The purpose of this ecological risk assessment (ERA) is to characterize and quantify the current and potential risks to biota exposed to contaminants of potential concern (COPCs) in the Housatonic River below the confluence of the East and West Branches (known as the 'Rest of River'), focusing on polychlorinated biphenyls (PCBs) and other hazardous substances originating from the General Electric Company (GE) facility in Pittsfield, MA. This information is synthesized, through a weight-of-evidence (WOE) approach, into a discussion of the nature and magnitude of the risks for the assessment endpoints, and the uncertainties associated with the characterization of these risks. Multiple lines of evidence for each assessment endpoint are evaluated, including where applicable or available: field surveys and studies; site-specific toxicity tests; and cmparison of effects thresholds to site-specific COC concentrations or doses (measured or modeled).