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ECOLOGICAL ENDPOINT MODELING: EFFECTS OF SEDIMENT ON FISH POPULATIONS
Rashleigh, B. ECOLOGICAL ENDPOINT MODELING: EFFECTS OF SEDIMENT ON FISH POPULATIONS. Presented at National Total Maximum Daily Loads Science and Policy Conference, Phoenix, AZ, November 13-16, 2002.
This research project sets out to design and conduct an assessment of the long-term ecological consequences of alternative watershed management choices. As the first project to be done at this scale using predictive ecological endpoints, we will seek to identify the appropriate components of such an analysis. We will use experience gained in the conduct of this analysis to identify key research and data needs for future analyses. We will extend this analysis beyond previous and ongoing studies in two ways: by incorporating biological endpoints, primarily properties of fish communities, and by introducing the concept of sustainability of ecological state under future scenarios contrasted with the present state of those same ecological resources. Requirements that are identified during the course of this study will permit the recommendation of specific capabilities that should be incorporated in a general modeling system currently under development to support environmental assessments. Finally, the analysis is intended to be of value for establishing environmental management choices that will be beneficial and those that would be detrimental to the sustainability of ecological resources. Specific objectives are listed below:
1. Develop watershed-based modeling systems to forecast the effectiveness of alternative management plans in meeting sediment-related, nutrient-related, pathogen-related, and toxics-related criteria and standards, and biologically-based criteria and standards; and
2. Develop and maintain a comprehensive technical support capability that directly links environmental TMDL exposure research activities and products for the EPA Office of Water, EPA Regional Offices, and the States to be used for implementation of policy, regulatory development, remediation, and enforcement needs.
Sediment is one of the main stressors of concern for TMDLs (Total Maximum Daily Loads) for streams, and often it is a concern because of its impact on biological endpoints. The National Research Council (NRC) has recommended that the EPA promote the development of models that can more effectively link environmental stressors (and control actions) to biological responses to support development of TMDLs. A quantitative approach relating sediment exposure to stream fish population dynamics is presented. An example is presented for a lithophilic, benthic insectivore fish species, such as a darter species, which is likely to be most sensitive to sediment impacts. First, equations are developed to characterize sediment exposure to the vital rates. Equations represent: the effects of suspended sediment and deposited sediment on feeding and food availability, and therefore survival; and the effect of deposited sediment on fecundity. Next, the vital rates are used in an age-structured population matrix model. A simple approach, a Leslie matrix, is presented. The matrix is used to examine the long-term population growth rate, which is given by the largest eigenvalue. A more realistic approach is an age-structured matrix that includes density-dependence. This type of matrix model is solved to give an equilibrium condition. Both the Leslie and density-dependent matrices are examined under the conditions of environmental stochasticity. These models can also be used to examine short-term population dynamics and to calculate sensitivity to different vital rates. The quantitative approach presented here can be used in TMDL development. A load allocation may be set based on the criterion that either the long term growth-rate is positive, or the equilibrium population size is above some minimum viable level. Results from the analysis of environmental stochasticity can be used to suggest a margin of safety for a TMDL. The use of models to simulate the response of ecological endpoints to sediment loading can lead to additional and improved management approaches for impaired streams.