Investigating the Source of PBDE Contaminant Exposure in Steelhead Trout within the Major Tributaries of the Nisqually River Basin

Steelhead trout (Oncorhynchus mykiss; anadromous form) are ecologically, economically, and culturally important in the Pacific Northwest. They are an important prey item for Puget Sound’s southern resident orca (Orcinus orca) or Southern Resident kill whale (SRKW) population and other marine mammals. Steelhead populations also support recreational and commercial fisheries, which bring economic benefits to the Puget Sound region. Furthermore, steelhead have an important cultural significance to Native Americans in Washington.

Steelhead trout like other salmonids rear in freshwater streams and require passage to marine waters. These freshwater habitats have faced pressure from urbanization, landscape development, and hydrologically altered rivers due to population growth in the Puget Sound region. The degradation of habitats from urbanization have long been known to affect salmonid species and efforts have been made to restore habitats of importance to salmonids. Habitat degradation goes beyond physical disturbance and is impacted by toxic contaminants from urban, residential, and agricultural landscapes which can degrade water quality. This toxic contamination can impact the freshwater food web steelhead rely on.

Steelhead populations across the Puget Sound region are less than 10% of their historical abundance leading to their listing as threatened under the US Endangered Species Act (ESA; Gayeski et al. 2011, Chen et al. 2018, Salish Sea Marine Survival Project 2018). Declines in Puget Sound steelhead populations are at odds with the statewide trend of population growth (Scott and Gill 2008). One cause of declining regional populations is the degradation and development of historical habitat used by steelhead, resulting in an estimated 9-30% reduction in habitat accessible to steelhead (Scott and Gill 2008). For example, the La Grande Dam may hydraulically limit the upstream travel of steelhead in the Nisqually River Basin (Figure 1), though there has been debate whether the canyon was already impassable. Furthermore, steelhead habitats are threatened by toxic contaminants from stormwater and wastewater (Chen et al. 2018). While documented studies of toxic contaminant exposure in steelhead are limited, other salmonid species are known to be exposed to environmentally relevant concentrations of toxics during their migration to marine waters (O’Neill et al. 2020b, Meador et al. 2010, O’Neill et al. 2015, Sloan et al. 2010).

A 2014 survey conducted by Washington Department of Fish and Wildlife (WDFW) of steelhead trout in the Skagit, Green/Duwamish, and Nisqually rivers identified polybrominated diphenyl ethers (PBDEs) at the highest concentration in Nisqually River steelhead (Chen et al. 2018). PBDE concentrations in Nisqually River steelhead exceeded critical body residues (CBRs) for increased disease susceptibility throughout the river system. PBDE contamination in steelhead was pervasive throughout the river system with 33-50% of fish sampled exceeding CBRs across the three sampling locations. In 2015, follow up sampling from the in-river smolt trap, identified approximately 33% of steelhead trout smolts had elevated levels of PBDEs (WDFW unpublished data). These findings were surprising due to the limited urbanized development within the Nisqually watershed. Furthermore, a previous study of juvenile Chinook salmon (Oncorhynchus tshawytscha) collected in the Nisqually River estuary did not document PBDEs at concentrations above CBRs for increased disease susceptibility (O’Neill et al. 2015).

To identify potential sources of PBDEs in the Nisqually River system a 2017 study was undertaken by WDFW and Ecology, which concluded PBDEs were entering the river system through the three major tributaries, the Mashel River, Muck Creek, and Ohop Creek (O’Neill et al. 2020a). Results of the survey found the Mashel River had elevated levels of PBDEs, congener patterns that differed from other river samples, and an enriched signal of the nitrogen stable isotope, ẟ15N. The town of Eatonville’s WWTP outfall is located upstream of the 2017 study’s upper Mashel River sampling site, suggesting this as the source of PBDEs entering the river (O’Neill et al. 2020a). Muck and Ohop creeks had distinct PBDE congener patterns from those found in the Mashel River, with higher concentrations of heavier PBDEs (Octa-BDEs, and Nona-BDEs). These sites also did not show an enriched signal of nitrogen stable isotope, ẟ15N, suggesting the source of the PBDEs was not wastewater (O’Neill et al. 2020a). The spatial scale of the survey limited its ability to conclusively identify the major source of PBDE inputs to the river system therefore a follow up survey of the tributaries was necessary to conclusively determine PBDE sources in the watershed.

Determining the source of PBDEs entering the Nisqually River is necessary to establish corrective management actions which may increase steelhead survival. The purpose of this study was to further investigate potential sources of PBDEs in the three major tributaries of the Nisqually River (Mashel River, Muck Creek, and Ohop Creek) and the impacts on the food web. The WDFW TBiOS team, in collaboration with the Department of Ecology, collected co-located biofilm and invertebrate samples from fourteen sites spanning the three tributaries. The specific objectives were to:

• delineate the locations of PBDE inputs to the Mashel River
• investigate the presence of PBDEs in Ohop and Muck creeks
• measure and describe the uptake of PBDEs in aquatic insects that are prey for juvenile steelhead trout.

Biofilms and invertebrate samples were used to determine potential locations of PBDE inputs to the three tributaries. Additionally, invertebrate samples provide a measure of PBDEs’ impact on the food web and provides insight on how PBDEs potentially accumulate in steelhead trout. Biofilms act as a natural passive sampler, due to their carbon content, and accumulate PBDEs over their growth period on the order of several months. Invertebrates are potential prey items to steelhead trout and provide an exposure vector as they accumulate PBDEs through feeding on biofilms. To determine the location of PBDE sources into the tributaries a synoptic survey of biofilms and invertebrates was performed. PBDE concentrations and congener patterns across the tributary systems were used to identify reaches with increased PBDE concentration indicating a potential input source. Stable isotopes of carbon and nitrogen were also analyzed in biofilm and invertebrate samples. These naturally occurring stable isotopes serve as chemical tracers of trophic status and diet (Caut et al., 2009; Olson et al., 2010; Ramos et al., 2011), as well as nutrient inputs from wastewater (Cabana and Rasmussen, 1996; Loomer et al., 2015; Schlacher et al., 2005). Regionally, the enrichment of nitrogen stable isotopes beyond the background river level were used to indicate the incorporation of nitrogen derived from wastewater inputs to juvenile salmon in the Snohomish River (O’Neill et al. 2020b). Stable isotopes also provide complementary information to the analysis of PBDEs, allowing the determination of source type (i.e., wastewater, legacy dump, etc.) of PBDEs to the river system.