NATURAL AND UNNATURAL CONDITIONS OF 



AQUATIC EXISTENCE 



Essential Conditions 



It is somewhat difficult for the mind to get a clear picture of 

 conditions existing below the surface of the water. This is doubtless 

 due to the fact that the light is largely reflected from that surface 

 and does not display to us clearly the objects that lie beneath. The 

 eye takes in at most limited areas, rather than the long stretches of 

 bottom territory. It finds them very differently constituted from 

 the surface of the dry land. We are inclined to consider the situation 

 more complex and difficult to understand than it really is. In truth, 

 the essentials of existence are the same in the water as on the land. 



Each organism demands for its existence a certain food supply 

 and it must have also a supply of oxygen available for respiration. 

 A shortage of oxygen in the water has precisely the same effect 

 upon fish that an insufficient supply of oxygen in the atmosphere 

 has upon air breathing animals. All degrees of oxygen deficiencies 

 may be observed in the water, as they are on the land in the atmos- 

 phere, and among the organisms of the water all grades of effect 

 produced may be observed that are familiar to us among air breath- 

 ing animals under similar atmospheric conditions. A shortage of 

 oxygen either kills or stupefies the fish, or, it may be, drives them 

 away from the abnormal environment to seek better conditions 

 elsewhere. 



Movements of Fish 



A shortage of food is promptly met by the migration of fish from 

 the region. Like other animals, they are quick to react toward an 

 approaching shortage of food, and having considerable activity may 

 desert a region entirely and move to some distant point without 

 attracting the attention even of those who are watching the stream 

 day by day. Fish also carry out natural movements of their own 

 in response to the reproductive instinct and to modifications in the 

 temperature of the water and to the amount of silt which it carries, 

 or, in other words, to the variation of the stream with the changing 

 seasons and periods of drought and rainfall. It is not easy to dis- 

 tinguish these migrations from those which are due to food shortage. 



