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Steller sea lions threatened by rising mercury levels

Steller sea lions (Eumetopias jubatus), once listed as endangered, have made a partial recovery in some regions of the North Pacific, particularly the Aleutian Islands. However, not all populations are rebounding proportionally, prompting a focused research effort to identify potential causes. One key area of investigation is the concerning rise of mercury levels in Steller sea lion pups.

Mercury: An environmental contaminant

Mercury (Hg) exists naturally in Earth’s crust but enters the environment in increased concentrations due to both natural processes (like volcanic eruptions) and human activities (such as mining and industrial processes).

Once in the environment, especially in aquatic systems, certain bacteria can change mercury into methylmercury, a more harmful organic form.

Mercury, and specifically methylmercury, enters the marine food web through consumption. Small organisms ingest it, larger predators consume the small organisms, and the mercury accumulates at higher levels in each successive step up the food chain. This process is known as bioaccumulation.

Adverse health effects of mercury

  • Immunotoxicity: Mercury exposure compromises the immune system of various animals. This weakens an organism’s natural defenses against disease and makes them more susceptible to infections.
  • Disrupted oxidative balance: Oxidative stress is a natural process where unstable molecules called ‘free radicals’ can damage cells. Organisms have systems to combat this stress, but mercury disrupts these systems, potentially accelerating cellular damage and contributing to health issues over time.
  • Vulnerability during development: Exposure via the mother to a developing fetus or pup is a significant concern. Developing bodies are highly sensitive to toxins, and mercury can lead to neurological and developmental problems with lasting impacts.

Mercury levels in Steller sea lions

To understand the complex dynamics of mercury in the marine environment, an international research team led by Dr. Todd O’Hara, Dr. Lorrie Rea, and Dr. Daniela Cisneros has conducted in-depth studies of Steller sea lions throughout the Aleutian Islands. Their multi-year investigation, spanning over a decade, aims to track mercury levels within this key indicator species.

The researchers have observed an alarming increase of over 50% in mercury levels found in Steller sea lion pups from specific regions along the island chain. This trend raises concerns about the long-term health of the species.

The team’s analysis suggests that mercury exposure is not uniform across the Aleutian Islands. There appears to be a distinct “hotspot” around the center of the island chain where Steller sea lion pups exhibit the highest mercury concentrations. This points to possible localized sources of contamination or specific oceanographic processes that concentrate mercury in this area.

“When we started this research in Southeast Alaska, we found very few pups with alarming levels of mercury,” said Dr. Lorrie Rea, a research professor at the University of Alaska Fairbanks (UAF) who plays a significant role in the Steller sea lion research program.

“Once we got to the Western Aleutian Islands, we found pups with mercury concentrations that were three to four times higher than the highest we saw in other regions. As we moved toward Russia along the Aleutian Island chain, the concentrations went down again, so it’s almost like a bell-shaped curve.”

The observed correlation between rising mercury levels within certain regions and the slower recovery of some Steller sea lion populations has prompted further investigation. Scientists are examining the hypothesis that mercury exposure could be one factor contributing to the uneven population trends seen across different parts of the Aleutian Islands.

It’s important to emphasize that while a correlation has been observed, this is not direct proof that mercury exposure is definitively causing population declines. Many factors can influence wildlife populations, and teasing out the individual impact of one variable can be complex.

Steller sea lions significance

The importance of research focused on Steller sea lions goes beyond understanding the health of a single species. Because these animals occupy a high position in the marine food web (meaning they eat many other animals), they act as sentinel species. This means that changes in their health, such as those related to mercury exposure, can alert us to broader environmental shifts with far-reaching consequences.

Steller sea lions and humans share common food sources within the marine ecosystem. As mercury bioaccumulates in species throughout the food chain, there’s a potential risk that it could reach levels in commercially fished species that may impact human health.

Although current data suggests that mercury levels in most commercially fished species fall within safe limits, consistent monitoring is crucial for several reasons:

  • Dynamic ecosystems: Marine environments are complex and changing. Ongoing monitoring allows for detecting any potential increases in mercury levels in fish over time.
  • Early warnings: Steller sea lions, as a highly sensitive sentinel species, may provide an early warning sign of changes in mercury contamination before it reaches concerning levels in human food sources.
  • Proactive management: Continued monitoring enables informed decision-making about fisheries management, helping to ensure the long-term safety of seafood resources.

Ongoing research and collaboration

Unraveling the causes behind rising mercury levels in this region is complex and likely involves a combination of natural and anthropogenic (human-caused) factors. The study underscores the importance of ongoing research into mercury cycling, its impacts on marine ecosystems, and the interconnected nature of environmental and human health.

Nevertheless, the success of this multi-faceted investigation highlights the value of collaboration between scientists, commercial fisheries, government agencies like NOAA, and researchers across international borders.


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