STREAM POLLUTION 
367 
Water standards for fishes and other aquatic organisms, moreover, are not 
identical with those standards which will define water as potable for human beings 
or satisfactory for industrial use. Water may be serviceable for many industries 
and yet not support fish life, or fishes may thrive in water which would be unsafe for 
human consumption due to the presence of particular bacteria as typhoid, or certain 
compounds harmful to man, as the western alkalis. 
The definition of waters as suitable for aquatic life is complicated still further 
by the fact that various species of fishes and other aquatic animals and even indi- 
viduals of different ages of the same species have different degrees of tolerance to 
deviations from the ideal environment, and to the cumulative effects of many stream 
pollutants. Consequently, the presence or even the survival for a time of fishes in 
waters suspected of pollution does not in itself constitute evidence that these waters 
are either satisfactory or safe for fishes. 
In spite of the various confusing factors which have been set forth in the pre- 
ceding paragraphs, it is essential in order to determine the extent and degree of 
pollution in any given stream to define as far as possible the limits of variation in 
the several components of those acpiatic complexes which desirable fishes will tolerate 
and in which they will still thrive. Neither minimal lethal nor arbitrary standards 
will suffice. The limiting values for the various substances in stream waters, with 
references to the effects on aquatic life, as presented here have been obtained through 
the correlation of data of four sorts: (a) The amounts of these substances found in 
natural waters where fishes were successfully maintaining themselves, (6) studies of 
streams which as far as could be determined were unpolluted and which, therefore, 
presented natural conditions, ( c ) the physiological responses of fishes and other 
aquatic animals to variations in the concentrations of these substances, and ( d ) the 
survival of aquatic forms when exposed to these substances over long periods under 
controlled conditions. These data have been drawn from the existing literature, 
and from field and laboratory studies by the staff of the Columbia (Mo.) field unit 
of the United States Bureau of Fisheries during the past 5 years (Ellis, 1935a). 
In the present consideration of water standards for fish and other acpiatic animals 
the dissolved and suspended substances have been divided into two groups, namely, 
those constituting the complex favorable to fishes in natural unpolluted waters, i. e., 
those substances to which the fresh-water fishes are physiologically adapted; and, 
those substances which are added from time to time to natural waters by man and 
his agencies, and to which the individual fish must adapt itself. There is, of course, 
some overlapping between the two groups since certain forms of pollution merely 
alter the amounts of specific substances normally found in streams, as in the case 
of the acid wastes from wire-nail mills, which effluents raise the acid ions, the iron 
and the sulphates, all of which occur in small quantities in most streams, to levels 
toxic or detrimental for aquatic forms, with, of course, disastrous results. 
Throughout the application of these data and standards it must be borne in 
mind that individual fishes and various species of fish have different degrees of 
resistance and tolerance. Consequently, some fishes may be found in waters where 
less favorable conditions than those here designated obtain, since both the minimal 
and maximal limits immediately compatible with life have been avoided, for these 
limits cannot be regarded as desirable or physiologically reasonable in determining a 
