Disturbing Trend in Mercury Contamination in Harbour Porpoises

The research is based on the examination of 2,589 harbour porpoise bodies between 1990 and 2021. It was conducted by the Cetacean Strandings Investigation Programme (CSIP) in England and Wales and by the Scottish Marine Animal Strandings Scheme (SMASS). Trace elements, including mercury, were analysed in a sub-sample of 738 animals. This is the first study in almost two decades to assess trace elements in UK-stranded marine mammals.

Researchers found that, despite global bans, mercury levels rose steadily over the past three decades. So, by 2021, the average mercury concentrations were almost twice as high as they were in the early 1990s and, most concerningly, one tenth of the porpoises had levels where serious health impacts would be expected.

By contrast, the levels of other trace metals (lead, nickel, chromium and cadmium) declined. This is in line with what might be expected given the tighter controls on these pollutants.

Why is mercury still increasing?

The paper explains that despite an overall reduction in European anthropogenic mercury emissions precipitated by reduction measures in the 1970s, Hg concentrations in the open ocean are believed to have tripled globally at depths of 100–1000m. So what may be happening is movement of previously discharged mercury (sometimes called legacy mercury) into the marine food chains in subsurface and surface waters. This could be in part caused by environmental changes linked to climate change.

Mercury and Health

The researchers also looked at the relationship between mercury and health, comparing porpoises that died of infectious disease with those that died acutely, for example in fishing gear. They found that the more highly mercury-contaminated porpoises were at more risk of dying from infectious disease.

Lead author Rosie Williams commented in an article she authored in ‘The Conversation’ that “Many factors, including nutritional stress and other pollutants like industrial chemicals called polychlorinated biphenyls (PCBs), also affect immune function. But our study strongly suggests that mercury is part of the problem.”

She added “Our findings highlight that mercury isn’t just a historical problem. It is a current, growing pressure on marine mammals that face multiple other stresses: bycatch, noise pollution, habitat degradation, climate-driven prey shifts and exposure to forever chemicals”.

Wider Implications

Dr Williams also stressed that there is a warning in these results for humans too because the processes that are leading these top predators in UK waters to become more contaminated may also be affecting the fish and shellfish that people are eating.

Comprehensive data on pollutants is generally lacking for the marine mammals that are still hunted and eaten in the Northeast Atlantic, such as pilot whales in the Faroe Islands, where contaminant levels—including mercury—have been a longstanding concern for human health.

Rising mercury levels and climate change are related issues. Burning less fossil fuel would reduce emissions of both CO₂ and mercury, while missing climate targets will send more mercury into marine systems.

Sources

Temporal Increases in Mercury Concentrations are Associated with Increased Risk of Death by Infectious Disease in Harbour Porpoises (Phocoena Phocoena). Environmental Science & Technology, 2025: https://pubs.acs.org/doi/10.1021/acs.est.5c08346

Artikel in ‚The Conversation‘: https://theconversation.com/mercury-pollution-in-marine-mammals-is-increasing-new-study-270123