Challenges of Fish Consumption-Based TMDLs

Fish consumption advisories are a common reason for placing a water body on the 303d list. However, it is not straightforward to determine the Total Maximum Daily Load (TMDL) that can enter the water body without causing a fish tissue target to be exceeded. Typically, TMDLs are keyed to state water quality standards that can be directly related to pollutant loading. With a fish tissue target, there is only an indirect relationship between the TMDL target and pollutant loading. Loads affect water column and sediment contaminant levels. The relationship of these levels to fish tissue contaminant levels depends on complex food chain pathways and movement of fish across gradients of contaminant concentrations. Thus, a thorough understanding of the diet and migration patterns of consumed fish species is imperative. An additional challenge posed by a fish-tissue based TMDL targets is in the selection of the appropriate fish species to use to evaluate attainment. In situations where multiple species of fish are consumed, USEPA guidance mandates the collection of site-specific fish consumption data, and an evaluation that includes the entire assemblage of consumed fish species. Additionally, given the need to understand how spatial and temporal changes in water column and sediment contaminant concentrations as a result of waste load reductions will impact fish tissue concentrations, sitespecific trophic transfer models that accurately incorporate the appropriate food web structure and migration patterns should be developed. The trophic transfer model of consumed species can then be used in the evaluation of the time to achieve a fish tissue target.