368 
BULLETIN OF BUREAU OF FISHERIES 
suitable environment for fishes any more than for man. An effort has been made, 
therefore, to present usable and reasonable standards of water suitability favorable 
to fish life; that is, standards defining waters in which a mixed fauna of fresh-water 
fishes of the common warm water types including desirable centrarcliids, cyprinids, 
catostomids, and silurids, as well as such tolerant forms as carp and gar, will thrive. 
Various lethal limits are also set forth. 
It must also be pointed out that these standards of suitability must be main- 
tained throughout the periods of low water, maximal temperature, and maximal 
liability to pollution, since a deviation in the amount of any of several substances, 
as dissolved oxygen, acids, or salts, to the critical level for only a few hours may so 
change conditions in a considerable portion of an otherwise favorable stream, that 
months or years may be required to reestablish the former fish fauna and normal 
balance of aquatic species. 
In a large series of field studies it has been shown that the natural, inherent 
water conditions of most streams can be ascertained satisfactorily for pollution studies 
as regards fisheries problems by determining repeatedly at different times of the night 
and day and at various seasons of the year the (1) dissolved oxygen, (2) pH, (3) ioniza- 
ble salts, (4) carbon dioxide, fixed and free, (5) total ammonia, and (6) suspensoids, 
since the determinations of these factors not only give specific data concerning partic- 
ular conditions, but also concerning several complexes which vary in even unpol- 
luted streams and which are definitely affected by many forms of pollution. 
From determinations made at many stations where good mixed fish faunae were 
present, it was found that the \alues from the above determinations in favorable 
waters, i. e., waters supporting good mixed fish faunae, fell within rather definite 
limits, and that deviations from these limits in our inland streams were almost always 
indicative of conditions unfavorable to aquatic life. However, these values alone, 
which cover only the more basic, inherent conditions which must be maintained in any 
stream if it is to support a good fish fauna and on which conditions of specific pollution 
are superimposed, will not suffice for the complete definition of water as favorable for 
aquatic life, since the absence of specifically toxic substances must also be demon- 
strated before the water can be finally approved as unpolluted. 
GENERAL FIELD METHODS 
Throughout these studies, both for the determination of water suitability 
standards and of pollution conditions, certain routine procedures were followed, in 
addition to the special investigations which the conditions in the particular locality 
required, in order that certain data from all localities could be compared fairly and 
without the skewing which results from haphazard sampling. 
The stations at which samples were taken in each locality were selected as 
representative of the various complexes of conditions presented. Whenever possible, 
samples were taken at intervals throughout the night and day and at different times 
during the season. Many of the stations included in this report have been visited 
repeatedly during the past 5 years. 
Although in this paper the detailed data concerning the findings on plankton, 
bottom organisms, fresh-water mussels, and fish population are not presented, since 
various general statements concerning findings on these animals have been included 
