3. Complex mixtures of PBTs are the norm, so interspecies differences in potency, as well as in 
bioaccumulation, for individual chemicals in the mixture must be factored into population level 
risk predictions. 
4. Is absence of overt mortality, even for ELSs, an adequate effects end point for preventing 
population declines caused by PBTs or non-PBTs? If not, how do we determine what is adequate 
for aquatic invertebrates, fish, amphibians, or avian and mammalian wildlife? The presently 
proposed toxic chemicals research will fill this fundamental gap only to the extent that specific 
chemicals are intensively used as foci for development of models and risk assessment methods. 
5. A national WQC methodology for different classes of PBTs needs definition, through use of a 
generic population model (or a suite of generic population models) of species characteristics, life 
stages, and toxicity effects that are most predictive of risks to populations, regardless of site 
conditions. This information will fill a gap which presently limits development of population 
level based, chemical-specific criteria, as well as determination of site-specific model and data 
requirements for application of the criteria. 
Two gaps relate to the need for demonstrating and verifying the usefulness ofpopulation models: 
6. Population models need to be developed and applied through case studies to explicitly 
demonstrate risk assessment requirements for prediction of adverse population impacts as a result 
of PBT toxicity-caused reductions in survival of aquatic organisms. 
7. The proposed approaches in this researdi plan rely heavily on the accuracy of population 
models. The utility of these approaches must be evaluated through field verifications. More 
broadly, it must be better understood how different model types and levels of complexities are 
necessary to achieve the desired reduction in uncertainties for specific assessment needs. Only 
one project in this program addresses this need. 
One gap relates to the importance of spatial scales for aquatic life assessments: 
8. Spatially explicit population models are often required for assessment of risks to aquatic- 
dependent wildlife populations that function in landscapes that integrate many lakes, wetlands, 
and streams. Because PBTs tend to distribute widely, if not uniformly, for long periods of time 
in aquatic habitats, uncertainty exists for when, and the extent to which, spatially explicit 
population models are requisite for assessment of risks to populations of aquatic organisms. 
References 
Aldenberg, T., Slob, W. 1993. Confidence limits for hazardous concentrations based on 
logistically distributed NOEC toxicity data. Ecotoxicol Environ. Saf 25:48-63. 
Ankley, G.T., Collyard, S.A., Monson, P.D., Kosian, P.A. 1994. Influence of ultraviolet light on 
the toxicity of sediments contaminated with polycyclic aromatic hydrocarbons. Environ. Toxicol. 
Chem. 13:1791-1796. 
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