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

assuming there is also some constant rate of terrestrial invertebrate subsidy in addition to a
residual aquatic community, a floor of 0.20, or 20% of fish ration, is reasonable. The model does
not include any additional impacts to fish via dietary exposure from contaminated prey, or any
potential synergistic or additive effects to the aquatic invertebrates that may be result from
multiple stressors (Schulz and Liess 2001).
Modeling spikes in invertebrate drift following insecticide exposure
“Catastrophic drift” of invertebrates, due to acute mortality and/or emigration of benthic prey
into the water column is frequently observed following exposure to insecticides (Davies and
Cook 1993; Schulz and Liess 2001; Schulz 2004). Drift rates within hours of exposure can be
more than 10,000 times the natural background drift (Cuffney 1984), and fish have been found to
exploit this by feeding beyond satiation (Haines 1981; Davies and Cook 1993). The duration and
magnitude of the spike in drift of prey is dependent in part on the physical properties and dose of
the pesticide; however, the spike is generally ephemeral and returns to natural, background levels
within hours to days (Haines 1981; Kreutzweiser and Sibley 1991). Likewise, the magnitude of
the spike is dependent in part on the benthic density of prey; the spike in drift from communities
that have been reduced by previous exposures is smaller than the spike from previously
undisturbed communities (Cuffney 1984; Wallace, Huryn et al. 1991). To reflect this temporary
increase in prey availability, the model includes a one-day prey spike for the day following an
exposure (Appendix 1). The model also accounts for this short-term increase in prey availability
by allowing fish to feed at a maximum rate of 1.5 times their normal, optimal ration.
Modeling recovery of salmonid prey
We selected a 1% recovery in prey biomass per day. Reports of recovery of invertebrate prey
populations, once pesticide exposure has ended, range from within days to more than a year
(Cuffney 1984; Kreutzweiser and Sibley 1991; Pusey, Arthington et al. 1994; Ward, Arthington
et al. 1995; Van den Brink, van Wijngaarden et al. 1996; Liess and Schulz 1999; Colville, Jones
et al. 2008). The dynamics of recovery are complicated by several factors, including the details
of the pesticide exposure(s) as well as habitat and landscape conditions (Liess and Schulz 1999)
(Van den Brink, Baveco et al. 2007). In watersheds with undisturbed upstream habitats,
recovery can be rapid due to a healthy source of invertebrates that can immigrate via drift and/or
aerial colonization (for adult insects) (Heckmann and Friberg 2005). However, in watersheds
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