REACTIONS OF FISHES TO ATMOSPHERIC GASES 241 



experiments were run with the two most sensitive species 

 Hybopsis and Notropis). The oxygen content of the water was 

 raised by the addition of the atmosphere from the tank (nitrogen 

 92 parts; oxygen 8 parts) and a small quantity of oxygen was 

 added at the other end. This made the difference in the oxygen 

 content of the two ends only 0.16 cc. per liter with the most 

 oxygen at the high nitrogen end. In the three experiments, 

 there were no turnings but a time preference for the low nitrogen 

 end. The graph of the reaction of Hybopsis gives no good evi- 

 dence that the fish reacted to nitrogen but only a suggestion 

 that they may react to a nitrogen gradient, and that they may 

 select the low concentration. However the fishes may have 

 been avoiding a slight oily odor which was detectable in the 

 high nitrogen atmosphere. Because of this and the small differ- 

 ence in nitrogen that could be obtained and the difficulty of 

 manipulation, the experiments were not carried further. 



3. REACTIONS TO COMBINATION OF FACTORS 



a. Boiled water 



The effect of boiling the water in the apparatus is shown in 

 table 4 (p. 217). The water lost most of its oxygen and much 

 of its nitrogen, the nitrogen content being reduced from 18.45 

 to 3.33 cc. per liter. The free carbon dioxide was reduced from 

 2.5 to 0.7 cc. per liter; 1.2 cc. per Hter of half-bound carbon 

 dioxide was lost in addition to the changes in salt content already 

 discussed. The fishes were then reacting to a difference in salts, 

 carbon dioxide and nitrogen and oxygen. When the higher 

 oxygen was chosen, the fishes of necessity selected the higher 

 nitrogen and the higher carbon dioxide. We have clear evidence 

 that the fishes selected the lower concentrations of carbon diox- 

 ide when the minimum was that of tap water but we have no 

 evidence concerning the optimum amount of carbon dioxide for 

 fish. It is therefore difficult to interpret the results of such 



Chart 4 Showing the relative intensity of reaction to the various factors 

 employed in experiments with Hybopsis. The control given is a graphic repre- 

 sentation of the average of all the controls. 



