378 
BULLETIN OF BUREAU OF FISHERIES 
in the stream. Effluents polluting streams alter this dissolved oxygen balance chiefly 
through increasing the oxygen demand. Pollutants may create oxygen demand in 
any of three ways or combinations of these; that is, by the addition of quantities of 
organic matter of a putrescible sort, among which may be mentioned domestic sewage, 
packing-plant wastes, beet-sugar waste, and hide vat liquor from tanneries; through 
the action of various reducing chemicals as certain sulphite wastes from paper mills, 
sulphide and iron wastes from mines, and certain spent dyes from leather works; and 
by killing large masses of aquatic vegetation which subsequently decompose. Many 
effluents which create oxygen demand are also harmful to aquatic organisms because 
of specific toxic effects. 
The determination of the biochemical oxygen demand of various effluents 
affecting the dissolved oxygen balance in the stream does not necessarily, therefore, 
give a true evaluation of the pollution hazards produced by these effluents ; for various 
species of warm-water fishes will live in water having high oxygen demand, due to 
domestic sewage or industrial wastes rich in organic material but without specific 
toxic substances, if the dissolved oxygen level be maintained above 5 p. p. m. and the 
aeration be sufficient to blow off the excess of other gases as carbon dioxide, methane, 
and sulphur derivatives. The biochemical oxygen demands of such effluents as have 
high oxygen demands must be taken into account, however, if these wastes are poured 
into streams, in order to compute the dilution required to prevent this oxygen demand 
from lowering the dissolved oxygen in the stream to an unfavorable level. 
Two examples of the effect of dissolved oxygen on the lethality of effluents free 
from specific toxic substances but presenting large pollution hazards because of high 
oxygen demands will suffice. The tan vat liquor from a tannery on the upper Missis- 
sippi River in spite of the high oxygen demand of this waste was not only tolerated 
readily when properly aerated and unmixed with the toxic chemical effluents from the 
plant by various warm water fishes but materially increased the production of 
plankton when added to the river water. Findings from the sewage treatment system 
of the city of Munich, Germany, also demonstrate that high biochemical oxygen 
demand in itself need not be a pollution hazard to fresh-water fishes. In this system, 
water from the River Isar is mixed with prepared sewage and impounded for the com- 
mercial raising of rainbow trout and carp. This procedure was very successful over 
a period of years in the production of commercial quantities of both trout and carp. 
The following statement from a recent English review of the work of this plan 
(Engineering, 1935) may be quoted in this connection: 
The proportion of fresh water to sewage should be at least 5:1, and the proportion of oxygen 
not less than 5 cubic centimeters per litre (7.1 p. p. m.). If these conditions are maintained, the 
micro-organisms will flourish, absorbing and destroying all impurities and providing the food for the 
completion of the process. In such circumstances the fish will suffer no ill effects, although, as is 
well known, trout are most susceptible to any impurity. 
The oxygen balance of streams may also be affected by oils which exclude oxygen 
from the surface of the stream and prevent proper reaeration of the water. Oil is 
rarely poured into streams in such quantities as to present this difficulty, however. 
Again sewage and industrial sludges are often particularly harmful in reducing the 
dissolved oxygen through their biochemical oxygen demands during the winter season 
when ice covers the stream surface and interferes with the reoxygenation of the water. 
