Fish may be able to tolerate greater concentrations of natural sedi- 
ments than mineral solids because: (a) The abrasive mineral particles 
are diluted by organic material in the natural sediment and therefore are 
not as damaging, or (b) the larger natural particles allow water to flow 
through the larger interstices and reach the gill surface. 
In freshwater systems, concentrations of 2 to 6 g aD Oe SiLile may 
persist for 15 to 20 days in flood-stage rivers (European Inland Fisheries 
Advisory Commission, 1964). Similarly, freshwater streams polluted with 
china-clay mining waste may carry burdens of 1 to 6 g 17!, continuously 
(Herbert and Merkins, 1961). Saline waters carry lower concentrations of 
suspended particles because of flocculation, dilution, and the ''salting- 
out'' phenomenon. However, Masch and Espey (1967), in a study of shell- 
dredging operations in Galveston Bay, Texas, recorded suspended solid 
concentrations of 4.15 g 1~! in the immediate vicinity of a dredge dis- 
charge. Concentrations were 0.3 g 17! suspended solids 838.2 meters from 
the discharge. Suspended solid concentrations may reach 1.2 g 171 during 
flood conditions, such as Hurricane Agnes in 1972, in the upper Patuxent 
River, Maryland. Values recorded during the summer of 1972 were generally 
between 0.08 and 0.14 g 17!, depending on local weather conditions and 
tidal scouring. Suspended solid concentrations capable of causing signi- 
ficant mortality in certain estuarine fish species at the 10- and 50- 
percent levels can be maintained in estuarine systems near dredging 
operations or during times of excessively high runoff. 
The relationship between suspended solid concentrations and their 
effect on mortality with increasing exposure time are shown in Table 4. 
Arithmetic plots (Figs. 6 and 7) of the LCjq and LCs5q data from Table 4 
allow an estimation of the severity of the impact of a given concentration 
of fuller's earth upon white perch and spot. For white perch, the concen- 
tration of fuller's earth needed to cause 90-percent mortality for a 48- 
hour exposure (not shown) was 25 percent, by weight, of the 12-hour LCo9g 
value. However, the LCj}9 value for white perch exposed to fuller's earth 
for 48 hours was only 2.2 percent of the 12-hour LC,g value. Thus, very 
low concentrations of suspended solids caused low, yet important levels of 
mortality during long exposure periods. 
For white perch (Fig. 6), the LC,}g duration of exposure had its effect 
primarily on the lower levels of mortality. Thus, using LCj9 data 
(Fig. 6), it was found that the visual approximation afforded an excellent 
estimate of inflection between 20 and 24 hours at approximately 4 g 17} 
fuller's earth. 
Mortalities beyond the 48-hour LC;g approached zero. Extrapolated 
LC,}g values for 72- and 96-hour exposures were 0.06 and 0.0045 g 19 
respectively, or well within the range of suspended material carried by 
"undisturbed" natural systems. 
Fish exposed to concentrations of suspended solids normally found in 
natural waters are not adversely affected by concentrations below a cer- 
tain threshold value. The concentration of 0.0045 g 17! is well below 
20 
