394 
BULLETIN OF BUREAU OF FISHERIES 
(death in 10 days or less, depending upon conditions of experiments) was found to 
be near 2.5 p. p. m. of ammonia. Some acclimatization to ammonia is possible, and 
it is well known that individuals of various species of fish may be found in water 
containing 3 to 10 p. p. m. of ammonia. However, the existing literature and the 
data from our experiments indicate that under average stream conditions with pH 
value 7.4 and pH 8.5, 2.5 p. p. m. of ammonia will be harmful to many individuals 
at least of the common aquatic species. 
Therefore, in view of the small amount of ammonia found in unpolluted natural 
flowing waters, 1.5 p. p. m. dissolved ammonia was considered the maximal amount 
of dissolved ammonia not suggestive of specific organic pollution. In flowing streams 
2 to 3 p. p. m. were almost always associated with definite organic pollution and 
values above 3 p. p. m. in our field studies were always traceable to sewage or factory 
effluents. 
SUSPENSOIDS 
The amounts of finely divided suspensoids in various waters and light penetration 
into these waters were determined by a photoelectric apparatus described by Ellis 
(1934). 
The suspensoids — that is, particulate matter m suspension in inland fresh 
waters — consists normally of erosion silt, organic detritus (as discussed under 
ammonia), bacteria, and plankton. Each component of this mixture, with the 
exception of plankton, may be greatly augmented by man’s agencies, as quantities 
of powdered rock, cellulose pulp, sawdust, semisolid sewage, and other debris are 
added to natural waters. 
Parts of some streams, as the Yellowstone and Missouri — draining areas in 
which natural erosion has been proceeding rapidly — have been muddy with their 
loads of erosion silt since before the earliest records by man and have as a result 
limited fish faunae. In a large proportion of the inland streams, however, erosion 
silt, organic detritus, and bacteria were formerly in balance over a considerable 
portion of the year and conditions favorable to aquatic life maintained, although now 
and then floods and other unusual conditions killed many aquatic animals in these 
streams by inundations of silt. With the advent of civilized man and unrestricted 
deforestation, agriculture, and other uses of the earth’s surface, the erosion problem 
has become gigantic, and the effects of the loads of erosion silt carried by these once 
relatively clear streams overwhelming on aquatic life in many places. 
Erosion silt and other suspensoids (disregarding any specific toxic action of 
suspensoid wastes) affect fisheries directty by covering the bottom of the stream with 
a blanket of material which kills out the bottom fauna, greatly reduces the available 
food, and covers nests and spawning grounds ; and also by the mechanical and abrasive 
action of the silt itself which may clog and otherwise injure the gills and respiratory 
structures of various aquatic forms, including many fishes and mollusks (Ellis, 1936a). 
The mechanical action of silt and other suspensoids may not be severe or even harmful 
to the gills and other structures of the free-swimming fish which move about above 
the bottom and are not mired down by the settling deposits of the suspensoids, if 
the amounts of the suspensoids in the water are not too great and if the action of the 
suspensoids is uncomplicated by other pollutants. Normal fish and many other 
swimming animals secrete continuously quantities of mucus which wash away 
