In 2017, a new global treaty was meant to bring mercury pollution under control. But three decades of data from UK harbour porpoises show mercury is still increasing, and is linked to a higher risk of dying from infectious disease.

When the Minamata convention came into force eight years ago, it was hailed as a turning point. The global treaty on mercury commits countries to reducing mercury from coal-fired power plants, industry and products, like batteries and dental fillings.

Yet mercury levels are still rising in many parts of the ocean. Human activities such as the burning of fossil fuels have already tripled mercury in shallower ocean waters (less than 1,000m in depth) since the industrial revolution. Warmer seas and shifting food webs are exacerbating the problem by increasing the rate of accumulation in the marine food chain.

In our new study, my colleagues and I analysed liver samples from 738 harbour porpoises that stranded along UK coastlines between 1990 and 2021. We found mercury levels increased over time and animals with higher levels are more likely to die from infectious disease.

Harbour porpoises are sentinels of ocean health because they are long lived (often for more than 20 years) and high up the food chain. This makes them more vulnerable to certain pollutants. The contaminants that build up in them are a warning for the marine ecosystem – and for us.

We measured trace elements as part of the UK’s strandings programmes in England, Wales and Scotland – the Cetacean Strandings Investigation Programme (CSIP) and the Scottish Marine Animal Stranding Scheme (SMASS). Stranded animals die from a range of causes, including bycatch in fishing gear and disease. When found washed up, a subset are sent to our London laboratory for post-mortem examination to help us better understand the population and the threats they face.

We sampled each animal to measure eight trace elements, including mercury, in their liver, which plays a critical role in the metabolism, detoxification and accumulation and tends to be where concentrations are highest. We analysed how concentrations changed over time, how they varied geographically around the UK, and whether levels were related to cause of death.

Over the last 30 years, mercury concentrations in porpoise livers rose by about 1% per year. By 2021, the average mercury concentration was almost double that of early 1990s. A worrying minority (about one in ten animals in the last decade) had mercury levels where serious health effects are expected.

In contrast, lead, cadmium, chromium and nickel declined, reflecting past bans and tighter controls on these pollutants (such as the ban on lead petrol).

We then investigated whether metal burdens were linked to health. Comparing porpoises that died of infectious disease with those that died of trauma, such as bycatch in fishing gear, we found that animals with higher burdens of mercury had a significantly greater risk of dying from infectious disease.

In parallel, we saw a steady increase in the proportion of porpoises dying from infectious disease and a corresponding decline in deaths from trauma. That doesn’t prove mercury is the sole cause. 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.

Why mercury is rising

Large amounts of mercury from past coal burning, industry and mining are already present in the oceans. Much of it sits in deeper waters acting as a source supplying shallower water and can take decades or centuries to be removed. This may explain why declines aren’t evident.

Climate change and overfishing are also disrupting marine food chains. This affects the formation and bioaccumulation (build up in tissues) of methylmercury (the toxic organic form of mercury), increasing levels in the fish that porpoise prey on. And global emissions have not stopped: coal power, cement production and sources such as dental amalgam still release mercury to the environment.

Read more: The five most poisonous substances: from polonium to mercury

Our findings highlight that mercury ins’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.

Because mammals share many aspects of physiology and immune function, the trends in porpoises offer a warning for human health too. If top predators in UK coastal waters are becoming more contaminated, the same processes may be affecting some of the fish and shellfish we eat.

Harbour porpoises are small, shy and easily overlooked. But their tissues are quietly recording the story of our chemical footprint in the sea. Right now, that story is telling us something uncomfortable: even after a global treaty, mercury pollution is still rising, and it is affecting the health of marine wildlife.

Mercury and climate change are two sides of the same problem: burning fewer fossil fuels cuts CO₂ and mercury, while missing climate targets risks driving more methylmercury into marine food webs. A safer ocean for porpoises and for people can be achieved by phasing out coal more quickly, reducing industrial emissions and moving away from mercury-containing products wherever safer alternatives exist.

The outlook for marine mammals can also be improved by addressing other human threats such as bycatch, underwater noise and other pollutants. None of this works without long-term monitoring, so continued investment in programmes, like the UK strandings network that underpinned our study, is essential to assess progress.

Don’t have time to read about climate change as much as you’d like?
Get a weekly roundup in your inbox instead. Every Wednesday, The Conversation’s environment editor writes Imagine, a short email that goes a little deeper into just one climate issue. Join the 47,000+ readers who’ve subscribed so far.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Rosie Williams, Zoological Society of London

Read more:

Rosie Williams was funded by the Natural Environment Research Council (NERC).