Mercury levels in Pacific skipjack tuna across different ocean regions
What it found
Skipjack tuna caught near Asia carried much more mercury than those from the central or western Pacific.
The difference was linked to low oxygen zones and human pollution sources.
What they found
Mercury varies by region
Tuna caught in the northwest Pacific near Asia had mercury levels about four to five times higher than those from the southwest or west-central Pacific. Levels in the east and central north were in between.
Low oxygen raises mercury
In parts of the ocean with less oxygen below the surface, bacteria produce more methylmercury closer to where the tuna feed. This natural process explained much of the difference between eastern and western fishing grounds.
Pollution near Asia
Computer models using only natural ocean conditions could not explain the very high mercury found in tuna near Asia. Adding estimates of human air pollution from that region matched the measurements, pointing to local industrial emissions as an extra source.
Fish size matters
Bigger skipjack tuna had more mercury in their muscle tissue. The researchers adjusted for this so they could compare regions fairly.
Diet played little role
Chemical markers showed that differences in what the tuna ate across regions did not explain the mercury patterns. The environment mattered more than the food web position.
What the authors conclude
- They conclude that both natural ocean chemistry and recent human pollution drive mercury levels in Pacific skipjack tuna.
- Their results suggest that current industrial emissions in Asia directly raise the mercury people are exposed to when eating tuna caught nearby.
- They recommend using skipjack tuna to monitor ocean pollution and call for similar large-scale studies on other popular tuna species.
How it was done
People eat a lot of tuna, which can contain methylmercury. Scientists wanted to understand why mercury amounts differ depending on where in the Pacific Ocean the fish are caught.
The researchers measured mercury in 661 skipjack tuna collected across six areas of the Pacific Ocean between 1997 and 2018. They used computer models to figure out what factors drove the differences they saw.
What it can’t tell you
- This work looked at wild-caught fish and ocean conditions rather than testing health effects in people.
- It cannot show exactly how much risk these mercury levels pose to someone eating tuna.
Who paid
- Conflicts
- The authors declare no competing interest.
- Authors work at
- Université de Bretagne Occidentale (UBO), France; Université Toulouse III - Paul Sabatier, France; The University of Tokyo, Japan
The paper
- Title
- Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs
- Type
- Study
- Summarised from
- Full text
- Cite
- Médieu A, Point D, Itai T, et al (2022). Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs. Proceedings of the National Academy of Sciences of the United States of America. doi:10.1073/pnas.2113032119Free full textPubMed 34983875DOI
Summary written 1 Oct 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error