Shelfor'd. — Suitability of Water for Fishes 29 



considering their behavior they are very sensitive to de- 

 composition products and their life and death resistance 

 to them is low. The less sensitive fishes are usually of less 

 food value. Food fishes usually live associated with or- 

 ganisms which, like themselves, are very sensitive to de- 

 composition substances, and these organisms are usually 

 absent when the fishes are. 



Indices are then of three types (1), results of the in- 

 spection of bottom, (2), results of chemical tests of the 

 water for decomposition products, and (3), the presence 

 or absence of index organisms of a semi-stationary char- 

 acter, such as snails, etc. Here we will concern ourselves 

 with the first two types only. The third is doubtless the 

 basis of common though restricted practice in judging 

 the suitability of waters for fishes, being the result of ex- 

 perience of individual naturalists, but is not as yet organ- 

 ized for general purposes. This is to be taken up ex- 

 perimentally by the writer in connection with the work 

 of the Illinois State Laboratory of Natural History to be 

 reported on later. 



Considering the first two we must comment separately 

 upon fresh and salt water. If a body of fresh water is 

 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. The terrigenous bottom 

 should usually be free from blackened debris for this 

 usually accompanies decomposition. It should be borne 

 in mind, however, that there is nothing deleterious about 

 humus provided the material in it has passed the early 

 decomposition stages. Thus darkened bottom usually, 

 though not always, indicates decomposition and bad con- 

 ditions. For many fishes an area of water more than 

 four feet deep is relatively unimportant. This much of 

 the inspection can readily be completed by a cursory 

 mapping of the different areas in the body of water. 

 Since most bodies of water contain sufficient vegetation 



