30 American FisheHes Society 



to supply insects and other food for more fishes than can 

 exist there, the amount of terrigenous bottom up to one- 

 third of that occupied by vegetation may be regarded as 

 an index of the suitability of the body of water for food 

 fishes. 



The second index is essential but must accord with the 

 first. The chemical character of the water must be such 

 that the fishes will not suffer from it or leave on account 

 of it. Carbon dioxide results from the decomposition of 

 organic matter. In the process oxygen is consumed so 

 that the presence of any quantity of carbon dioxide near- 

 ly always indicates lack of oxygen. Fishes are very sen- 

 sitive to carbon dioxide, turning back from increase of 

 one or two cubic centimeters per liter of the gas in solu- 

 tion. Their reactions are especially striking when car- 

 bon dioxide is accompanied by lack of oxygen ( Shelf ord 

 and Allee '12). Likewise low oxygen and high carbon 

 dioxide in combination are more rapidly fatal than any 

 other combination of these two factors (Wells '13). 

 While exact figures cannot be given it is probable that the 

 carbon dioxide content of water over breeding grounds 

 (terrigenous bottom) should not average more than one 

 cubic centimeter per liter, nor exceed 5 cubic centimeters 

 during the summer months. Such amounts are not usual- 

 ly accompanied by lack of oxygen. Thus the amount of 

 carbon dioxide may be taken as an index of the suitability 

 of the water. 



In salt water the more complex conditions make addi- 

 tional indices necessary. As in fresh water, bottom con- 

 ditions are important to fishes which use them for breed- 

 ing. Clean rock, sand, or gravel bottom in one to six 

 feet of water (at low tide) and free from darkened de- 

 composing matter and foul odor is probably essential to 

 the eggs of many salt water species because of the poison- 

 ous character of decomposition products. Thus in gen- 

 eral the relative absence of decomposition products from 

 the shallow water of any bay or enclosure may be re- 

 garded as favorable to demersal fishes like the herring. 



In the sea where such vast areas are connected the be- 



