 
tected, may be considered under two heads: first, the natural, or un- 
controllable ; and, second, the artificial, or those connected with the 
interference of man. Where the former alone are responsible there 
may be a hope of a return to original abundance; man’s influence acts 
persistently and with increasing effect throughout long continued years. 
There are two classes of natural causes of variation: first, those 
induced by physical conditions ; and, secondly, the dependence of the 
fish upon, or the relations of fishes to, their fellow-inhabitants of the 
sea. The action of man iseither direct or indirect. The direct agen- 
cies are those of overfishing and the pollution of the water. The indi- 
rect consist of the obstructions to the movements of the fish, the dis- 
turbance of the balance of nature, by unduly fostering or destroying 
certain classes of animals, and by breaking up the schools of fishes 
during a critical period, and preventing their spawning. 
We havealready considered under the heads of migrations and move- 
ments of fishes the subject of variations in abundance, depending upon 
migration, or change of place, where, although the fish may be scarce in 
one locality, they are proportionally more abundant in another, the 
actual number in the sea remaining the same. At present we are con- 
sidering the subject of diminution in actual number of fish. It will 
be more convenient to consider fhis subject of variations in the abun- 
dance of marine fishes under ‘the next head, of dangers and fatalities, 
where I propose to go into more details. 
