lethal to most species. Physical comparison of the two mineral solids 
suggests that the differences in the lethal effects of these substances 
(Table 5) may be due in part to particle-size distribution (see App.). 
Suspended solid particles of differing composition vary greatly in 
lethal effect on fish (European Inland Fisheries Advisory Commission, 1964; 
Rogers, 1969). Using a variety of solids (kaolinite, diatomaceous earth, 
natural glacial till, and incinerator fly ash), Rogers (1969) concluded 
that the lethal effect of a suspended solid is dependent on particle shape 
and angularity rather than on particle size. However, it is not known 
how particle angularity causes rapid (24 to 96 hours) mortality in test 
fish. 
Common symptoms in dead and moribund test fish are extensive hemor- 
rhaging of minute blood vessels over the entire body surface and packing 
of the gills with sediment. Microscopic examination of fresh gill prep- 
arations from recently dead fish showed no hemorrhaging associated with 
exposure to kaolinite or fuller's earth. Rogers' (1969) hypothesis could 
not be evaluated because particles of kaolinite and fuller's earth were 
flat and platelike, and the sand grains in the natural sediments were 
relatively smooth. 
Respiration studies show some effects of suspended mineral solids on 
fish. Rogers (1969) noted increased survival among test fish when he bub- 
bled air into his test chambers. This suggests that exposure to suspended 
solids leads to anoxia. The relationship between particle angularity and 
the lethal effect may result from angular particles having a greater 
affinity for the gill surface, thus causing anoxia by covering or abrading 
the respiratory epithelium. 
Natural sediment suspensions were less harmful than mineral solid 
suspensions for every species tested (Table 5). The lethal effect of the 
natural mud probably was due to clogging of the gill interstices, as 
opposed to the coating effect of fuller's earth particles. Ellis (1937) 
described several ways in which particles could cause asphyxiation in 
fish; e.g., coating by fine particles and clogging by larger particles 
- such as those found in natural mud. 
Although Ellis' discussion did not include details of fish gill mor- 
phology, investigations by Muir (1969) and Cameron and Davis (1970) sup- 
port the hypothesis that suspended particulate matter is lethal to fish 
at concentrations well in excess of those observed in nature. The lethal 
effect of finely divided solids on fish depends on several factors although 
the cause of death is the same--anoxia. Fine particles generally will not 
eause death unless the particles are angular. Larger particles trapped by 
the primary and secondary lamellae of the gill block the minute circulation 
channels between the secondary lamellae. This leads to "dead space" at 
the primary site of gas exchange; limited oxygen diffusion occurs in these 
dead spaces. A coating of clay particles over the entire gill surface 
would be less likely to permit gas exchange. 
19 
