SUGGESTIONS AS TO INDICES OF THE 



SUITABILITY OF BODIES OF 



WATER FOR FISHES 



By Dr. V. E. Shelford, 

 University of Illinois, Urbana, III. 



The relations of fishes to environment are very com- 

 plex due to the great complexity of the environment it- 

 self. This complexity seems to grow greater as our 

 knowledge of it is increased and our perspective widened. 

 The decomposition of organic matter in water under the 

 action of bacteria yields many substances, even under 

 primeval conditions, the physiological effects of which 

 have been too little studied to make a statement of their 

 importance practicable at the present time. When we 

 look to the field of contamination and the products which 

 it yields the complexity is increased many fold. Still, in 

 spite of this, we believe that it is practicable to use the 

 presence of certain conditions as indices of the suitabil- 

 ity of an entire great complex for food fishes. 



In such a discussion we must keep in mind the fact 

 that in fresh water the majority of food fishes deposit 

 their eggs on the bottom. The eggs of many marine fishes 

 rest on the bottom, but a considerable per cent, have pela- 

 gic eggs. It is to the bottom that the dead bodies of organ- 

 isms sink and decompose and, accordingly, at or near 

 the bottom the poisonous products of decomposition oc- 

 cur in greatest quantity. Decomposition of the bodies of 

 plants and animals results finally in gases such as am- 

 monia, carbon dioxide, hydrogen sulfide, methane, etc., 

 which diffuse rather slowly to the surface and into the 

 atmosphere. Thus the extent to which they occur is de- 

 pendent upon the amount of decomposition and the circu- 

 lation of the water. It must further be borne in mind 

 that the same processes of decomposition which result in 

 these gases consume oxygen and as a rule there is insuf- 

 ficient oxygen for eggs and young fishes and in many 



