EXPERT REACTION: Warm waters and overfishing may be increasing the levels of mercury in our seafood

Embargoed until: Publicly released:
Who's down for sushi with a side of mercury? International researchers suggest that our warming oceans and changing diet due to overfishing could be increasing the levels of methylmercury (MeHg) in some fish we like to chow down on. They say that popular 'food-fish' such as cod and tuna are some of the worst affected, with mercury concentrations in Atlantic cod increasing by up to 23 per cent between the 70s up to the noughties. Additionally, the team say that the temperatures could contribute to an estimated 56 per cent increase in MeHg in Atlantic bluefin tuna. The team says this increase exists still despite a decrease in the concentration of the toxin in seawater since the late 1990s.

Journal/conference: Nature

DOI: 10.1038/s41586-019-1468-9

Organisation/s: Harvard University, USA

Funder: Financial support for this study was provided by the US EPA (contract EP-H-11-001346); the US National Science Foundation (OCE 1260464); and the Nereus Program sponsored by the Nippon Foundation. Statements in this publication represent the professional views of the authors and should not be construed to represent any determination or policy of the US EPA.

Media Release

From: Springer Nature

Climate change and overfishing affect toxin accumulation in fish

Ocean warming and dietary shifts due to overfishing may increase the levels of methylmercury (MeHg) in some fish consumed by humans, according to a study published online this week in Nature. This is despite a decrease in seawater concentrations of the toxin since the late 1990s.

Many people rely on seafood for nutrition, but fish are also a source of exposure to the neurotoxin MeHg. To mitigate the risks of MeHg exposure, a global treaty (the Minamata Convention) to reduce anthropogenic mercury emissions was introduced in 2017. However, how ongoing changes to marine ecosystems might affect the accumulation of MeHg in fish that are frequently consumed by humans (such as cod and tuna) was not considered when global targets were set.

Amina Schartup, Elsie Sunderland and colleagues set out to understand the impacts of increasing temperatures and overfishing on fish MeHg concentrations. They used more than 30 years of data on ecosystem, sediment and seawater MeHg concentrations from the Gulf of Maine, in the northwestern Atlantic Ocean. MeHg concentrations in the tissue of Atlantic cod increased by up to 23% between the 1970s and 2000s. The authors attribute these changes to shifts in diet as a result of overfishing, with cod having a greater reliance on prey such as larger herring and lobster, which have higher concentrations of MeHg than other prey fish consumed in the 1970s.

The authors also analysed the effects of recent temperature changes on MeHg accumulation in Atlantic bluefin tuna. They found that the effects of seawater temperature rises since a low in 1969 could contribute to an estimated 56% increase in MeHg concentrations in this species. Warming has previously been linked to increases in MeHg concentrations in some fish, but the extent of these changes in wild species has been poorly understood.

Although global mercury emissions have reportedly plateaued, this study indicates that ocean warming and fishing have a role in modulating mercury concentrations in fish.

Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Dr Jenny Fisher is a researcher in the Centre for Atmospheric Chemistry and a lecturer in the Schools of Earth & Environmental Sciences and Chemistry at the University of Wollongong

Methylmercury is a potent neurotoxin, with serious implications for human health.

Human exposure to methylmercury generally comes from eating predatory fish like tuna, which in turn are exposed through their own diets.

Before it gets into fish, mercury typically first enters the environment through emission to the atmosphere from both natural processes and human activities, like burning coal.

From there, most mercury is deposited to the ocean where it can be converted to the methylmercury form and enter marine food webs.

This new study used detailed ecosystem and food web modelling for the northwest Atlantic to find that methylmercury in some fish species has increased over recent decades because of overfishing and ocean warming.

This increase is somewhat surprising, given that the amount of mercury in seawater in the North Atlantic region has actually declined during this period due in large part to substantial decreases in emissions of mercury to the atmosphere in North America.

To the best of my knowledge, this sort of detailed ecosystem modelling has not been done for waters around Australia.

However, one thing we do know is that unlike in North America (and Europe), in recent years atmospheric mercury emissions have been increasing rather than declining in Australia – and indeed across the southern hemisphere.

This would suggest that if ocean warming is similarly affecting ecosystems here, these changes may be further exacerbated by increasing seawater mercury concentrations.

Last updated: 08 Aug 2019 11:40am
Declared conflicts of interest:
None declared.

News for:


Media contact details for this story are only visible to registered journalists.