Contaminant biotransport by Pacific Salmon in Lake Michigan: analysis of salmon and stream-resident fish in Great Lakes tributaries

Pacific salmon (Oncorhynchus spp.) can deliver a significant pulse of biomass, including its bioaccumulated contaminants, to tributaries during spawning runs. Thus, salmon transport contaminants accumulated in the Great Lakes (e.g., persistent organic pollutants [POPs], total mercury [THg]) to tributaries that otherwise lack point source pollution. We used a combination of observational surveys, experimental manipulations, and modeling, to (1) assess the extent of salmon-mediated biotransport across the upper Great Lakes; (2) determine pathways by which stream fish become contaminated by salmon; and (3) forecast areas at significant risk from salmon biotransport. Resident stream fish (e.g., brook trout Salvelinus fontinalis) in salmon spawning reaches had higher POP concentrations than fish in upstream reaches lacking salmon, but the extent of contamination varied among lake basins and streams. In contrast, THg concentrations in the same fish did not differ between reaches with and without salmon spawners but exhibited considerable among-site variability. In general, resident fish in Lake Michigan tributaries were the most contaminated by POPs, suggesting a direct relationship between salmon-derived contaminant inputs and resident fish contaminant levels. Experimental exposure to salmon carcasses and eggs for 50 days increased brook trout POP concentrations by 50 times. Eggs are elevated in POPs but depleted in THg compared to whole salmon, suggesting that resident fish contaminant levels reflect direct consumption of eggs rather than indirect food web pathways. Our model suggests that salmon-mediated bioaccumulation is primarily influenced by the size and duration of salmon runs, and secondarily by factors including individual consumption rates, temperature regime, and background pollutant levels. Overall, our research provides increased understanding on the physical, chemical, and biological controls of salmon contaminant biotransport in the Great Lakes region. This research will help inform management decisions in this region with respect to legacy pollution, dam removal, stream connectivity, fish stocking, and non-native species in stream ecosystems