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

change in first-year survival rate for each of the salmon life-history strategies modeled. We also
addressed mixture toxicity of the three insecticides in the model using exposure concentrations
derived from EPA modeling estimates and from NMFS’ modeling estimates of off-channel
habitats. Model output is displayed in Tables 74-77.
The percent changes in lambdas increased as concentrations of the three carbamates increased.
Increases in direct mortality during the first year of life produced large impacts on the population
growth rates for all the life history strategies. Model results for stream-type Chinook salmon
showed significant impacts at lower concentrations than the other modeled populations. This
result is primarily due to the size of the standard deviation of the unexposed population. Percent
changes in lambda were deemed significant if they were outside of one standard deviation from
the unexposed population. The relative sensitivity of the life history models producing the
greatest to the least changes in population growth rate for equivalent impact on survival rates
was coho salmon, ocean-type Chinook salmon, stream-type Chinook salmon, and sockeye
salmon. We note that the choice of LC50 is an important driver for these results. Therefore, an
LC50 above or below the ones used here will result in a different dose-response. We selected the
lowest reported salmonid LC50 from the available information to ensure that risk is not
underestimated. However, if the actual environmental 96 h LC50 is lower, then the model will
under-predict mortality. If the actual environmental acute LC50 is higher, then the model will
over-predict mortality.
These results indicate that exposure of salmonid populations to carbaryl, carbofuran, and
methomyl for four days at the reported LC50s would have severe consequences to the
population’s growth rate. If exposure occurred every year for each new cohort, population
growth rate would be reduced and recovery efforts would be slowed. For those natural
populations with current lambdas of less than one, risk of extinction would increase
substantially, especially if several successive generations were exposed. For each of the
combinations of species and insecticide, we denoted the relative concentration at which the
percent change in lambda is deemed significantly different from the unexposed populations e.g.,
a 9.1% change in lambda is estimated at 190 μg/L carbaryl for ocean-type Chinook salmon.
These population effect thresholds assume exposure to all the juveniles in the population.
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