85] STUDY OF SOUTHERN WISCONSIN FISHES—CAHN 85 



shallow water environment is not due to enemies in the deep. Hence 

 protection can not account for the migration toward the shore. 



The suggestion of Hubbs that the species seek the deep water as an 

 avoiding reaction against large objects falls down when the species is 

 found returning to the shallows under cover of twilight and associating 

 itself more closely with large objects than any fish with the exception of 

 those characteristically bottom thigmotactic forms such as many of the 

 species of Boleosoma, Cottus, Etheostoma, etc. Since the fish return to 

 their deep water station again during the day, it would be hard to account 

 for this reversal of their reaction if the behavior were merely a negative 

 response to large objects. 



In order to determine in the laboratory whether the young fish tend 

 to avoid large bottom objects, the same tank used in the work on changing 

 temperature was used. This time three large stones were placed near one 

 end, and the fish permitted to swim around in the tank. The results as 

 shown in figure 16 indicate that the fish pay no attention to the presence 

 of large objects on the bottom. In the case of a piece of floating wood, 

 however, the fish show a marked aversion to its vicinity. This however, 

 is not surprising, and can be considered as nothing more than a normal 

 reaction to an object in the path of progress. It could hardly be expected 

 that a fish or any other animal would do anything other than avoid such 

 an object. 



This brings us to the last of the factors investigated by the writer as 

 a possible cause of the migrations which Labidesthes performs, the change 

 in hydrogen-ion concentration of the water. It was previously noted that 

 there is a slight change in the H-ion concentration of the lake water from 

 top to bottom, the pH readings (colorimetric) being higher at the surface 

 than at the bottom (see table 3) indicating a lesser (more alkaline) 

 condition near the surface. Inasmuch as the silversides is confined to the 

 surface waters, it is these surface conditions that are important. The 

 question under consideration becomes: is there any change in the H-ion 

 concentration over the deep water that could cause or initiate the shore- 

 ward movement as evening approaches. 



With the aid of the assistant, a series of readings of H-ion concentra- 

 tions were made over the shallow and deep waters, a duplicate, checked 

 series of phenol red indicators being used. Two samples of water were 

 taken in each case, and if there occurred a material difference between the 

 two readings, a third was immediately taken. In order that the light 

 conditions remain constant throughout the work both day and night, a 

 small black box was fixed to the end of a pocket flashlight, and the light 

 sent into the box through a pale blue glass, to give as nearly a white light 

 as possible. Both batteries were checked at the beginning and end of the 

 work each day. A hole in the top of the box permitted the insertion of the 



