Fish Culture in Inland Heaters 257 



tions have now changed greatly, and no one can tell when to expect, 

 even in remote woods, a sawmill or some mine or industrial plant, 

 which drains or -throws its refuse into streams and lakes. With the 

 automobile these remote waters are now relatively accessible, and 

 over-fishing is a condition that cannot be ascertained by an inspection 

 of the waters. Probably the greatest handicap is that the time avail- 

 able for such considerations is very limited, few have the training 

 to make such determinations, and finally the amount of waters need- 

 ing examination are so extensive. There are thus many difficulties 

 which usually prevent the proper study of this problem. 



Indexes to Fish Habitats. Many efforts have been made to find 

 simple accurate methods of measuring the suitability of waters for 

 fish. Shelford and his students (Shelford and Allee, '12; Shelford, 

 '14, 'i8a, '23; Wells, '18) have given much attention to this 

 important subject, particularly to the chemical conditions, and the 

 influence of the chemicals on fish. As Shelford ('14) points out, 

 the common methods are three in number: i, inspection of the 

 bottom, largely to determine the presence of organic debris ; 2, 

 chemical tests, largely for decomposition products ; and 3, the 

 presence of index organisms known to indicate certain conditions. 



Shelford ('14, pp. 29-30) states that: " If a body of fresh water 

 's to support the most desirable fishes it should have an area of 

 clean sand, gravel or other terrigenous bottom covered by from six 

 inches to two feet of water and an area of emerging and submerged 

 vegetation to supply food. It is probable that for the best results 

 these three areas should be about equal. . , . Since most 

 bodies of water contain sufficient vegetation to supply insects and 

 other food for more fishes than can exist there, the amount of 

 terrigenous hottoin up to one-tJiird of that occupied by vegeto^tion 

 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 

 sufifer from it or leave on account of it. Carbon dioxide results from 

 the decomposition of organic matter. In the process oxygen is con- 

 sumed so that the presence of any quantity of carbon dioxide nearly 

 always indicates lack of oxygen. Fishes are very sensitive to carbon 

 dioxide, turning back from increase of one or two cubic centimeters 

 per liter of the gas in solution. . . . Thiis the amount of carbon 

 dioxide may be taken as an index of the suitability of the water." 

 '(Cf.'G. M.' Smith, '23, p. 135.) 



Wells C18, p. 568) has further shown by experiment that: " The 

 more resistant species are found in ponds, shallow, muddy-bottomed 

 lakes, or in the stagnant pools of streams. These are the fishes 

 which one sees in aquaria. They are able to withstand increased 

 temperature and wide fluctuation in the oxygen and carbon dioxide 

 content of the water, and to some extent are able to live in the 

 presence of the excretory products of their own metabolism. The 



