Contaminants, especially persistent bioaccumulative toxic chemicals (PBTs), are a continued public and ecological health risk in the region. Most waterways in the Bay-Delta region are listed as impaired by mercury and other harmful chemicals and concerns over human exposure and health effects have resulted in consumption advisories for some locally caught fish. Bioassessments are a common approach for studying contaminant exposure and effects in aquatic ecosystems. Bioassessment can be used in combination with other indicators to assess the impact of contaminants on the condition of a resource or habitat. The weight-of-evidence approach uses a "triad" of bioassessment, chemical concentrations and toxicity testing to evaluate contaminant impacts.