44 i\ e\v Yokk State 



estimate their relations to the different kinds of life which are 

 involved in the stream. Furthermore, the processes are time con- 

 suming and expensive so that their general application to water 

 bodies is attended with evident difficulty. 



Chemical Tests 



Chemical tests determine for us more or less readily and rapidly 

 the constituents of a given water mass. This again is somewhat 

 tedious and expensive. Furthermore, we do not by any means 

 always know the effect upon living organisms of a given chemical 

 substance, and, as has been found by experimental procedure, the 

 effects of various substances differ according to the concentration 

 present, and also, in relation to the other chemicals which may be 

 associated with this original substance. Consequently, the situation 

 is exceedingly involved and much more study will be necessary before 

 it is profitable to use such tests widely and generally in the testing 

 of water bodies. 



Those who desire to find a more rapid means of testing 

 approximately what the character of a given aquatic environ- 

 ment may be, naturally seek some sort of superficial and readily 

 determined feature as an ear-mark, and have found this in the 

 relation of the environment to fish life. This is a rough and ready 

 method of determination and not without its merits, though there 

 are distinct limitations to its employment. If one follows along the 

 bank of a stream or lake, or traverses its surface in a boat, and finds 

 large fish and schools of smaller ones, he inclines to infer from the 

 number and distribution of these animals the condition of the water. 

 However, the habit of relying on a single criterion in judging such a 

 complex situation as a natural environment is dangerous. Perhaps 

 there is no other way to explain an abundance of fish life than the 

 satisfactory condition of the water for fish existence, though it is 

 easily possible that, unless the observation is repeated at different 

 times, the condition may be the result of accident rather than a 

 general characteristic of the situation. 



Effect on Fish of Changed Conditions 



On the other hand, the total absence or extreme rarity of fish life 

 in any body of water would be good evidence that the aquatic 

 environment was unsatisfactory for them, for it has already been 

 explained that over-catching will account for a reduction in the 

 supply of food and game fish, but if it affects at all other types, 

 would rather tend to multiply than to reduce their numbers. How- 



