Efforts to recover populations of threatened Chinook salmon in the Pacific Northwest may be hindered by exposure to contaminants in the juvenile life stage. Here we leverage a large dataset collected by the Washington Department of Fish and Wildlife’s Toxic Biological Observation System to test for accumulation of 219 contaminants of emerging concern (CECs) in juvenile Chinook salmon and identify chemicals that could be affecting salmon physiology, behavior, or fitness at current exposure concentrations. We also highlight results obtained from the region’s two most urbanized watersheds, the Green/Duwamish and Puyallup/White, to demonstrate how these data can inform decision making to protect juvenile salmonids. We found that juvenile Chinook salmon sampled in the Puyallup/White watersheds had the highest average concentrations of CECs, and that six chemicals appeared to be ubiquitous and were found in all five river systems spanning a gradient of urbanization. In the subsequent detailed studies in the Green/Duwamish and Puyallup/White River systems, we observed patterns in accumulation of per- and polyfluoroalkyl substances (PFAS) that revealed regions where certain analytes could be entering the rivers. In contrast, spatial patterns for pharmaceutical and personal care products (PPCPs) were less clear and indicated diffuse sources throughout the migration corridor, such as stormwater or wastewater. We recommend these results be used to target areas for source tracing studies for PFAS, and that future studies test for PPCP residues in commercial fish feed. Finally, the detection of multiple emerging contaminants in almost all composite fish samples, 11 of which exceeded available biological effects thresholds, reinforces the global call for green infrastructure projects that target source control and the removal or reduction of emerging contaminants from stormwater and wastewater.

All data compiled for this manuscript were collected by the Washington Department of Fish and Wildlife (WDFW) between 2013 and 2023 and include 141 composite samples comprising 772 individual subyearling Chinook salmon collected between April and June during the peak of their seaward migration. Fish were captured using beach seines, lampara seines, and an in-river screw trap, with additional collections made directly from hatchery ponds at the Soos Creek Hatchery in Auburn, WA. All collections were conducted under an ESA Section 4(d) permit obtained by WDFW for the collection of listed Chinook salmon. Hatchery-origin fish were identified using fin clips, otolith thermal marks, and coded wire tags; all others were presumed to be of natural origin and are hereafter referred to as wild fish.

All chemical analyses were completed by the commercial laboratory SGS AXYS Analytical Laboratory (Sydney, B.C.). Multiple established extraction and analytical methods were used to test for a range of CECs including 141 PPCPs, four alkylphenols, 34 current-use pesticides, and 40 PFAS. These methods are referenced in the manuscript and Document S1 alongside further discussion of quality control screening criteria for % recovery, lab flags, chemicals observed in the method blanks, and reporting limits. Analytical methods documents are available from SGS AXYS upon request.