12 New Yokk State 



Furthermore, it is very evident that the absence of fish at a given 

 point need not necessarily be explained by their movements at all 

 The readiest way to account for the situation is to furnish the ever- 

 ready excuse that the stream has been over-fished. Knowing- the 

 situation superficially, the public is ready to accept this explanation, 

 and there is no doubt that .t applies with full force to certain locali- 

 ties and to many kinds of fish. There are, however, some elements 

 in the situation which are not always considered. 



Effects of Catching Fish 



In the first place, over-catching may reduce the supply of game 

 and food fish, but it hardly has any such distinct effect upon the 

 numerous other kinds, especially the intermediate and smaller 

 species that are naturally abundant in most waters. Accordingly, 

 if the reduction in the fish population were due to over-catching, 

 one would expect to note the absence of those species which have 

 been followed up by anglers and commercial fishermen, and at the 

 same time to record the presence of the other sorts. In fact, our 

 game and food fishes are almost exclusively large forms that spend 

 their energies preying upon the smaller species, so that if the food 

 fishes were eliminated, the smaller forms would meet with less 

 competition and would become more abundant, and more conspic- 

 uous, also, for they would not be driven to sheltered places of 

 concealment, but would have opportunities to range freely through 

 the water, as they could not if the large types were preying upon 

 them. Accordingly, the absence of the smaller types of fish is 

 clearly indicative of the fact that in some respects at least the con- 

 ditions of the environment are not satisfactory. It may be that the 

 food supply is lacking, or that the chemical environment is preju- 

 dicial to health and normal existence, or finally that some factor 

 has interfered with the carrying out of the reproductive functions. 



Effects of Changed Environment 



Every one of these conditions certainly obtains in individual cases. 

 The reproductive period is perhaps the most vulnerable point in the 

 life history of the fish and the one at which unfavorable conditions 

 affect its numbers most rapidly and seriously. The erection of dams 

 not provided with fishways interposes barriers beyond which the 

 fish cannot go in its effort to reach head waters and small tributaries 

 where many species spawn. These dams also establish a constant 

 level for the stretch of the stream above the barrier. In this way 



