Each year, millions of Pacific salmon embark on an impressive journey from the ocean back to their freshwater spawning locations as they complete their life cycles. This migration significantly impacts food webs and ecosystems along their route. Whether they perish naturally or are eaten by other species, salmon bring a mix of vital nutrients and harmful contaminants that they have gathered while living in the sea. Researchers from UConn, the University of South Dakota, the U.S. Geological Survey, Natural Resource Consultants, the University of Saskatchewan, the University of Missouri, and Regis University collaborated to investigate the movement of these nutrients and contaminants and examine trends over 40 years as the salmon population evolved. Their findings have been published in the journal Nature.
Every year, millions of Pacific salmon take an awe-inspiring trip from the ocean to their freshwater spawning areas as they reach the end of their life cycles. This migration affects food webs and ecosystems along the way. Regardless of whether they decompose or are consumed by other animals, these salmon carry with them both nutrients and toxins that they accumulated during their time in the ocean. A team of researchers from several institutions, including UConn and the U.S. Geological Survey, aimed to track the journeys of these nutrients and contaminants and analyze changes in the salmon community over a 40-year period. Their results are now available in the journal Nature.
Jess Brandt, an assistant professor at UConn’s College of Agriculture, Health, and Natural Resources, led this study. It focused on the migratory patterns of the five main Pacific salmon species: Chinook, chum, coho, pink, and sockeye. The research team combined fish biomass estimates for the 1976-2015 period with nutrient and contaminant levels found in various scientific studies.
Energy and nutrients that have beneficial effects when they transfer between systems are called ecological subsidies. In contrast, harmful substances that move along with these nutrients are known as the “dark side of subsidies,” according to Brandt.
“This research explores both the ‘bright and dark sides’ of Pacific salmon contributions. We typically examine them as separate topics in relation to animal transport, but nutrients and contaminants are interconnected,” adds Brandt.
Initially, the researchers merged species biomass estimates with nutrient and contaminant data to determine the material movements resulting from salmon migration. Brandt mentions that these movements have not been quantified at such a broad, continental scale before, and the volume of material moved by Pacific salmon was striking.
“On average, about 119 million Pacific salmon returned to North America annually during the 40-year study period, moving thousands of tonnes of nutrients and kilograms of contaminants. The nutrient fluxes attributed to Pacific salmon are among the largest reported for any large animal groups migrating or dying in clusters,” explains Brandt.
The researchers also explored how shifts in the Pacific salmon community over the years affected the transport of nutrients and contaminants.
They discovered significant growth in the Pacific salmon community during these four decades, both in terms of total biomass and fish numbers. According to Brandt, the quantity of nutrients and contaminants transported in 2015 was 30% greater than in 1976, with pink salmon representing nearly 80% of this increase.
“We were intrigued by how changes in the Pacific salmon community structure influenced the transport of nutrients and contaminants over time,” says Brandt.
To grasp the importance of these findings, it’s useful to examine the life characteristics of various Pacific salmon species. Brandt notes that a species’ position in the food chain (or trophic level), its duration in the ocean, and its size at spawning are critical factors influencing contaminant accumulation. Environmental contaminants can concentrate in food webs, escalating from prey to predator through biomagnification. This means that higher-level consumers have more significant contaminant burdens.
Brandt emphasizes that, overall, contaminant levels in Pacific salmon are low compared to many other fish species. However, variations exist among the different salmon types. Pink salmon, for instance, consume lower on the food chain, spend less time in the ocean, and return smaller, while Chinook salmon feed higher up, remain in the ocean for longer, and are the largest of the five species, affecting their contaminant load accordingly.
“This led us to investigate how much of each chemical a species carries—its loading potential—and how these nutrient and contaminant loads compare. We found that higher trophic level salmon, particularly Chinook, bear higher ratios of contaminants to nutrients, whereas pink salmon are associated with more nutrients relative to contaminants,” states Brandt.
Although there are differences among the species, pink salmon transported the highest total amounts of contaminants due to their vast population numbers. Brandt explains, “We also evaluated which species made the most significant contributions to nutrient and contaminant transport within the Pacific salmon community. Despite pink salmon having lower contaminant concentrations, their dominance in the community means they transport the largest portion of contaminants. Their rising numbers mean that even with unchanged contaminant levels, a higher quantity of fish results in greater overall contaminant transfer.”
Finally, the researchers examined the balance between the nutritional benefits and contaminant exposure for animals consuming salmon, including humans. They assessed the advantages of omega-3 fatty acids from salmon against the potential health risks from the identified contaminants. According to Brandt, “The findings for each species suggest that consuming salmon provides net benefits to people, which likely extends to overall ecosystem health as well.”
By analyzing nutrients and contaminants together, the study sheds light on the environmental consequences of migratory species like salmon.
“When we separate the studies of nutrients and contaminants, we miss crucial aspects of the whole picture. We hope this research encourages further investigation into migratory species as vital transporters of both nutrients and contaminants and explores the trade-offs between these inputs for broader ecological contexts,” concludes Brandt.
