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 cent 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). (The Conversation)
