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Salmon swimming up stream

Early industrialization of the Great Lakes region has led to the release of numerous pollutants into the land, air, and water, which have been shown to be harmful to humans and wildlife in specific concentrations. Bioaccumulation and biomagnification, the processes by which pollutants build up in individual species and populations higher on the food chain, are relatively well documented and understood.

Less is understood, however, about biotransportation—the process by which migratory species, such as Pacific salmon, transfer these pollutants into environments that may not have localized sources of contamination—and its potential impacts on surrounding fish and wildlife as well as people who consume those fish and wildlife.

The Angle

Dominic Chaloner, a research associate professor at the University of Notre Dame, led a team of scientists to assess the extent to which Pacific salmon migrating from the Great Lakes into tributaries during their spawning runs increase contaminant levels in local fish populations. The team focused on mercury and persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs), given their prevalence in the Great Lakes Basin.

The Nitty-Gritty

To perform their assessment, Chaloner’s team collected samples from 15 tributaries to Lakes Huron, Superior, and Michigan. At each site, they gathered up to 15 fish, macroinvertebrates, sediment samples, and salmon eggs, and used standard laboratory methods to determine POPs and mercury concentration levels. Then, they developed a bioaccumulation-bioenergetics model to predict growth, size, and contaminant accumulation in different species using various assumptions, including the amount and types of salmon tissue and eggs consumed by local fish. To help ensure modeled results were reliable, they compared them to field samples where the research team experimentally controlled the introduction of salmon-mediated contaminants.

The Results Are In…

The team reached four principal conclusions regarding the role that Pacific salmon play in moving contaminants from one location to another and transferring those contaminants to local fish populations.

  • Different chemicals have significantly different biotransport rates. Mercury, for example, is present in the Upper Great Lakes at relatively consistent levels and introduced through atmospheric deposition, which was determined to have a larger role in depositing pollutants locally than salmon-mediated biotransport. When it comes to POPs, however, biotransport can be a significant contributor.
  • Salmon eggs, rather than tissue, appear to be the most likely source of POP transfer to local fish. The amount of POPs that build up in local fish populations depends on diet and physiology. Of the species studied, brown trout were most susceptible to increased pollutant concentrations, followed by brook trout because they eat more salmon eggs. Mottled sculpin, on the other hand, were the least susceptible.
  • POP contaminant levels at the lake basin scale influence local fish contaminant levels—even in more pristine tributaries with salmon runs. In other words, in lake basins with generally higher PCB levels, like Lake Michigan, local fish, in what might be regarded as otherwise unpolluted tributaries, often have higher POP concentrations from salmon-mediated biotransportation.
  • Biological variables such as diet and growth rates, rather than physical and chemical variables, appear to influence POP levels in local fish populations.

What Does It All Mean?

When persistent organic pollutants like PCBs are present in the Great Lakes, they accumulate in Pacific salmon, which then transport these chemicals into tributaries during spawning runs. These pollutants can then be transferred to local fish populations, and ultimately to people, through the food web.

The knowledge gained through Chaloner and his team’s assessment can help fisheries managers make more informed decisions about fish consumption advisories and tailor them to different lake basins based on the prevalence of POPs. For the Great Lake basins with higher PCB levels, like Lake Michigan, resident fish in tributaries are more likely to have higher PCB levels when salmon can migrate upstream. Furthermore, when stakeholders evaluate enhancing upstream passage for migratory salmon through dam removals or other bypass mechanisms, contaminant transfer should be an additional consideration when assessing environmental consequences of maintaining or removing barriers to passage.

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Photo’s provided at the courtesy of Brandon Geirg

Because of a recent study funded by the Great Lakes Fishery Trust, natural resource managers have new information to consider when developing fish consumption advisories and managing dams. The study, led by Dominic Chaloner, a research associate professor, and Brandon Gerig, a graduate student in the Stream and Wetland Ecology Laboratory at the University of Notre Dame, definitively shows that Pacific salmon can transfer persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs) from the Great Lakes into tributaries during spawning runs. These contaminants can then move through the food web to resident fish populations, wildlife, and potentially humans.

Great Lakes fisheries management and decision making rely heavily on an understanding of the delicate balance between potential tradeoffs and desired outcomes. In this case, the decision to allow salmon to migrate into tributaries to support a recreational fishery may have undesirable effects through increasing contaminant exposure in different geographic regions. The research underscores complexities of Great Lakes management and decision making and provides new information for managers to consider as they develop fish consumption advisories and evaluate how maintaining or removing dams can affect contaminant levels in the environment.

