81] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 81 



fish come from the deep. By referring once more to table 18, one will find 

 that there is a slow but very definite time gradient, from 7:45 on August 

 8 to 6:50 on September 7. One notes, further, that on cloudy days, when 

 the light intensity is lessened, the hour of inshore migration is notably 

 earlier, as on August 18. These facts, coupled with the known fact that 

 Labidesthes sicculus is a surface inhabitant, point to a conclusion that the 

 species is positively phototropic. Is there any further evidence on the 

 question? This brings us again to Hubbs' statement that at night the 

 fish lie motionless as if asleep. Had he happened to visit the lake on a 

 clear moonlight night, he would have seen a sight that would have caused 

 him to alter his conclusions. 



During the permanently deep water stage of the life history of the fish 

 the young do lie motionless at night — motionless, that is, except for the 

 fin movement necessary to sustain their position. But with the change in 

 habits which propels them shoreward at dusk, certain other changes 

 occur. If the night be dark and calm, with no moon — or at best a new or 

 old moon — the fish come in and behave essentially as they do in the 

 earlier stage of their history over deep water: they lie suspended, motion- 

 less. But let the moon be two thirds or more full, and the shallows becomes 

 the scene of one of the most startling activity in the fish world. The 

 silversides seem to go crazy, as if they were moon-struck. They dart about 

 at a most startling speed, dashing here and there, leaping out of the water 

 again and again, bumping into each other, splashing, circling, behaving in 

 a most exaggerated manner. If the water be calm, the surface becomes 

 entirely agitated by their activity so that a myriad of tiny waves dance 

 upon its surface, and the gentle splashing of the "breaks" is the character- 

 istic night sound of the lake. Such activity goes on during the entire night 

 if the light holds. But let the sky become clouded and the moon be hidden, 

 and the activity immediately dies down. That this is a phototropic reaction 

 can not be doubted. If the moon is not sufficiently bright, no activity 

 results; if the moon be full, but so low that the light is refracted from the 

 surface instead of penetrating into the water, no reaction occurs. But let 

 the point of refraction be passed and the activity begins at once. Again, 

 late at night, let the point of refraction be again reached by the setting 

 moon, and activity ceases. It is a phototropic reaction in response to a 

 certain minimum amount of light. Thus one can explain not only their 

 activity on bright moonlight nights, but their incessant activity during 

 the daytime. 



A series of simple experiments was designed to see if this activity is 

 a phototropic reaction. On the night of August 10, with no moon present, 

 the fish were up in the shallows, lying quiescent. A bright, focused beam 

 from a pocket flash-light was thrown upon the water. To begin with, there 

 were no fish in the circle of light (two feet in diameter) ; however, the fol- 

 lowing results were obtained: 



