89] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 89 



In order to ascertain the pH preferences of the fish the galvanized iron 

 tank was again used. This was filled with water and well mixed so as to 

 give a uniform pH value throughout the tank. The fish were placed in 

 the tank and their normal movements noted. Then the pH readings were 

 taken at different points in the tank to make sure that the environment 

 was uniform. At one end of the tank a small amount of acetic acid was 

 added, this lowering the pH readings as the H-ion concentration increased 

 with the increased acidity of the water. The behavior of the fish was noted 

 and the pH of the water was taken at the point where the fish turned back 

 in its avoidance reaction. The results of this procedure are given in figure 

 17. The fact that the fish react against a slight increase in the acidity of 

 the water is shown by their returning to the less acid end of the tank as 

 soon as they come in contact with the increasing zone of water of a higher 

 acidity. The change from the pH of 7.9 is noted by the fish at once, and 

 the pH value of 7.7 and 7.65 are evidently the critical points. The fish 

 does the best it can to avoid the increasing acidity by retreating before 

 the advancing zone of pH 7.65, and the experiment ends with the fish 

 flat up against the end of the tank farthest away from the acid, with further 

 retreat impossible. 



Essentially similar results were obtained by the use of carbonate in- 

 stead of acid, thus increasing the alkalinity of the water. These results are 

 given in figure 18. It is evident from a glance at this figure that the fish 

 avoid water of a higher pH than 8.2 if possible. It is further evident that 

 their avoidance to increasing alkalinity is less decided than to an increasing 

 acidity as evidenced by their frequent lingering along the advancing pH 

 8.2 line. As in the case of acidity, the fish are evidently doing their best 

 to avoid the radical change in pH until retreat is impossible longer. In 

 this connection it is well to note that the critical point is very close to 

 pH 8.2, which is also the pH value of normal sea water. This may be 

 significant when one recalls again that this is a fresh water representative 

 of an otherwise strictly marine family, and becomes more marked when one 

 compares the reactions against higher alkalinity and higher acidity; it is 

 to be noted that the reaction against higher acidity is less decisive. 



In order to see how the fish would react in a tank with high alkalinity 

 at one end and high acidity at the other, a third series of experiments 

 was run under combined conditions. The results are very similar to those 

 of the two preceding experiments combined. The fish retreat before in- 

 creasing acidity and tolerate greater alkalinity in preference to greater 

 acidity, as shown in figure 19. It will be seen that as long as any real 

 choice was possible, the fish chose a pH value very close to 7.7. 



From these experiments it must be concluded that: (1) the fish are 

 very sensitive to pH changes in the water; (2) their range is from pH 8.3 

 to pH 7.5, with the optimum approximately 7.65 to 7.7; (3) the experi- 



