Page 830 - The Toxicology of Fishes

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810 The Toxicology of Fishes

Suspended Solids and Sediment
Although the toxicity of synthetic pyrethroids in clean water is extraordinary, the observations in ﬁeld
trials have found that pyrethroids have not had as draconian an impact on ﬁsh populations as once feared.
Their toxicity is dramatically inﬂuenced by the presence of particulate matter in the water column,
probably through adsorption of the very lipophilic toxicant molecules to the suspended matter, sediment,
and possibly dissolved organic matter, as well (Coats et al., 1989; Smith and Stratton, 1986). Bioavail-
ability of the pyrethroid fenvalerate in water was shown to be drastically reduced in bioassays that
measured LC values for mosquito larvae in clean water and four different concentrations of humic acid
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in the water. Up to a sixfold difference was observed in the 24-hour LC values, depending on the
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concentration of humic acid added (Coats et al., 1989).

Toxicokinetics
Considerable research on the toxicokinetics of synthetic pyrethroids has been conducted in ﬁsh, in hopes
of ﬁnding explanations for the extreme susceptibility of ﬁsh to this class of insecticides compared to
birds and mammals and compared to other biodegradable insecticides (e.g., organophosphates and
carbamates):
• Are pyrethroids taken up more rapidly than other organic chemicals by the ﬁsh gills?
• Is there an especially effective distribution of pyrethroid residues to the nervous system in ﬁsh?
• Are ﬁsh deﬁcient in their capability for detoxiﬁcation of pyrethroids?
• Do ﬁsh excretory systems have poor efﬁciency of elimination of pyrethroids?

Uptake
The most deﬁnitive studies of uptake rates by ﬁsh gill have been conducted by Jim McKim at the U.S.
Environmental Protection Agency (USEPA) lab in Duluth, MN. Working with him, Bradbury and Coats
(1989a) studied the uptake efﬁciency of H-fenvalerate in rainbow trout in a passive diffusion model for
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xenobiotic uptake and distribution (McKim and Heath, 1983). The average uptake efﬁciency was 28.6%,
which was much lower than the majority of other organic chemicals studied in that system. The calculated
logP of 7.2 for fenvalerate was similar to that calculated for mirex (logP of 7.5); the gill uptake efﬁciency
for the rainbow trout was also low for mirex (20%). The implication is that mirex and fenvalerate are
so lipophilic that they are not rapidly absorbed across the ﬁsh gill (Bradbury et al., 1986), and they are
both taken up more slowly than the numerous other classes of chemicals tested (McKim et al., 1985)
which were less lipophilic. It is possible that an optimum lipophilicity exists for rapid uptake by ﬁsh
rather than the oft-assumed positive correlation between lipophilicity and uptake rates in ﬁsh.

Distribution
The radiolabeled fenvalerate also allowed for study of the distribution of residues within the rainbow
trout body. The largest proportion of residues were found in the bile, with the fat deposits next, followed
by the liver, gill, kidney, and red blood cells. All of those locations are to be expected for a lipophilic
xenobiotic, and the concentration in the brain (21 parts per trillion) was lower than most other tissue
levels. The distribution does not seem to especially favor selective partitioning or delivery to the brain
(Bradbury et al., 1986).

Detoxiﬁcation
The biotransformation of several synthetic pyrethroids has been examined; those studied have included
fenvalerate, permethrin, and cypermethrin. Identiﬁcation of the most prevalent metabolites revealed that
oxidation products were common, primarily due to ring hydroxylation and side-chain oxidation reactions

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