82 ILLINOIS BIOLOGICAL MONOGRAPHS [82 



15 seconds after light was thrown, 2 fish were within the circle. 

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Thus, within the short period of a minute and a half 42 fish were attracted 

 into the circle of illumination. This experiment was performed on 32 

 nights under varying conditions, always with the same general results — 

 the fish always came to the light. How strong was the attraction of the 

 light on the fish? By walking out on the pier, a distance of 100 feet, shining 

 the light in the water as I walked, I was able to lead the fish entirely around 

 the pier again and again: they would follow as long as the light was there, 

 with an increasing number of individuals in the procession as new fish 

 found the illuminated area. By sitting in the back end of a row boat with 

 an assistant at the oars, I found that I could lead the fish for quite a dis- 

 tance out into the lake, and once, on a very hot, quiet night, I was able to 

 lead a little school of five silversides entirely across Oconomowoc lake, a 

 distance of seven-eighths of a mile, in three hours and ten minutes. Seven- 

 teen fish started the trip; five arrived, the others getting lost en route. On 

 all other occasions the school abandoned the light as deep water was 

 reached, and here it was found that a difference in temperature of 4.3°C 

 existed between the point where they started and the point where they 

 turned back. How far inshore would they follow the light? With the fish 

 normally over 30 cm of water I have repeatedly lead them inshore into 

 water 1 cm or less in depth — up among the stones where there was scarcely 

 enough water to float their slender bodies. Further: I have often had them 

 leap clear out of the shallow water, up onto the dry land, when they were 

 unable to follow the light any farther because of insufficient depth. The 

 conclusion must be drawn that the fish are very strongly positively photo- 

 tropic and that light intensity plays an important part in their migrations. 

 The writer has been unable to demonstrate any measurable difference in 

 the oxygen content of the upper water and over the deep water which 

 might account for the diurnal-nocturnal movement. Repeated analysis 

 has failed to yield any clue that oxygen is a determining factor in the migra- 

 tion. This conclusion is further borne out by Mr. Juday, who tells me that a 

 demonstratable change in the oxygen content of the water during the day 

 as compared with the night is almost unknown in his experience. 



As has been already stated, a change of food habits occurs at the time 

 the shoreward migration begins. Up to this time the food of the young has 

 been almost entirely entomostraca, and entirely plankton organisms. An 

 examination of the stomachs of 250 young Labidesthes taken prior to the 

 beginning of the inshore migrations by night, shows the following results: 



