USGS study finds mercury and stress affect northern elephant seals

The US Geological Survey said in an announcement published Tuesday 03 November 2026 that a 10-year collaborative study found interactions between mercury contamination and stress hormones in adult northern elephant seals. Researchers examined how mercury and cortisol may act separately or together to affect the endocrine and immune systems.

USGS researchers collected tissue samples from northern elephant seals and quantified 10 blood biomarkers. The biomarkers represented thyroid function, immune function and reproduction. The researchers related the biomarkers to mercury contamination in blood and muscle.

The study found that mercury contamination and cortisol concentrations were related to physiological responses in adult female northern elephant seals. The thyroid hormone thyroxine, or tT4, and the antibody immunoglobulin E were associated with mercury and cortisol concentrations interactively.

The thyroid hormones tT3 and rT3 were associated with mercury concentrations and cortisol in an additive manner. Immunoglobulin M, the pro-inflammatory cytokine IL-6 and the reproductive hormone estradiol were negatively related to mercury concentrations in seal muscle. They were not related to cortisol.

Estradiol concentrations in late molting seals decreased by 50% across the range of muscle mercury concentrations. Late molting is the time in the year when implantation of the fertilized egg occurs.

The US Geological Survey said the findings showed negative consequences for animal health. Mercury bioaccumulation and extrinsic stressors can affect the ability of marine mammals to maintain homeostasis, fight off pathogens and disease, and reproduce. The agency said these effects can have significant individual and population level consequences.