that threshold. The concentration threshold for a given species may be 
determined by the ability of the fish to cleanse the gills by the cough- 
ing reflex, or by continuous secretion and sloughing of a protective mucus 
sheet. 
The LC,9 and LC59 curves (Fig. 7) for spot in suspensions of fuller's 
earth do not show a marked inflection point. The extrapolated 72- and 
96-hour LC,g values for spot (0.135 and 0.017 g 171, respectively) would 
be within the range of concentrations found in natural waters. 
White perch were exposed to suspended solids at 0.65 g 1>? for as long 
as 5 days (O'Connor, Neumann, and Sherk, in preparation, 1976). From an 
extension of the LCsg and LCjg curves relating exposure duration to concen- 
tration, concentrations of 0.65 g 1-1 would be expected to cause 10-percent 
mortality in less than 5 days. A threshold concentration between 0.67 
and 0.65 g 17! appears to exist. Higher concentrations may cause death, 
at least at the 10-percent level, during exposure of 48 hours; lower con- 
centrations are unlikely to cause death. However, fish exposed to 0.65 
g 17! for 5 days showed sublethal physiological changes in blood charac- 
teristics and damage to gill tissues (O'Connor, Neumann, and Sherk, in 
preparation, 1976). 
V. CLASSIFICATIONS 
Fish species used in these experiments may be placed in the following 
three groups according to their toleration of suspended solid concentra- 
tions. Classification is subjective and based on LCjg values of fuller's 
earth, because 10-percent mortality in addition to natural mortality rates 
is a more realistic maximum than a 50-percent mortality limit (Ricker, 1954). 
(a) Class I: Suspension-Tolerant Species. The concentration of 
fuller's earth required to attain the 24-hour LCjg value is equal 
to or in excess of 10 g 1~!. Tolerant species were the mummichog, 
striped killifish, and spot. Toadfish, hogchoker, and cusk eel 
were tested for suspension. tolerance, but concentration-dependent 
mortality curves were not determined. The tolerant species 
commonly inhabit the mud-water interface where suspended solid 
concentrations are high (Masch and Epsey, 1969); e.g., the killi- 
fish, hogchoker, and cusk eel frequently burrow into the bottom 
and remain covered for extended periods of time (Hildebrand and 
Schroeder, 1928). The toadfish is a relatively inactive bottom 
dweller (Hildebrand and Schroeder, 1928). 
(b) Class II: Suspension-Sensitive Species. LCjqg values for 
24-hour exposure to fuller's earth were between 1 and 10 g 17}. 
The sensitive species (white perch, bay anchovy, and juvenile 
menhaden) were tested at Hallowing Point (Sherk, O'Connor, and 
Neumann, 1973); their habitat preference with tolerance to 
fuller's earth was not correlated. Although specific LCj,g values 
could not be determined, three important commercial species (the 
striped bass, croaker, and weakfish) were in this general class. 
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