STREAM POLLUTION 
377 
Besides it is recognized in the various fields of animal husbandry, including aquiculture 
and pisciculture, that if any variety of animal is to be successful and thrive merely 
sublethal conditions are not adequate. 
The application of this observation concerning sublethal conditions is particularly 
important in pollution investigations, for a reduction of dissolved oxygen increases 
the lethality of many stream pollutants, especially those injurious to the gills (v. i.). 
For example, it was found that certain concentrations of various metallic salts and 
RESPIRATION 
RATE PER MIN. 
Figure 10. — Changes in the respiration rate of a 4^-inch yellow perch, Perea flavescem, correlated with changes in the dissolved 
oxygen content of the surrounding water, temperature 18 degrees centigrade. 
acid wastes were consistently more injurious to fish when the dissolved oxygen con- 
tained in the water thus polluted was comparatively low, but still sublethal, than 
when the dissolved oxygen was high. This difference in lethality correlated with 
dissolved oxygen obtained in spite of the fact that these specific pollutants neither 
reacted with nor were removed by dissolved oxygen regardless of the amount of 
oxygen present. 
The dissolved oxygen content of unpolluted streams normally varies with at 
least four major sets of factors, namely, (a) physical conditions such as stream flow, 
stream fall, and temperature, which influence the saturation of water with oxygen 
from the air; ( b ) oxygen produced by aquatic plants; (c) oxygen removed by aquatic 
organisms both plant and animal; and (d) the oxygen demand of the organic detritus 
