166 FISH-CULTURAL ASSOCIATION. 
areas to their natural limits, but if the agency of man has so 
modified the natural conditions that the proper food of the 
young fish during their river life is no longer found, or occurs 
in much less than the necessary abundance, then the effort to 
increase the supply by artificial propagation and planting will 
prove a dismal failure. 
How far the pollution of our rivers by sewerage, gas tar, ref- 
use chemical products, etc., has changed the original conditions 
of our rivers, is a matter inviting exhaustive and critical inves- 
tigation. 
Fourth—A rational code of laws, relating to the fisheries, may 
exert an important conservative influence, by imposing such re- 
strictions upon the time and methods of capture, as will permit 
some considerable portion of the shad and herring which enter 
our rivers, to reach their spawning grounds and deposit their 
eges without molestation. 
By the observance and enforcement of the conditions above 
indicated, we may reasonably expect to greatly increase the 
average annual production of our river fisheries, but we can 
never hope to eliminate great unequalities in the product of the 
fisheries in different seasons. 
Natural conditions, apparently beyond the control of man, 
will determine disastrous and discouraging failures one season, 
and the next a teeming abundance in the same river. 
The influence of water temperatures, in determining the pres- 
ence. or absence» of; certain: species ‘of fish! inj) certainy areasvor 
water, has been observed both in regard to the ocean and the 
river species which are the object of commercial fisheries. Ob- 
servation of water temperature and its relations to the migra- 
tions of fish, have not been continued long enough to justify us 
in formulating conclusions, but the drift of investigation and 
observations goes to show that there is for each species a normal 
temperature in which it prerers to be, and that its migrations 
are determined by the shifting of these areas of congenial tem- 
perature under the influence of the seasons. 
Observations, now continued for several vears, have led to the 
conclusion that, in the case of the shad, the normal tempera- 
ture, toward which it is ever moving, is about 60 degrees, Fahr. 
