38 New Yobk Stvtb 



their position, as well as to undergo for a limited period unfavor- 

 able conditions without really adapting themselves to the situation. 

 For other reasons, also, study of the fish in the stream 

 alone gives unreliable results. In response to the complaint 

 that the pollution of streams has affected the abundance of 

 fish life, it has often been urged that the real reason for 

 the scarcity of the fish fauna is to be found in the multitude of 

 fishermen and the methods that they follow, rather than in the 

 unfavorable conditions of the aquatic environment. One hears 

 strong arguments advanced in the effort to establish that contention, 

 and it may well be true. In many regions the number of fishermen 

 has increased beyond the possibility by natural methods of repro- 

 duction to supply the draft upon the waters. Men have also caught 

 more fish than was reasonable or legal, and have used nets, traps, 

 dynamite and other means of obtaining fish to such an extent that 

 the fish supply in certain waters has been reduced pretty nearly to 

 zero. There are other factors also which tend to reduce at a given 

 time the number of fish in a certain stream or pond. One may refer, 

 first, to the migrations that fish carry out for reproductive purposes 

 or in response to seasonal changes of water temperature and depth, 

 as well as those which result from mechanical disturbances and 

 from the addition of chemicals and wastes of various kinds. 



Hence to base any conclusion upon the number of fish which 

 happen to be present in the particular region at a given point of time 

 is evidently to run the risk of serious error. A stream may be well 

 adapted to support fish life, and adequately supplied with the con- 

 ditions necessary for the existence of the fish, even though it be 

 entirely without, or nearly entirely without, a fish fauna. It is how- 

 ever, not difficult to distinguish between those places in which the 

 absence of fish may be due to the pollution of the stream and those 

 other points at which the lack of a fish population is to be explained 

 on some other basis. 



Natural Aquatic Life 



The method for distinguishing between such cases in general will 

 be readily understood when one considers the general biological 

 condition of the water. Each water body contains, under natural con- 

 ditions, a varied assortment of living organisms, provided it furnishes 

 the possibility of existence for the fish themselves. These other 



