107] STUDY OF SOUTHERN WISCONSIN FISHES— CAHN 107 



directly with the increase in temperature, one can explain the mortality 

 as being due to three factors: 1) the thermocline; 2) warm water; and 

 3) insufficient oxygen available. The thermocline is responsible for 

 the situation. (Figs. 6, 7.) 



A second migratory rhythm was discovered when it was found that 

 the fish in Oconomowoc lake under ordinary conditions were nearer the 

 surface at night than during the day. That this is not a phototropic 

 reaction is indicated by the fact that they can easily be attracted to the 

 surface during the hours of daylight, and that they will remain under the 

 ice during the winter for hours at a time under the brightest of light 

 conditions, if food is available. This suggested the idea that this daily 

 movement might be a feeding rhythm. Since the fish were found to be 

 feeding entirely upon entomostraca at the time (February 2) a series of 

 water samples was taken at different depths at noon and again at the 

 same depths at midnight. The four chief organisms found were Daphnia, 

 Cyclops, Dioptomus and Bosmina. The water was pumped up by means 

 of a garden hose and was strained through the Birge and Juday plankton 

 catcher kindly loaned me by Mr. Juday. The amount of water strained was 

 15 liters from each depth. Figure 24 shows the distribution of these four 

 organisms at the different depths during the day and at night. It will be 

 noted that in every case the organisms migrate upward at night, the 

 amount of the vertical change being the least in the case of Cyclops and 

 greatest in Daphnia and Dioptomus. Since the cisco is dependent upon 

 these organisms for food, and since it is well known that these minute 

 entomostraca have such a daily rhythm, and, lastly, since the water 

 conditions are uniform in the winter and the fish show no suggestion of 

 negative phototropism, it must be concluded that this daily movement 

 is a feeding rhythm following that of the entomostraca. Repeated observa- 

 tions and examinations have borne out this conclusion. 



BEHAVIOR 



I have stated in the discussion of Labidesthes sicculus that, were it not 

 for the fact that the silversides lie quiescent during the dark nights it would 

 be the most active of all our fishes. But this title belongs to the cisco which 

 is active, apparently, from the time it hatches until it dies. I have had the 

 cisco under observation in the laboratory for nine months; I have spent 

 two weeks each winter for nearly twelve years watching the behavior of 

 the fish through the ice and I have never yet seen one quiet for a period 

 even approaching a second. This constant activity is perhaps due in a 

 large measure to the food habits of the fish; since their chief item of diet is 

 entomostraca and since these are eaten in enormously large numbers as 

 has been shown, it follows that great quantities of water must be passed 

 through the gill rakers in order to obtain this food in quantities such as 



