(c) Class III: Highly Sensitive Species. Twenty-four-hour LC, 
values were less than or equal to 1 g 1-! of fuller's earth. 
Highly sensitive species were Atlantic silverside (24-hour LCj9 
value 0.57 g LPB) p juvenile bluefish, juvenile menhaden, and 
young-of-the-year white perch (Sherk and O'Connor, 1971). Juve- 
nile bluefish and juvenile menhaden tested at the Bayside Labo- 
ratory failed to survive in concentrations of 0.8 g 17! for more 
than 18 hours. Young-of-the-year white perch suffered 100- 
percent mortality in 0.75 g 17! fuller's earth in 20 hours. 
The lethal effects of suspended solids on fish species change at dif- 
ferent stages in the life history. Juvenile white perch are more likely 
to be killed by low concentrations of suspended solids than are adults. 
The basis for such age specific differences in tolerance is unknown. 
When fish are exposed to lethal concentrations of fuller's earth, the gill 
filaments and the secondary lamellae act as a sieve to trap particles 
which clog the gill, resulting in asphyxiation (Ellis, 1937). The physical 
dimensions of the gill increase with the increasing size of the fish 
(Muir, 1969). As the fish grows and the gill dimensions increase, the 
openings in the gill filter also increase. Thus, large fish may trap 
fewer particles, thereby decreasing the lethal effect of a given concen- 
tration of suspended solids. 
Another factor that may explain the link between fish size and toler- 
ance is that small fish have a higher metabolic rate than large fish 
(O'Connor, Neumann, and Sherk, in preparation, 1976). Small fish demand 
more oxygen per unit body weight than large fish and therefore are less 
tolerant to gill clogging. The combined effects of a higher metabolic 
rate and a finer, more efficient filter render juveniles more sensitive 
to solids than adults. 
VI. SUMMARY AND CONCLUSIONS 
Static bioassays using fuller's earth suspensions and natural sediment 
suspensions produced significant mortalities in five common estuarine 
fishes--white perch, spot, Atlantic silversides, mummichog, and striped 
killifish. Concentrations typically found in estuarine systems during 
natural events such as storms and flooding, as well as during dredging 
and dredged material disposal, are within the range of the lethal concen- 
trations of fuller's earth determined experimentally. Therefore, the 
possible adverse impact on fish populations of activities producing 
suspended sediment should be considered. Most fish are capable of 
avoiding or temporarily leaving a hostile environment. 
The particulate mineral solids were similar in size distribution to 
natural sediments likely to be found in estuarine systems. Suspensions 
of natural sediments affect fish in the same way as fuller's earth but 
higher concentrations of the natural sediments are required to produce 
the same level of mortality. Death is caused by anoxia resulting from 
blockage of small passages in the gills or abrasion of the gill tissue. 
The effect of finely divided solids is dependent upon concentration, 
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