RESISTANCE OF FISHES. 329 



were kept in running tap water and there was practically no 

 mortality. In all some fifteen species were used. In a few 

 instances, few individuals were available and in these cases the 

 data will not be presented unless particularly suggestive. 



IV. REACTIONS OF THE FISHES. 



The experiments were of six kinds, this number resulting from 

 the different combinations of gases, those being selected which 

 it seemed would be most likely to give significant results. They 

 were (i) low oxygen (0.1-0.15 c.c. per liter) and high CO 2 (35-50 

 c.c. per liter) ; (2) low oxygen and alkaline w T ater (0.5 c.c. N/2O 

 HC1 made neutral); (3) low oxygen and medium CO 2 (15-17 c.c. 

 per liter); (4) low oxygen and low CC>2 (about I c.c. per liter); 

 (5) medium oxygen (3-4 c.c. per liter) and high CO?; and (6) high 

 oxygen (8-10 c.c. per liter) and high CO;;. 



All the concentrations used are found at times in natural 

 waters. Birge and Juday ('n, pp. 144-169) report concen- 

 trations of carbon dioxide for Wisconsin lakes, which vary from 

 about -7 c.c. to nearly +35 c.c. per liter. They also report 

 concentrations of oxygen varying from o to 15 c.c. per liter. 

 On page 89 (/. c.) they report concentrations of carbon dioxide 

 as high as 47.8 c.c. per liter, in the ground waters of the state. 

 Alice (unpublished) in tests of the water in the series of ponds 

 along the south shore of Lake Michigan has found 40 c.c. per 

 liter of carbon dioxide. In analyses of the water of these same 

 ponds, I have found concentrations of oxygen varying from I c.c. 

 to 12 c.c. per liter. 



It is probable that the extreme concentrations, as low oxygen 

 (o.i c.c. per 1.) and high carbon dioxide (35-50 c.c. per 1.) are 

 not in any sense general conditions, but concentrations not so 

 extreme are of frequent occurrence. Birge and Juday state 

 that conditions of low oxygen and high carbon dioxide may exist 

 for long periods in the deeper waters of the Wisconsin lakes, 

 but the influence of such concentrations upon general fish distri- 

 bution is probably not so great as that of smaller but more 

 widely distributed concentrations. 



The presence of high and low concentrations of carbon dioxide 

 is affected by many factors such as vegetation in the water, 



