258 Roosevelt Wild Life Bulletin 



stream fishes proper can not do this, and therefore die when placed 

 for any length of time in standing water." 



" In general, the resistance of fishes is correlated with the 

 environment in which they are found. The more resistant species 

 are found in ponds and shallow lakes w^hile the least resistant fishes 

 occur in the swift streams and in cold, deep lakes." 



Very recently Shelf ord ('23) has again discussed rather fully the 

 influence of hydrogen ion concentration upon fishes and other 

 aquatic animals, and points out that the chemical methods for their 

 determination are not yet perfect to such a degree as to determine 

 the relative value of this factor. The perfection of the various 

 chemical methods to a point of devising relatively simple field 

 methods wall be of great value in fish culture, as w^ell as for other 

 purposes. 



Methods of using index organisms or communities of organisms, 

 that is, different kinds of plants and animals whose characteristic 

 presence in w^ater indicate its chemical and other conditions, seem 

 destined to become of considerable importance in determining fish 

 habitats, because of their ready application to field conditions. 



Temperature. The usual fish cultural test of suitability of w-aters 

 for receiving young fish has been the temperature of the water. As 

 concerns certain species of fishes the range of temperature in which 

 each species will thrive, or the extremes of which it can endure, has 

 been approximately determined. But temperature alone is not the 

 all-controlHng factor. There are other essential factors to which, 

 in some cases at least, temperature is secondary, as, for instance, 

 when a fish " seeks " a water condition which is colder than that 

 which it leaves, although the temperature in the latter place may be 

 well within its particular " temperature range." In such instances 

 some other factor, as oxygen, may be the controlling factor, for it 

 is know'n that lowered temperature signifies greater absorption of 

 oxygen, at least under certain attendant conditions, as for instance 

 in the rapid w^ater of a stream of clear or pure w^ater. 



Space will not permit of any extended discussion of bio-physico- 

 chemical conditions which are intimately bound up with temperature 

 conditions, and in this connection it is hardly necessar\', for after 

 all, temperature is often a fairly good guide. Yet in an investiga- 

 tion all of the factors must be considered in order to learn the 

 significance of many phenomena connected with fishes, the causes 

 of which are obscure. 



There are at least two artificial conditions which modify biological 

 capacit}-, but the extent and the effect of which are not always 

 clearly known. One is that of pollution and the other physical 

 obstructions. 



Pollution. Pollution has been accused of all sorts of harmful 

 effects upon fish, and in many instances justly so. But in the 

 words of Evermann ('94), " we know^ that vast quantities of sawdust 

 and vast amounts of refuse from paper-mills and other factories 

 are let into our streams, and we know, perhaps, that great mortality 



