THE SEA FISHERIES OF EASTERN NORTH AMERICA 59 



son's fishing. This is not unfrequently illustrated in the driving ashore 

 of the menhaden by the blueiish in immense masses, while the bluefish 

 themselves in their ardent pursuit are stranded at the same timer. A 

 similar pursuit of the mackerel by the bluefish is often noticed. The 

 bluefish themselves are, by an act of retributive justice, pursued and 

 driven ashore by schools of porpoises and horse mackerel or tunnies. 



Human agencies.— The influence exerted by man in determining 

 the abundance or the movements of fishes, apart from their actual cap- 

 ture, is manifested in various ways, although more particularly in the 

 case of the anadromous fishes than any other. Whenever any impassa- 

 ble obstruction is laid across a river, ascended by anadromous species, 

 as shad, salmon, &c, for the purpose of reproduction, the exclusion from 

 their breeding grounds has very soon a marked effect. Usually, for the 

 first two or three years not much difference is appreciable, as these 

 species require three or four years to mature after passing down the 

 river before they return to their starting point. There will therefore be 

 three years of successive returns of schools, and after that there will be 

 no young fish to keep up the supply, which will be confined to the older 

 individuals returning in the vain attempt to find spawning beds. At 

 the expiration of six or eight years the supply will probably cease en- 

 tirely, and there will be no further run in the river. In this event the 

 remedy is the removal of the obstructions by taking down the dams or 

 barriers, or introducing a fishway, and planting the young fish above 

 the former obstruction ; at the end of three or four years the mature 

 individuals will make their appearance again. 



Nets constitute an obstruction of less moment than dams, since they 

 are of temporary application and constantly liable to be torn or de- 

 stroyed by the elements, or removed by legal enactments. 



The disappearance of fishes to a greater or less degree from certain 

 localities has frequently been ascribed to such agencies as the sound 

 from the paddles of steamboats, the firing of cannon, &c. How far 

 this is of any moment remains to be seen. A variation in abundance 

 of fish is not unfrequently caused indirectly by man in destroying or fos- 

 tering predaceous species. It has not unfrequently happened that one 

 species of fish has greatly multiplied in consequence of the capture by 

 man of some special enemy. There is no doubt whatever that the num- 

 ber of blueiish caught during the summer season for market purposes 

 permits a vast increase in the number of menhaden, scup, sea bass, 

 and other fishes which would otherwise be devoured. 



Many such cases could readily be adduced, and suggest extreme cau- 

 tion in the adoption of measures for protecting certain fishes from 

 natural enemies, without a careful inquiry as to the possibility of indi- 

 rect results not anticipated. A noticeable instance has been furnished 

 by Mr. Whitcher, the distinguished commissioner of fish and fisheries 

 of the Dominion of Canada. 



