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

Fresh et al. 2007) or document fish growth rates below maximal potential growth rates when
prey are limited (Dineen, Harrison et al. 2007).
One study, in particular, tested the hypothesis that single applications of the OP insecticide
chlorpyrifos (0.5, 5, 20 μg/L) to outdoor ponds (littoral enclosures) would reduce the abundance
of invertebrates and cause diet changes that would result in reduced growth rates of juvenile fish
(Brazner and Kline 1990). The results are direct, empirical evidence that support this hypothesis.
Growth rates of fathead minnow larvae were reduced significantly in all chlorpyrifos-containing
treatments due to reduction in prey abundance. At 15 d post-treatment, the reductions in growth
rate compared to control fish were the most pronounced and coincided with the greatest
reductions in invertebrates. Stomach contents of minnows were identified throughout the
experiment. By day 7 mean numbers of protozoans, chironomids, rotifers, cladocerans, mean
total number of prey being eaten per fish, and mean species richness were greater in unexposed
treatments compared to some of the other treatments. On day 15, most of the differences were
more pronounced. The results strongly support the conclusion that foraging opportunities were
better in untreated enclosures and unexposed larvae grew significantly more compared to
chlorpyrifos-treated enclosures. Furthermore, the reductions in prey items in diets mirrored the
reduction in prey items in the enclosures. We did not find any study results with N-methyl
carbamates, but we make the inference that concentrations of the three N-methyl carbamates that
are sufficient to reduce aquatic prey would also lead to reduced fish growth. This further
supports the hypothesis that reduction in prey abundances translates to reductions in subsequent
ration as well as individual growth. The authors concluded that “low levels of contaminants that
induce slower growth in young-of-the-year fish through food chain effects or other means may
eventually reduce the survival and recruitment of these fish.”
Collectively, the lines of evidence strongly support the overall hypothesis. Thus, we carry
reduced prey impacts to the next level of analysis (i.e., the population-level). We conducted
population modeling exercises based on reduced abundances of salmonid prey, presented in the
next section, (Effects to Salmonid Populations from the Proposed Action).
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