50 New York State 



destroyed most easily and most completely, and consequently the 

 times at which the character of the stream life is determined. The 

 lower the flow of the stream, the greater the probable effect of the 

 pollution upon fish. With varying conditions in the effluent, such 

 as alkalinity at one period and acidity at another, that time should 

 be selected for the experiment which gives conditions least favorable 

 for fish life. Similarly, the period of the minimum percentage of 

 oxygen in the water indicates the moment of lowest resistance on 

 the part of the fish, while other elements, such as the shortage of 

 food, which tend to reduce the vitality of the fish, are also import- 

 ant factors in determining the effect of wastes on fish life. It will 

 be noted that a number of these conditions are probably accentuated 

 at times when the water is covered by a coating of ice and conse- 

 quently the most serious effects of stream pollution will probably be 

 found when in the winter season the diminished stream flow and 

 the ice cover tend to emphasize the injurious character of wastes 

 discharged into the stream. 



The foregoing discussion is sufficient to indicate the extreme 

 limits of usefulness of the minnow tests. Much more accurate ex- 

 periments have been made on the susceptibility of fishes to various 

 specific influences and it has been possible to determine thereby 

 some important questions concerning their individual responses to 

 particular chemical substances. These experiments have been 

 carried out in the laboratory. They require complicated apparatus 

 and students have been misled in applying them because of the 

 unsuspected introduction into the natural situation of other factors 

 than those on which the experiment was based. It is evident that 

 lowered vitality due to confinement may easily be confused with 

 lowered vitality due to the influence of the chemical used in the 

 experiment. Furthermore, in order to cover the wide range of 

 substances which may enter into consideration in connection with 

 industrial wastes, the amount of time involved in experimentation 

 is extreme and the expense not an inconsiderable factor, so that the 

 method can hardly find universal application. Yet one must empha- 

 size the fact that these experiments are important and in the hands 

 of Shelford and his students have yielded very valuable results for 

 the general orientation of the worker in dealing with the problem. 

 Indeed, one may say they are indispensable and should be generally 

 employed wherever it is proposed to test a new problem or to 

 determine whether a proposed modification of some industrial waste 

 will render it innocuous to aquatic life. 



