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

elatively low survival EC50 values when exposed to current use insecticides (Johnson, Jepson et
al. 2008), including carbaryl (Peterson, Jepson et al. 2001; Johnson, Jepson et al. 2008).
The selection of the 10 th percentile is a reasonable choice in keeping with general risk
management practices of protecting the aquatic community. The standard procedure the U.S.
EPA Office of Water uses in establishing Aquatic Life Criteria is to protect to the 5th percentile
of a species sensitivity distribution that includes all genera of aquatic species for which valid
data are available (EPA 1995). Their procedure involves combining species data into a genus
mean value so that multiple tests on one sensitive species do not unduly influence the outcome,
and evaluating a specified group of taxa. As we had already selected for the sensitive end of the
spectrum by using salmonid prey items, and did not collapse species data into a genus mean, we
used the 10 th percentile. In the probabilistic risk assessment for carbofuran (EPA 2005), OPP
evaluated effects on the 5 th , 50 th , and 95 th percentiles of separate species sensitivity distributions
for fish and invertebrates, although it was not clear in the document which was the preferred
percentile for risk management.
Modeling availability of unaffected prey
Reductions in benthic invertebrate densities can lead to long-term reductions in prey availability
and reductions in fish growth (Davies and Cook 1993). That said, prey densities are not usually
reduced to zero (Wallace, Lugthart et al. 1989). Therefore, it is assumed that regardless of the
exposure scenario, prey abundance would not drop below a specific “floor” of prey availability.
This floor is included in the model to reflect an assumption that a minimal yet constant terrestrial
subsidy of prey and/or an aquatic community with tolerant individuals would be available as
prey, regardless of pesticide exposure and in addition to the constant recovery rate (see below).
Therefore, even in extreme exposure scenarios, some prey will be available, as determined by the
value assigned to the floor; in some highly degraded systems this may or may not be the case.
No studies have quantified this floor for the purpose of estimating prey availability, but several
studies have documented reductions in overall benthic insect densities of 75-98% (Wallace,
Lugthart et al. 1989; Anderson, Hunt et al. 2003; Anderson, Phillips et al. 2006). Because
benthic densities are typically correlated with drift densities (Hildebrand 1974; Waters and
Hokenstrom 1980), these reductions likely result in similar reductions of prey. Therefore,
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