87] 



STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 



87 



So far I have considered only the shoreward migration of the evening, 

 and have said little as to the outward migration of the morning. This 

 morning movement is very much harder to study, as it is less definite, 

 less of a mass movement, than is the evening migration, and this in itself 

 indicates what might be termed "reluctance" on the part of the fish to 

 leave the shallows, and also because the departure of a few fish from a great 

 school is less easily noted than the first arrivals in an area theretofore free 

 of the species. The region into which the evening movement comes is 

 relatively limited; that into which the morning migration goes is relatively 

 unlimited. Yet a study of the H-ion concentration changes during the 

 early hours of dawn gives a very good clue as to the cause of this second 

 phase of the daily shifting. The results of the changes in H-ion concentra- 

 tion along the shore and over the deep water, made in the same way as the 

 night readings, are given in Table 20. 



Table 20 



CHANGES IN HYDROGEN ION CONCENTRATION IN THE MORNING 

 OVER SHALLOW AND DEEP WATER OCONOMOWOC LAKE, 1923 



As compared with Table 19 showing the evening changes, certain 

 marked and important differences are to be noted. Conspicuous among 

 these is the fact that the H-ion concentration along the shore begins to 

 change considerably earlier than over the deep water, as indicated by the 

 rise in the pH values as early as 3:30 on August 8. The fact that the pH 

 values for September do not change as early as those in August must be 

 explained as being due to the light conditions as affecting the respiratory 

 activity of the aquatic vegetation. It is a well known fact that photo- 

 synthesis takes place in the presence of very little light; hence the night 

 intake of oxygen and output of carbon dioxide are reversed in the early 

 morning hours long before the sun gets up, resulting in the output of oxygen 



