Mortality normalization of chronic growth data for point estimate determination
The US EPA seven day chronic Pimephales promelas (EPA/600/4-89/001), Cyprinodon variegatus, Menidia beryllina, and Mysidopsis bahia (EPA/600/4-87/028) protocols use a final mass measurement as an estimate of chronic toxicity. The mean final mass per surviving organism is determined per replicate at the conclusion of the bioassay and used for all statistical analysis. Calculating point estimates using final mass measurements determined in this way can be problematic. Ignoring all mortality when determining the final mass can bias the growth data. The sampled population for growth determination are no longer randomly distributed, having been preselected based on their resistance to the toxic constituent. Two techniques for normalizing this growth data were examined using effluent and reference toxicant data from seven day chronic Pimephales promelas bioassays. The first technique combined the mortality and growth endpoints by defining the final mass as the total replicate mass divided by the number of organisms present at test initiation. The second method employed an initial mass as an estimate of the final mass for all dead larvae. The growth endpoints were calculated as indicated in the protocol (EPA/600/4-89/001) and with both normalization procedures. The endpoint determination methods were then evaluated using linear interpolation (ICp). Both normalization methods dramatically improved the linear interpolation`s ability to calculate effect levels which could not be calculated in non-normalized data sets. Comparative differences in point estimates between the two normalization techniques indicate that combining the mortality and growth endpoints by dividing the replicate mass by number of organisms at initiation can, and in most cases will, overestimate toxicity.
- OSTI ID:
- 212080
- Report Number(s):
- CONF-9511137--; ISBN 1-880611-03-1
- Country of Publication:
- United States
- Language:
- English
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