Carbaryl, Carbofuran, and Methomyl - National Marine Fisheries ...

e killed outright by the stressors of the action leading to reduced prey availability or the
complete absence of prey. These same areas also have off-channel habitats where fry seek
shelter and food and those areas are highly susceptible to the highest concentrations of the three
insecticides. Therefore, we expect reductions in a population’s abundance where transitioning
yolk-sac fry are exposed to the stressors of the action. All salmonid ESUs share this common
life stage transition and therefore are at risk.
Death of returning adults
We discussed and analyzed with models the importance of juveniles to population viability.
However, we did not address possible implications of returning adults dying from exposure to
the stressors of the action. An adult returning from the ocean to natal freshwaters is important to
a population’s survival and recovery for many reasons. Notably, less than one percent of adults
generally complete their life cycle. For populations with lambdas well below 1, every adult is
crucial to a population’s viability. We expect that some sensitive adults will die from short-term
exposures before they spawn within some of the populations, particularly those that spawn in
intensive agricultural watersheds and urban/suburban environments where elevated temperatures,
other AChE-inhibiting insecticides, and low pHs are present. The lower end concentrations that
kill 50% of exposed salmonids fall in the hundreds of μg/L. For methomyl, we expect that fewer
adults would die compared to carbofuran and carbaryl based on differences in toxic potencies.
We still expect that sensitive individuals exposed to concentrations below the LC50 will die.
These concentrations are expected in habitats receiving drift from aerial applications. The
persistence of these concentrations will vary with habitat, but for those habitats with lower pHs
and minimal flow sensitive adult salmonids are expected to be killed. We cannot quantify the
number of adults lost to a given population in a given year. For those populations where each
adult salmonid is important to viability, we expect reductions in both productivity and
abundance.
Synergistic toxicity
With certain combinations and specific concentrations of carbaryl and carbofuran, synergism
occurs, translating into increased inhibition of AChE and in some cases increased rates of
mortality among exposed salmonids. We have no data either supporting or refuting synergism
for methomyl. We have no predictive models for synergistic toxicity. However, where we
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