Legacy habitat contamination as a limiting factor for Chinook salmon recovery in the Willamette Basin, Oregon, USA.

In the western United States, the long-term recovery of many Pacific salmon populations is inextricably linked to freshwater habitat quality. Industrial activities from the past century have left a legacy of pollutants that persist, particularly near working waterfronts. The adverse impacts of these contaminants on salmon health have been studied for decades, but the population-scale consequences of chemical exposure for salmonids are still poorly understood. We estimated acute and delayed mortality rates for seaward migrating juvenile Chinook salmon that feed and grow in a Superfund-designated area in the Lower Willamette River in Portland, Oregon. We combined previous, field-collected exposure data for juvenile Chinook salmon together with reduced growth and disease resistance data from earlier field and laboratory studies. Estimates of mortality were then incorporated into a life cycle model to explore chemical habitat-related fish loss. We found that 54% improved juvenile survival-potentially as a result of future remediation activities-could increase adult Chinook salmon population abundance by more than 20%. This study provides a framework for evaluating pollution remediation as a positive driver for species recovery.