RESISTANCE OF FISHES. 345 



i. e., the disappearance of the fishes from the area so long as 

 other fishes do not come in from the outside. Furthermore the 

 fishes might continue to reproduce more or less normally, with- 

 out preventing depopulation, if the adverse conditions caused 

 the young fishes to leave the area. 



\Ye can say then that in the long run the fish population of an 

 area will vary directly as the concentration of adverse conditions, 

 or that (i) the moving out of the fishes from the area, and (2) 

 the turning back of outside fishes which tend to enter, will at 

 some concentration of adverse conditions inevitably result in 

 complete depopulation. Furthermore, so long as such reactions 

 are possible, the only part played by resistance will be that the 

 least resistant fishes, since they are usually also more sensitive, 

 will move out or turn back first. In neither case need the matter 

 of dying time be taken into consideration. 



SUMMARY. 



1. Fishes die from lack of dissolved oxygen or excess of dis- 

 solved carbon dioxide. 



2. Oxygen in large amounts (10 c.c. per 1.) antagonizes the 

 detrimental effect of high carbon dioxide (50 c.c. per 1.). 



3. The action of detrimental concentrations of carbon dioxide 

 and of oxygen is first to stimulate, and if detrimental enough, 

 to later cause coma. 



4. Low oxygen (o.i c.c. per 1.) in alkaline water causes death 

 sooner than low oxygen in slightly acid water. This suggests 

 that the fishes have a CO 2 optimum. 



5. The resistance of the fishes to fatal concentrations and 

 combinations of oxygen and carbon dioxide varies with the 

 individual, with the species, and with the size (i. e., weight). 



6. Small fishes are more resistant per unit weight than are 

 large ones. This fact is not particularly important ecologically 

 or economically. 



7. Ecologically and economically, it does not matter whether 

 the fishes in a given body of water die within a minute, an hour, 

 a week, or do not die at all but merely fail to reproduce success- 

 fully; the final result must in any case be the same, i. e., the 

 disappearance of the fishes from the area, unless new stock be 

 constantly added. 



