372 
BULLETIN OF BUREAU OF FISHERIES 
From figure QA it may be seen that during the ivarm season the waters at 96 
percent of the good fish faunae stations carried 5 p. p. m. or more dissolved oxygen, 
and that in all of the 5,809 cases good, mixed fish faunae were not found in waters 
carrying less than 4 p. p. m. dissolved oxygen. These data collected from localities 
where the fish had had opportunity to choose for themselves point very strongly to 5 
p. p. m. as the lower limit of dissolved oxygen, if the complex is to maintain a desirable 
fish faunae under natural river conditions. 
E. MISSISSIPPI, IMPOUNDED WATER 
PERCENT HASTINGS POOL (MINN.) 
OF CASES H STATIONS, 118 DETERMINATIONS 
■*0 - 
30 - 
F. MISSISSIPPI, IMPOUNDED WATER 
LAKE DAVENPORT AND MOLINE POOL (IOWA. ILL.) 
13 STATIONS, 220 DETERMINATIONS 
SO 
40 
DISSOLVED OXYGEN 
Figure 7.— Continuation of the dissolved oxygen compj 
G. MISSISSIPPI, IMPOUNDED WATER 
PERCENT LAKE PEPIN (MINN. AND WIS.) 
OF CASES 22 STATIONS. 286 DETERMINATIONS 
•40 - 
H. MISSISSIPPI, IMPOUNDED WATER 
LAKE KEOKUK (IOWA. ILL.) 
2 4 6 6 10 12 14 
PARTS PER MILLION 
sons, stippled graph A in figure 6, being the standard. 
It maj 7 be suggested in the absence of other data that the aggregation of the good 
fish faimae in waters containing 5 p. p. m. or more dissolved oxygen does not constitute 
proof that this amount of oxygen was required by these fishes, and that some other 
factor or factors delimit these complexes where good fish faunae were found. However, 
experimental data presented in another portion of this section support the view that 
dissolved oxygen is definitely one of the determining factors in these favorable 
complexes. 
A comparison of graphs C and D in figure 6, with the composite A, however, 
brings out the fallacy, for river waters at least, of the suggestion which has been made 
at times that the determination of dissolved oxygen or dissolved oxygen in terms of 
oxygen demand is sufficient alone to define water as suitable or unsuitable for fish 
life. Graph 6C gives the distribution of the dissolved oxygen value for all stations in 
