7. Complex, multi-stressor models, such as required for photo-induced PAH toxicity to fish 
during embryo-larval stage of development, need to be developed and applied to determine the 
magnitude of ecological risks which are presently highly uncertain. 
Other important gaps: 
8. In some cases, populations of chronically exposed aquatic organisms have demonstrated an 
evolved tolerance or genetic resistance to toxicity through chronic exposure. The potentially 
enormous and irreversible consequences of rapid evolutionary change suggest the importance of 
better understanding, predicting, and managing these anthropogenic impacts on aquatic and 
wildlife populations. In addition, technological advances now permit an identification of the 
genetic changes that may provide the key to understanding the mechanisms by which populations 
and species adapt or become extinct. 
9. A gap exists in the development of biological indicators that can lead to diagnosis of 
developing toxicity problems in aquatic ecosystems before population impacts are observable. 
For example, commonly measured biochemical effects, such as P450 enzyme induction, are 
sometimes used as diagnostic indicators of exposure to specific categories of chemicals. 
However, these measurements are of limited use because their relationships to organismal, much 
less population-level, risks are not well understood. 
10. NHEERL toxicology research plans organized outside of Goal 2 should consider criteria and 
aquatic ecological risk assessment needs in order to prevent gaps for the utilization of the 
research products to meet aquatic stressor data and methods requirements. 
Population Models 
Several gaps relate to the needs for information for developing population models: 
1. The greatest limitation to the application of population matrix models is the paucity of high 
quality data on mortality and fecundity rates, and our limited understanding of density-dependent 
feedbacks and other ways populations compensate for losses due to stressors. Generalized 
population models are helpful for identifying information needs, but scenario-specific population 
models often will be limited by the lack of data about the populations of interest. Specific case 
studies have been chosen because of their relative wealth of population parameter information 
and the potential to identify the types and formats of data required for population models when 
applied to other species/region/stressor(s) combinations. The collection of basic life history 
information on fish and wildlife species is a gap not addressed by this effort outside of a few 
species covered under case studies. 
2. Although life history infoimation is being gathered by State and Federal resource agencies and 
others, it is not always in an adequate form for use in a population model. In addition to 
proposing methods for population modeling, we need to work with resource agencies to 
influence the format of life history information being gathered to improve its utility in risk 
assessment. 
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