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

experimental concentrations tested, number of treatments and replicates used, solvent controls,
etc.- are needed. The slope of the observed dose-response relationship is particularly useful in
interpolating incidences of death at concentrations below or above an estimated LC50. The
variability of an LC50 is usually depicted by a confidence interval (95% CI) or standard
deviation/error and is illustrative of the degree of confidence associated with a given LC50
estimate i.e., the smaller the range of uncertainty the higher the confidence in the estimate.
Without an estimate of variability, it is difficult to infer the precision of the estimate.
Furthermore, survival experiments are of most utility when conducted with the most sensitive
life stage of the listed species or a representative surrogate. In the case of ESA-listed Pacific
salmonids, there are several surrogates that are available for toxicity testing including hatchery
reared coho salmon, Chinook salmon, steelhead, and chum salmon, as well as rainbow trout 2 .
The available toxicity data include a varitety of salmonids. Unfortunately, slopes, estimates of
variability for an LC50, and experimental concentrations frequently are not reported. In our
review of the BEs, we did not locate any reported slopes of dose-response curves, although some
of this information was presented in some of the corresponding Science Chapters. Consequently,
we must err on the side of the species in the face of these uncertainties and select LC50s from the
lower range of available salmonid studies. We selected LC50s and associated slopes as input in
the population modeling exercises discussed later. We evaluate the likelihood of concentrations
that are expected to kill fish and apply qualitative and quantitative methods to infer populationlevel
responses of ESA-listed salmonids within the Risk Characterization section (Figure 2).
Growth of individual organisms is an assessment endpoint derived from standard chronic fish
and invertebrate toxicity tests summarized in the BEs. It is difficult to translate the significance
of impacted growth derived from a guideline study to fish growth in aquatic ecosystems. The
health of the fish, availability and abundance of prey items, and the ability of the fish to
adequately feed are not assessed in standard chronic fish tests. These are important factors
affecting the survival of wild fish. What is generally assessed is size or weight of fish measured
2 Rainbow trout and steelhead are the same genus species (Oncorhynchus mykiss), with the key differentiation that
steelhead migrate to the ocean while rainbow trout remain in freshwaters. Rainbow trout are therefore good
toxicological surrogates for freshwater life stages of steelhead, but are less useful as surrogates for life stages that
use estuarine and ocean environments.
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