“This is another example of where there is no simple prescription,” Chaloner says. “What you do in any given watershed should reflect the context of that watershed. Managers will have to prioritize between different outcomes of management tools based on the priorities for individual watersheds.”

Since their introduction in the 1960s, Pacific salmon have become an important part of the Great Lakes ecosystem and economy. Although continuing changes in food web dynamics present new challenges for managers, stocking Pacific salmon helped restore a delicate balance to the food web by reducing invasive alewife populations. However, their introduction has not been without impacts. Pacific salmon are prone to bioaccumulation and biomagnification, the processes by which pollutants accumulate and increase in concentration in species higher on the food chain. As salmon migrate, they carry these pollutants with them into areas without localized sources of pollution and transfer them to local fish populations. In some instances, the transfer of harmful pollutants may occur in watersheds that anglers perceive as being pristine.

Biotransportation is the process by which migratory species can transfer pollutants from one environment to another and introduce those contaminants to local populations. It is the culmination of bioaccumulation, biomagnification, biomovement, biodeposit, and biouptake.

  • Bioaccumulation and biomagnification: Pollutants build up in individual species and populations higher on the food chain, such as Pacific salmon.
  • Biomovement: As these fish migrate from the Great Lakes into tributaries on spawning runs, they can carry these pollutants in their tissue and eggs.
  • Biodeposit: These pollutants are deposited in tributaries as Pacific salmon lay their eggs and die.
  • Biouptake: Resident species, such as brown trout, consume the eggs, accumulating the pollutants in their bodies.

Understanding this process helps understand how pollutants move through the environment and food web—providing another layer of information to consider when developing fish consumption advisories and evaluating dam removals.

Joe Bohr, an aquatic biologist with the Michigan Department of Environmental Quality who coordinates fish contaminant monitoring for the state, observed these trends when sampling in less-polluted areas.

“The question came up in the Pere Marquette River where there is no known point source for PCBs, but we were finding levels in brown trout that were higher than we would expect,” said Bohr. “They were elevated compared to what we see in brown trout in rivers without access to the Great Lakes. Our suspicion was that they were being brought in by salmon and potentially steelhead, too.”

Chaloner’s team was interested in these observations as well. To better understand the extent and sources of salmon-mediated contamination, they developed a sampling approach that included 15 tributaries to Lakes Huron, Michigan, and Superior. There, they collected fish, macroinvertebrates, organic material, and salmon eggs in systems with and without salmon runs.

Overall, the study found that resident fish from rivers with salmon runs were 23 times more contaminated than those where salmon were not present. It also found significant differences in contamination levels among species and locations. For example, when salmon were present, PCB concentration levels were 57 times higher for brown trout, 17 times higher for brook trout, and eight times higher for mottled sculpin.

Salmon Presence Map

The team also studied the mechanism by which contaminants are transferred, concluding that biological, rather than chemical or physical, variables determine contaminant transfer from salmon to resident populations. Furthermore, the team deduced that salmon eggs can be an important vector of contaminant transfer for POPs, but salmon tissue is not likely to be a significant source of mercury in the Upper Great Lakes because of high background levels throughout the region.

Understanding that contaminant levels can vary within a lake basin when there is a salmon spawning run can help the state as it develops fish consumption advisories.

“It may lead us, where we can, to sample resident fish in rivers where salmon have access that we haven’t sampled before,” explains Bohr.

The results may also provide important information for dam management as stakeholders weigh the potential outcomes of management actions such as where to introduce salmon and which barriers to remove or maintain.

“At a very basic level, the study acknowledges an issue here, just as we acknowledge things like upstream colonization of habitat by invasive sea lamprey when we pull barriers down,” says Gerig. “We should also consider contaminant biotransport by migratory fish when stakeholders assess whether to remove barriers in a given watershed.”

While the research team focused its study of persistent organic pollutants on PCBs in salmon, the results suggest that migratory fish can serve as a vector of contamination into tributaries. This may also hold true for other species and other POPs, which managers and other stakeholders should consider when evaluating outcomes of management actions.

“The issue of upstream biotransport of contaminants is of particular interest to Great Lakes barrier dam owners and their regulators,” says Scott DeBoe, Consumers Energy Environmental Services Division and Great Lakes Fishery Trust Scientific Advisory Team member “This research is valuable in that it provides new details on biological and health impacts and so helps regulatory agencies make informed decisions with respect to the key issues of river connectivity and fish passage.”