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 bluefish in immense masses, while the bluefish | 
themselves in their ardent pursuit are stranded at the same time. 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 cf 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 theriver. 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 specialenemy. There is no doubt whatever that the num- 
- ber of bluefish caught during the summer season for market purposes 
te i 
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 caz- 
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. 
