Page 829 - The Toxicology of Fishes

Toxicology of Synthetic Pyrethroid Insecticides in Fish: A Case Study 809

endrin, and chlordane. Initial field trials, however, showed the pyrethroids to be less potent than expected
from lab studies. It was determined that the pyrethroids, with their extremely low water solubility and
high affinity for particulate matter in solution, did not remain bioavailable for uptake by the fish in the
field ponds. When the pyrethroid molecules bound to the suspended solids or the sediment, the resultant
toxicity was orders of magnitude less than predicted by the clean-water assays. A similar effect was
reported for a dose–response experiment in mosquito larvae (Coats et al., 1989). Some studies have
investigated interactions between insecticides and particulate matter in the water and their effect on
toxicity (Coats, 1980; Coats et al., 1989; Herbrandson et al., 2003). Mixtures of chemicals in the aquatic
systems can also impact organisms in ways that are difficult to predict (Lydy et al., 2004); pyrethroids
have been demonstrated to contribute to greater than additive toxicity in fish (Denton et al., 2003).

Formulation
One of the early indicators of the significance of the bioavailability of pyrethroids in fish toxicity tests
was revealed in acute toxicity tests that compared technical-grade synthetic pyrethroids with their
emulsifiable concentrate (EC) formulations (Coats and O’Donnell-Jeffery, 1979). The 24-hour LC 50
values for rainbow trout in static-exposure bioassays were noted to be two- to ninefold lower for the
formulated materials compared to the pure technical-grade active ingredients. The emulsifiable formu-
lation kept the pyrethroids in solution longer compared to the technical materials, and the pyrethroids
quickly adsorbed to the glass (and probably to the outside of the fish), removing them from solution.
The tendency for pyrethroids to bind to glass and plastic was later confirmed quantitatively (Sharom and
Solomon, 1981). The utilization of a flow-through apparatus provided better comparison of formulated
and technical fenvalerate in a toxicity test. The 24-hour LC values were twofold higher for the EC
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formulation, but by day 7 the incipient LC values were demonstrated to be the same. Residues were
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measured over the time course, and the technical-grade fenvalerate was taken up faster than the EC. The
resulting times to mortality were also significantly different such that, for comparable exposure concen-
trations, the time to death was shorter for the fish exposed to the technical grade (Bradbury et al., 1985).

Water Parameters
The ionic characteristics of the water can exert influence on the toxicity of pyrethroids to fish. Because
of the role of ATPases in fish osmoregulation (Dange, 1986) and because pyrethroids had been demon-
strated to inhibit ATPases in squid axon (Clark and Matsumura, 1982), it was hypothesized that inter-
ference with osmoregulation was a secondary mode of toxic action of pyrethroids in fish. Water hardness
was shown to be a factor in bluegill susceptibility to fenvalerate (Dyer et al., 1989). The 48-hour LC 50
values ranged from 0.9 to 1.9 µg/L for bluegill fry. The LC values were twofold higher in very soft
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water (6 mg CaCO per L), compared to those obtained from harder water (>36 mg/L). Residue analysis
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of the living and dead fry showed that neither net uptake rate nor final body burden changed significantly
with hardness. When salinity was examined as a possible factor, a 50% increase in toxicity was recorded
when salinity was raised from 12.5% to >25% in the bluegill fry (Dyer et al., 1989). A second approach
to the question was developed through the utilization of radioactive ions with bluegill and fathead
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minnow. The uptake and depuration of Na, Cl, and Ca were investigated in individual experiments
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over a range of fenvalerate concentrations (Symonik et al., 1989). The pyrethroid exposure did result in
ionic imbalances for each of the ions, raising the possibility that the osmoregulation system may be
stressed by the insecticide, but the whole-body ion analysis method did not allow any more specific
conclusions about this effect. Together, the two studies indicated that stressing of the ionic regulation
system may be a contributing secondary mode of action of pyrethroids in fish.
Temperature
A notable negative temperature coefficient exists for the susceptibility of fish to pyrethroids. The
phenomenon is described as an enhanced toxicity at lower temperatures and had previously been long
observed in insect toxicology. It is very unusual for any chemical class to be more toxic at lower
temperatures; DDT and the pyrethroids are the major examples known to date.