Despite international efforts to reduce mercury emission to the environment, mercury contamination of tuna is stubbornly high

Mercury, a potent neurotoxin, poses significant health risks to humans, particularly to developing fetuses, young children, and adults exposed to it. Methylmercury, the form found in fish, is highly absorbable and can accumulate in organisms up the food chain. Predatory fish such as tuna, swordfish, and halibut often contain high concentrations of mercury due to biomagnification. Such fish is the main mercury exposure source for humans and therefore a global health concern. A significant part of mercury pollution primarily stems from human activities like coal burning, waste combustion, and cement production. The Minamata Convention on Mercury, enacted in 2017 by 140 countries, aims to mitigate mercury emissions globally, yet its direct impact on methylmercury concentrations in marine environments remains unclear. Understanding the factors influencing mercury levels in tuna, especially amidst climate change, is crucial for policymakers to minimize human exposure to this toxic substance. Most anthropogenic mercury releases are estimated to have occurred during the past five centuries. While anthropogenic mercury emissions have decreased since the 1970s in regions like Europe and North America, they have surged in Asia since the 1980s.

An international team of researchers recently published a comprehensive study assessing mercury contamination trends in tuna to evaluate the effectiveness of the Minamata Convention. Over a decade-long effort, they analysed nearly 3,000 tuna samples collected from the Pacific, Atlantic, and Indian Oceans between 1971 and 2022. 

Focusing on skipjack, bigeye, and yellowfin tuna—constituting 94% of the global tuna catch—they observed that mercury levels remained largely unchanged despite global emissions reductions. In certain Pacific regions, skipjack tuna even exhibited increased mercury levels, correlating with rising emissions from Asia.

The researchers attribute tuna mercury contamination to ocean mixing processes, which stir up mercury deposited in deep ocean layers over decades. This legacy mercury resurfaces due to ocean dynamics, resulting in a delayed response to emission changes. However, the study's simplistic model lacks spatial resolution and excludes aquatic food webs, hindering the examination of local and regional impacts on mercury biomagnification. These intricate processes, coupled with the ongoing effects of climate change on ocean temperatures, further complicate predictions.

Consequently, the researchers suggest that only the most ambitious reduction policies would lead to a tangible decrease in tuna mercury contamination, aligning with the Minamata Convention's objectives.

The original publication

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Atmospheric Mercury Network, AMNet

European Monitoring and Evaluation Program, EMEP

UN environment programme: Global mercury assessment

UN Environment Programme: Minamata Convention on Mercury

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