258 VICTOR E. SHELFORD AND W. C. ALLEE 



b. Adult and juvenile fishes 



We have had only a Httle opportunity to note the differences 

 in behavior of adult and juvenile fishes. Wiegelt ('85) found 

 that young fishes were more sensitive to ammonia than adults. 

 As a rule, in our experiments the younger and smaller fishes 

 were more easily affected by the various stimuli employed than 

 the adults of the same species. In a general way, the adults and 

 where used, the young also of the hardier fishes (Abramis, Umbra, 

 and Ameiurus) react clearly in a negative manner to carbon 

 dioxide, acid, and with less vigor, to lack of oxygen. In all 

 probability the adults of the food and game fishes react in a 

 manner comparable to the young, but with the equipment at 

 hand, we were unable to obtain recordable results. 



Among other observations of this kind, a large Lepoh'iis was 

 placed in the tank with the smaller individuals in an experiment 

 with a carbon dioxide gradient in tap water. During the entire 

 period of experimentation, the young fishes went back and forth, 

 both turning back from the center and spending a longer time 

 in the tap water. The large fish came to rest in the carbon 

 dioxide end, and although clearly affected, as shown by gulping 

 and rising to the surface, remained in the carbon dioxide for 

 twenty-six-and-one-half minutes before encountering the tap 

 water end. During the next seven minutes, it remained most 

 of the time in the tap water but after that went back and forth 

 for seven minutes at a very rapid rate and without stopping in 

 either end. In a thirty-minute control, the same fish did not 

 cross the center at all. The greater speed of this large fish prob- 

 ably carried it to the end of the tank before it could be expected 

 to turn, after having been affected by entering the carbon dioxide 

 water. In such a case it could not have been possible to obtain 

 recordable results. The smaller size of the tanks, in proportion 

 to the size of the fish probably makes the surroundings much 

 more unnatural. In connection with the study of adult fishes, 

 we conclude that for the adults of the food and game fishes, the 

 tanks should be about three to five times as long as ours and 

 probably twice as wide and deep. We must, however, leave the 

 matter as a special subject for investigation. 



