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

incorporates empirical data when available. Since growth and toxicity data are limited,
extrapolation from one salmon species to the others was done with the assumption that the
salmon stocks would exhibit similar physiological and toxicological responses. Sigmoidal doseresponse
relationships based upon the AChE inhibition EC50 values and their slopes are used to
determine the level of AChE activity (Figure 2A, 2B, 2C) from the exposure concentration of
each pesticide exposure or pulse.
A linear relationship based on empirical data related AChE activity to feeding behavior (Sandahl
et al. 2005, Figure 2D). Feeding behavior was then assumed to be directly proportional to food
uptake, defined as potential ration (Figure 2E). The potential ration expresses the amount of
food the organism can consume when prey abundance is not limiting. Potential ration over time
(Figure 2F) depicts how the food intake of individual fish changes in response to the behavioral
effects of the pesticide exposure over the modeled growth period. Potential ration is equal to
final ration if no effects on prey abundance are incorporated (Figure 4). If effects of pesticide
exposure on prey abundance are incorporated, final ration is the product of potential ration
(relating to the fish’s ability to capture prey, Figure 2) and the relative abundance of prey
available following exposure (Figure 3). Next, additional empirical data (e.g., Weatherley and
Gill 1995) defined the relationship between final ration and somatic growth rate (Figure 4C).
While the empirical relationship is more complex (e.g., somatic growth rate plateaus at rations
above maximum feeding), a linear model was considered sufficient for the overall purpose of
this model. Finally, the model combines these linear models relating AChE activity to feeding
behavior, feeding behavior to potential ration, and final ration to somatic growth rate to produce
a linear relationship between AChE activity and somatic growth rate (Figure 4D). One important
assumption of the model is that the relationships are stable, i.e., do not change with time. The
relationships would need to be modified to incorporate time as a variable if, for example, fish are
shown to compensate over time for reduced AChE activity to improve their feeding behavior and
increase food uptake.
Juvenile salmonids are largely opportunistic, feeding on a diverse community of aquatic and
terrestrial invertebrate taxa that are entrained in the water column or on the surface (Higgs et al.
1995). As a group, these invertebrates are among the more sensitive taxa for which there is
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