EXCLUSIVE: Warming oceans raise mercury concerns

The world’s oceans are experiencing record warmth, raising fresh questions about how changing marine conditions could affect the seafood people eat, including the amount of mercury found in commercially caught fish such as tuna.

Scientists are already studying how warmer waters are altering marine ecosystems, from fish behaviour and feeding patterns to the areas where species are found. Those changes could also influence how mercury moves through ocean food webs and accumulates in fish.

Kevin McCay, chief operating officer of Safe Catch, told World Fishing & Aquaculture that the relationship between ocean temperature and mercury is complex and cannot simply be reduced to warmer water producing higher mercury levels.

“Warming doesn’t create mercury, but it can change the conditions that influence how mercury moves through the marine food web and accumulates in fish,” McCay said.

One factor is fish metabolism. As ocean temperatures rise, oxygen levels can fall, potentially increasing the amount of energy fish need and changing their feeding behaviour. McCay pointed to a 2019 study of Atlantic bluefin tuna that reported mercury concentrations increasing by about 3.5% per year between 2012 and 2017, despite declining mercury emissions.

However, he stressed that the findings should not be applied universally. A 2024 analysis of almost 3,000 tropical tuna samples collected between 1971 and 2022 found mercury concentrations were broadly stable, although regional variations were recorded.

The potential impact of El Niño is similarly complicated. A stronger event could alter ocean circulation, upwelling, thermocline depth and the distribution of prey — all factors that can influence where tuna feed and where fleets fish.

McCay highlighted research published in 2022 that found roughly a fivefold spatial difference in mercury concentrations in Pacific skipjack tuna. The study linked much of the variation to the location of methylmercury-rich water and oxygen-depleted layers.

“If a fleet begins sourcing from a different part of the Pacific, the mercury profile of the catch could change because the fish are coming from a different environment, even without a uniform increase in mercury across the ocean,” he said.

Those shifts could have consequences beyond food safety. Changes in fish distribution can affect where vessels operate, how far they travel and the cost of bringing seafood to market. McCay said he would expect ‘more volatility than outright shortage’ as ocean conditions become increasingly dynamic.

For consumers, he argues that the answer is not necessarily to eat less seafood. Instead, better measurement and monitoring may become increasingly important.

“The better answer is to understand that mercury isn’t uniform, even within the same species, and to look at how the seafood they’re buying is actually tested,” McCay said.

The US Food and Drug Administration’s action level for mercury in commercial fish is 1.0 parts per million. Safe Catch sets lower limits for its products and says it tests individual tuna for mercury before accepting them.

“If you want to know how much mercury is in a fish, test the fish,” McCay said.

As oceans continue to warm and marine ecosystems respond, that approach could become increasingly relevant. Researchers and the seafood industry will need longer-term monitoring that connects mercury levels with fish species, size, location, feeding behaviour and changing ocean conditions.

For consumers, the key question may ultimately be less about whether warmer oceans automatically mean more mercury — and more about how accurately the seafood supply can keep pace with a rapidly changing marine environment.