400 
BULLETIN OF BUREAU OF FISHERIES 
of the mud itself and by the paucity or absence of bottom fauna, often represented only 
by dead shells and other remains. Dredgings readily demonstrated such gross 
pollution by silt, sewage, pulps, tars and heavy oils, mine tailings, rock powders from 
quarry operations, and such organic wastes as starch effluents, sizings from cloth mills, 
and distillery slops. 
Downstream from a region of gross pollution the biological analyses of the 
bottom faunae were often particularly significant when compared with the bottom 
faunae in similar situations in comparable unpolluted waters (if possible of the same 
stream) in showing the selective effects of some pollutants, and the downstream extent 
of pollution. 
Many chemical wastes have high specific gravity and consequently tend to follow 
the stream bottom for some distance before complete mixing is accomplished by stream 
current action. Often downstream dredgings showed complete absence of bottom life 
of any type for some distance below the outlets of flumes carrying such chemical 
wastes, and the limits of the zones in which the bottom faunae had been killed out 
yielded valuable information concerning the points at which the effluent had been 
diluted to a non toxic level. 
ACTION OF POLLUTANTS ON FISHES 
Regardless of the effects on the environment, many pollutants affect fishes 
directly by some specific action on the living organism itself. These pollutants can 
be grouped, therefore, according to the locus of injury and method of action of the 
various active substances which the effluents contain (Ellis, 1936b). 
INJURIES TO GILLS AND EXTERNAL STRUCTURES 
The higher concentrations of almost all effluents and all lethal concentrations of 
some types of pollutants kill by their actions on the gills of the fish before little, if 
any, of the material causing the death of the fish passes beyond the gills. Such 
substances kill, therefore, by a combination of chemical and physical injuries rather 
than by true toxic action. The salts of several heavy metals, some acids, and some 
special chemicals as trinitrophenol combine readily with the mucus secreted by the 
fish’s skin, mouth, and gills, forming insoluble compounds. 
If the dilution of the pollutant of this type be great enough, or if the supply of 
the pollutant be limited so that it acts on the fish only a short time, the secretion of 
additional mucus may wash away the precipitated compound as fast as it is formed, 
oi 1 may carry away the larger masses of the precipitate before serious damage to the 
fish results. If, however, the concentration and exposure time are large enough 
(very small quantities of several heavy metal salts suffice, v. i.), the precipitated 
insoluble compound covers the body, the lining of the mouth, and the gills. (See 
fig. 21.) Disastrous results follow the covering of the gills with these precipitates, 
and death supervenes very rapidly from the respiratory failure. In our experiments 
with salts of heavy metals and various other compounds producing death by precipi- 
tation of insoluble matter on the gills, the precipitate was found to cause death as 
the result of a combination of three conditions which individually or collectively 
disturbed the proper functioning of the respiratory and circulatory mechanisms. 
