CONDITIONS OF EXISTENCE 45 



fishes are usually of less food value. Food fishes usually live asso- 

 ciated with organisms which, like themselves, are very sensitive 

 to decomposition products, and usually disappear with the 

 fishes. 



Indices are of three types, (i) results of the inspection of the 

 bottom, (2) results of chemical tests of the water for decomposi- 

 tion products, and (3) for fishes the presence or absence of index 

 organisms of a semi-stationary character, such as snails, etc., 

 see p. 52. Here the first two types only will be considered. 



If a body of water is to support desirable game fishes it should 

 have an area of terrigenous bottom covered with from 6 inches to 

 2 feet of water for breeding grounds and an area of submerged 

 (Chara, etc.) and of emerging vegetation to supply food. It is 

 probable that for the best results these three should be about 

 equal. The terrigenous bottom shoiild be comparatively free from 

 putrescible material. Humus which does not contain putrescible 

 material or even the roots of plants may be used by a few game 

 fishes for breeding. The amount of terrigenous (non-putrescible) 

 bottom up to one third that occupied by vegetation and muck is a 

 rough index of the suitability of an ordinary pond or lake (see 

 Fig. 7, p. 58) for game fishes and associated organisms. In river 

 bottom lakes and bayous floods may remove putrescible material 

 and leave bottoms composed chiefly of silt upon which luxuriant 

 vegetation springs up. Forbes has shown that productivity of 

 carp, and fishes generally, bears some direct relation to the area 

 fairly well supplied with submerged vegetation. The second index 

 must be appHed to such waters. 



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 large quantity of 

 carbon dioxide nearly always indicates lack of oxygen. 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 three cubic centimeters per liter, nor ex- 

 ceed six cubic centimeters during the summer months. Such 



