THE SEA FISHERIES OF EASTERN NORTH AMERICA. 61 



I). — Numbers and abundance of fish. 



That fish 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 euormous. 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 to a 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 it is 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 5 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-iuhabitants of the 

 sea. The action of man is either director 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, aud by breaking up the schools of fishes 

 during a critical period, and preventing their spawning. 



We have already considered under the heads of migrations and move- 

 ments of fishes the subject of variations in abuudauce, dependiDgupon 

 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 remainiug the same. At present we are con- 

 sidering the subject of diminution in actual number of fish. It will 

 be more convenient to consider this subject of variations in the abun- 

 dance of marine fishes under the next head, of daugers and fatalities, 

 where I propose to go into more details. 



