Gerig BS, Chaloner DT, Janetski DJ, Moerke AH, Rediske RR, O’Keefe JP, de Alwis Pitts DA & Lamberti GA (2018) Environmental context and contaminant biotransport by Pacific salmon interact to mediate the bioaccumulation of contaminants by stream-resident fish. Journal of Applied Ecology 1-14.
During the 1960s Pacific salmon were introduced into the Great lakes region to reduce invasive fish species in order to return native fish populations. Salmon are also a great game fish. However, the Great Lakes have experienced tremendous amounts of pollution and contaminants from the surrounding industry. These contaminants enter the food web through algae and smaller fish. As salmon consume contaminated prey out in the open waters of the Great Lakes they build up concentrations of contaminants. This process is known as bioaccumulation.
What contaminants are we talking about?
PCB, or polychlorinated biphenyl, was a compound found in many coolant fluids up through the 1960s. This likely carcinogen was banned by the EPA in 1978. But because PCB does not breakdown very quickly, the persistent organic pollutant still persists across many landscapes and within many species..
Mercury is found in low concentrations in most waters. As the heavy metal is absorbed up through the food chain, via bioaccumulation, it can reach dangerous concentration levels. High levels of mercury can pose a threat to surrounding ecosystem as well as public health. Consumption of foods with high concentrations can lead to mercury poisoning. This poisoning can have a variety of symptoms and affect vision, hearing and muscle control.
Why are we interested in salmon?
Salmon, even in lakes, are migratory animals. The life cycle of salmon can be broken down rather simply. Adults spend the vast majority of their time feeding in the ocean, or in this case the open-waters of the Great Lakes. Then when the time is right, the adults swim up rivers to spawn the next generation of salmon. Traveling upstream and spawning is a daunting task. Once spawning is completed the adults perish and only their carcasses and eggs remain.
Salmon migrations can have extraordinary effects on the river and the surrounding forested ecosystems as salmon eggs and carcasses contribute nutrients that previously did not exist in the rivers. For example, the nitrogen remaining from salmon carcasses promote the productivity of many forested landscapes across the Northwest that would otherwise be nitrogen-limited.
Gerig and colleagues were curious as to how salmon, who spend a large majority of their adult life in open, contaminated waters, affect the riparian fish populations living in more protected and secluded streams.
Gerig and colleagues looked at 13 different watersheds in the Great Lakes region. Within each watershed they examined one river that salmon were known to spawn in and one river where spawning doesn’t occur. They also collected a variety of characteristics associated with the watershed to determine whether contaminant concentrations were primarily related to the salmon input or simply the surrounding landscape.
The rivers are occupied by other species than just the salmon. Brown trout, brook trout, rainbow trout, and mottled sculpin also reside in these rivers. In order to see the effect salmon may have on overall level of contaminants in the rivers, all five species of fish were harvested from each river and nitrogen isotopes were measured. Each species had identifiable isotopic compositions of nitrogen. Therefore the researchers were able to infer whether or not if the resident fish populations were consuming either salmon eggs or carcasses. From isotopic compositions and sampling of individuals, Gerig and colleagues were able to determine how salmon may contribute both PCBs and Mercury to local watersheds.
The researchers found that PCB concentrations increased in resident fish species when salmon were present, but mercury decreased or stayed the same. Where the contaminants are distributed within the salmon is an important part of this story. Mercury concentrations were 15 times higher in the body of the salmon compared to the concentrations in the eggs. Whereas PCB concentrations were 1.2 times higher in the eggs than the salmon body. Resident fish species are interacting with the salmon primarily by consuming deposited salmon eggs. While these eggs are rich in protein and provide nutrients to the resident fish populations they also lead to the bioaccumulation of PCB in the resident fish populations; whereas mercury concentrations were similar in resident fish regardless of whether or not salmon were present.
River-residing fish may be experiencing higher concentrations of PCBs rather than mercury based on their dietary preferences. Many wildlife and land managers already have strategies in place to deal with environmental contamination but different strategies will have to be put in place to manage the biologically transported contamination such as this new contaminant transport vector, the Pacific salmon. Some examples include identifying where large salmon runs are occurring or use adaptive stocking strategies to minimize the contaminant transfer to the local watersheds.
While salmon were introduced in the 1960s for just reasons, introducing non-native species can have its implications. This study shows that migratory organisms can have significant consequences for the recipient ecosystems they travel to, in this case the rivers and watersheds of the Great Lakes. Thus it is critical to be conscious of all the pathways in which our ecosystems can be affected by pollutants and contaminants.