Controls of Contaminant Biotransport by Pacific Salmon to Great Lakes Tributaries

My dissertation evaluated factors that influence contaminant the transfer and uptake of Pacific salmon (Oncorhynchus spp.) derived contaminants to stream-resident fish in tributaries of the upper Laurentian Great Lakes. This research has broad relevance to fisheries of the Great Lakes because Pacific salmon are considered an important sport fish while the Lakes are replete with contaminants, especially persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs), and heavy metals such as mercury (Hg). My dissertation demonstrated that Pacific salmon transport contaminants to tributaries of the Great Lakes and influence the contaminant burden of stream-resident fish. Using a combination of observational, experimental, and modeling approaches, I identified the key factors related to how contaminants transported by salmon are transferred and accumulated by stream-resident fish. My research provides strong evidence for contaminant biotransport and transfer of POPs, but not Hg, by salmon. Therefore, not all contaminants are equivalent, and the trophic pathway to contamination is a primary driver of the impact of salmon-mediated contaminants on stream-resident fish. In CHAPTERS 2 and 3, I show how environmental context influences the transfer of PCBs biotransported by Pacific Salmon. Specifically, the lake-basin of origin for the salmon spawner, the flux of PCBs supplied by salmon, and the species identity of the resident fish strongly influenced the magnitude of contaminant biotransport. Differences among resident fishes are likely driven by differences in diet composition, consumption rate, and physiology. In contrast, the lack of a Hg effect suggests that salmon are not a significant source of Hg to resident fish. Moreover, because salmon eggs are enriched in POPs, but depleted in Hg, widespread consumption of eggs may be the primary pathway for biouptake of salmon-derived contaminants by resident fish. The experimental approaches described in CHAPTERS 4 and 5 provide strong inference that egg consumption results in elevated POP concentrations, consumption of salmon tissue leads to increased Hg concentrations, and that consumption of salmon eggs but not salmon tissue enhances the growth of resident fish. The contrast in Hg response between my mesocosm (CHAPTER 4) and salmon addition experiments (CHAPTER 5) was likely driven by the consumption of salmon tissue in the mesocosm experiment, suggesting that resident fish in streams receiving salmon runs rarely consume carcass material, especially if eggs are available. I integrate my inferences from CHAPTERS 2-5 in CHAPTER 6 using a bioenergetics-bioaccumulation model that provides a mechanistic explanation of salmon-mediated contaminant biotransport. The model suggests that variation in the trophic pathway, individual diet and consumption rate, and location in which salmon accumulate their contaminant burden interact to regulate the transfer and uptake of salmon-derived contaminants by stream-resident fish. Overall, my dissertation demonstrates that environmental context, reflecting the contaminant type, species identity, and trophic pathway to contamination, determines the magnitude of salmon-mediated contaminant biotransport and uptake. Consequently, consideration of the recipient food web and route of exposure is critical to understanding the fate of biotransported contaminants in stream ecosystems.