A comparison between dolphins that died from infectious diseases and those that died from trauma, such as bycatch, revealed that animals with higher mercury levels were more susceptible to mortality from infectious diseases. A steady increase was also noted in dolphin deaths from infectious diseases, contrasting with a decrease in deaths from trauma. Although these results do not prove that mercury is the sole cause, they strongly suggest that it is part of the problem, along with other factors such as food shortages and industrial chemical pollutants affecting the immune system.
Rosie Williams, an environmental researcher, for "The Conversation".
Mercury levels remain high due to its accumulation in the oceans from past industrial activities like coal burning and mining, and it can take decades or even centuries to be eliminated. Climate change and overfishing also affect marine food chains, increasing the formation of "methylmercury"—the organic and toxic form of mercury—in the fish that dolphins consume. Furthermore, emissions from coal used in power plants and cement production, as well as some other industries like dental fillings, continue to increase ocean mercury pollution. Rising sea temperatures and changes in marine food networks increase mercury accumulation in the food chain.
The Minamata Convention, adopted in 2017, marked a major turning point in international efforts to reduce mercury. These accumulations serve as a warning to the marine ecosystem and to humanity as well.
Samples were collected under programs for monitoring stranded marine animals on UK shores, as well as the investigation program for whale mortality. Animals that wash ashore after death are examined to understand the causes of their stranding and the threats they face. Samples were taken from the animals' livers to measure levels of eight trace elements, including mercury, due to the liver's vital role in metabolism, detoxification, and mineral storage.
Infectious Diseases
The study showed that mercury concentrations in dolphin livers increased by 1% annually over 130 years, and by 2021, the average concentration had doubled compared to the early 1990s. In contrast, a decrease was observed in the levels of lead, cadmium, chromium, and nickel, reflecting the success of banning and controlling these pollutants. Nevertheless, mercury levels remain high in many sea areas. Human activities, such as industrial and gaseous emissions, contribute to increasing mercury concentrations in the oceans, which have tripled in shallow waters (depth less than 1000 meters) since the Industrial Revolution. Rising sea temperatures and changes in marine food networks also increase mercury accumulation in the food chain, exacerbating the problem.
An Important Indicator
During the study, the research team analyzed liver samples from 738 stranded dolphins along the UK coast between 1990 and 2021. The results showed that mercury levels in the liver are increasing over time, and animals with higher mercury levels were more prone to death from infectious diseases. Dolphins are an important environmental indicator of ocean health, as they have a long lifespan (often over 20 years), making them more susceptible to accumulating pollutants.
In 2017, an international convention was adopted aimed at controlling mercury pollution, known as the Minamata Convention on Mercury. However, data from three decades of studying dolphins in the UK show that mercury levels are still on the rise, and this is linked to an increased risk of death from infectious diseases in these animals. When the Minamata Convention entered into force eight years ago, it was considered a major turning point in international efforts to reduce mercury. The Convention obliges member states to reduce mercury emissions from coal-fired power plants, various industries, and from products containing mercury, such as batteries and dental fillings.
