88 ILLINOIS BIOLOGICAL MONOGRAPHS [88 



and intake of carbon dioxide. With the intake of C0 2 the change in the 

 H-ion concentration of the water begins and this is indicated by the slowly 

 rising amplitudes of the pH values, showing an increase in alkalinity. Since 

 the surface water over the deep is less abundant in photosynthesizing 

 organisms, the larger aquatic plants being entirely absent, the change in 

 pH values is measurably slower. It is to be noted further that on cloudy 

 days (August 18) the pH values change more slowly and at a later hour, 

 due to the absence of sufficient light to start the photosynthetic processes 

 of the water plants. 



The morning off-shore migration of the silversides starts with a definite 

 change in the H-ion concentration of the shoal water. This becomes 

 evident if one compares the different pH values with the hour at which the 

 fish start out. 



Table 21 

 HOUR OFFSHORE MIGRATION BEGINS AND pH OF WATER AT THAT TIME 



Date Time of start of outgoing pH of the water at that time 



8/9 4:20 7.65 



8/10 4:05 7.7 



8/11 4:08 7.7 



8/14 3:48 7.65 



8/17 3:58 7.7 



8/18 5:20 7.65 



8/22 4:10 7.75 



8/26 4:22 7.7 



9/5 3:55 7.65 



9/15 5:10 7.6 



Average : 7 . 68 



Thus it is seen that the fish leave the shore as the pH reading reaches 7.65 

 to 7.7, the average being 7.68, which can not be read by the colorimetric 

 method. This gives a clue to the cause of the morning movement: it is 

 an avoiding reaction to a higher pH reading, that is, to water of increasing 

 alkalinity. Since the water in the shaol environment shows a higher pH 

 reading during the day than does the water over the deep, and since the 

 water over the deep shows a lower pH value at night than does the shallow 

 water, it must be assumed that the fish are reacting negatively to a high 

 pH value and negatively to a low pH value, indicating a marked sensitivity 

 to H-ion change and a limited range of tolerance. This conclusion is borne 

 out in a series of laboratory experiments. It is impossible to account for 

 the morning migration by light, temperature, food, oxygen or protection, 

 but one can account for it as a reaction to changes in the hydrogen-ion 

 concentration of the water. 



