Toxic contaminants in juvenile Chinook salmon (Oncorhynchus tshawytscha) migrating through estuary, nearshore and offshore habitats of Puget Sound

Juvenile Chinook salmon (Oncorhynchus tshawytscha) can encounter a wide range of water quality conditions, from relatively clean to highly contaminated, as they migrate from rivers into Puget Sound. During this life stage, as they transition into saltwater, they are particularly sensitive to stressors such as toxic contaminants. This study was designed to provide a synoptic assessment of contaminant exposure for major populations of juvenile Chinook salmon from Puget Sound as the fish migrate from their freshwater to marine habitats. Overall, the study estimated exposure of salmon to toxics chemicals in 1) the estuary habitats of major rivers entering Puget Sound, 2) the nearshore marine habitats associated with those rivers, and 3) the offshore marine habitats of the major basins of Puget Sound. The study addresses the general hypothesis that chemicals released into Puget Sound from human activities and development reduces the health and productivity of salmon and their food supply. Specifically, we hypothesized that juvenile Chinook salmon residing and feeding in the more urbanized and industrial estuary, nearshore marine, and offshore habitats of Puget Sound are exposed to higher concentrations of toxic contaminants than those in less developed habitats. In addition, we hypothesized that the elevated contaminant concentrations in the more urban areas are high enough to affect juvenile Chinook survival through reductions in growth, disease resistance, and altered hormone and protein levels.

Fish were sampled in spring and summer of 2013 from five major Puget Sound river systems (i.e., river estuaries plus associated marine nearshore) and four marine basins in Puget Sound. In each river system, sampling sites included one location in the lower estuary and two locations along adjacent nearshore marine shorelines. The marine basins included fish offshore habitat from Admiralty Inlet, Whidbey Basin, Central Basin, and South Basin. We analyzed whole bodies for persistent organic pollutants (POPs), stomach contents for polycyclic aromatic hydrocarbons (PAHs), and gills for metals in fish collected from estuaries, nearshore marine shorelines and offshore habitats in the basins of Puget Sound. Tissue residues were compared with published adverse effects thresholds to evaluate the potential health effects on juvenile salmon from exposure to these contaminants. Finally, for the whole body analyses, we compared body burden of POPs in fish from different habitats to assess the degree to which POPs were being accumulated in the river and estuary, nearshore, or offshore habitats (i.e., routes of exposure).

The levels of organic contaminants we observed in juvenile Chinook salmon from estuary and nearshore habitats, measured as POP concentrations in whole-body fish samples or as PAH concentrations in stomach contents, supported our hypothesis that salmon residing and feeding in the more urbanized and industrialized environments are exposed to higher concentrations of contaminants than those in less developed habitats. However, for salmon collected in offshore habitats of the marine basins our hypothesis was not supported. Fish from the more developed Central Basin of Puget Sound did not have elevated POPs and PAHs concentrations compared to those from the less developed Whidbey Basin and South Basin. As juvenile Chinook salmon migrated from river systems to offshore waters of Puget Sound, all fish continued to accumulate substantial amounts of POPs, as evidenced by the higher total mass of POPs in their bodies (i.e., POP body burdens measured as ng/fish) and after four months of feeding in the offshore habitats, fish from all basins had uniform concentrations of POPs (i.e., the mass of POP compared to the mass of fish tissue measured as ng POP/g tissue ww). In general, concentrations of POPs in fish from offshore basins were intermediate between those measured in fish from non-developed and developed river systems, indicating that the offshore was more contaminated than the undeveloped river systems habitats but less contaminated than the developed river systems habitats. The levels of copper and lead were also elevated in gill tissues of fish from the more developed nearshore marine habitats but the concentration of cadmium, nickel and zinc were not elevated in the more urban and industrial habitats. Fish body size did not show strong association with contaminant uptake; location was consistently the primary factor associated with contaminant levels.

Levels of PCBs and PBDEs in whole body tissue samples from fish collected in the Snohomish, Green/Duwamish and Hylebos/Puyallup river systems, and PCBs in fish from the offshore habitat of the Whidbey Basin and the Central basin were high enough to potentially cause adverse effects, including reductions in growth, disease resistance, and altered hormone and protein levels. Additionally, PAHs in stomach content of Chinook salmon were elevated in salmon from the nearshore habitats of the Snohomish and Green/Duwamish systems, at concentrations high enough to potentially increase variability in growth, and to alter plasma chemistry and lipid class profiles. Moreover, approximately one-third of the salmon we sampled from Puget Sound, regardless of the degree of development, had contaminant concentrations associated with adverse effects, indicating that a significant proportion of juvenile Puget Sound Chinook salmon are at risk for some type of health impairment due to contaminant exposure, potentially affecting their marine survival.

Analysis of contaminant body burden (ng/fish) in salmon from estuary, nearshore, and offshore habitats revealed that along the migratory pathway, salmon accumulated the majority of the mass of POPs in their bodies from offshore habitats, indicating that sources of POPs to fish migrating to the Pacific Ocean is not limited to contaminant exposure in developed rivers and nearshore habitats. POP contaminant loading from urbanized river system areas and other sources is reaching non-urbanized offshore habitats where juvenile Chinook salmon may feed for several months, sometimes accumulating concentrations high enough to potentially impair their health. These findings suggest that controlling the initial release of contaminants to river system and other sources may be necessary to protect offshore habitats and their associated pelagic species, including Chinook salmon.

Although juvenile Chinook salmon in estuary and nearshore habitats accumulated a lower mass of POPs (i.e., body burden measured as ng POP per fish) than salmon in offshore habitats, salmon in estuary and nearshore habitats of developed river systems often had POPs concentrations (ng POP per g of fish tissue) above adverse effects concentrations. Analysis of contaminant body burden (ng/fish) in fish from estuary and nearshore habitat of individual river systems revealed that the habitat along the migratory pathway where salmon are exposed to POPs (i.e., the route of contaminant exposure) depended on the river system and the contaminant. Thus, management efforts to reduce contaminant exposure in river systems must be prescriptive to the individual river system and contaminant of concern.

The results of this study augment previous sampling initiated as early as 1998, and will be used to establish a solid time series of contaminant conditions in juvenile Chinook salmon that can be used to fulfill the Toxics in Fish Vital Sign goal of tracking time trends of fish health. Future monitoring of contaminant exposure in juvenile salmon should include chemicals of emerging concern in the Puget Sound ecosystem. Additionally, the geographic scope of the monitoring should be expanded to include other river systems that contribute to the production of Puget Sound Chinook salmon, such as salmon populations from the Hood Canal, Nooksack, and Stillaguamish river systems.