Seafood dishes including cod and tuna could be wiped off the menu due to global warming.
Researchers say an increase in ocean temperature is leading to a rise in levels of harmful mercury in fish.
While regulations of mercury emissions has reduced the amount found in fish, spiking temperatures are driving levels back up.
Scientists have developed a model simulating how different environmental factors impact levels of the toxin, known as methylmercury in fish.
They are calling for a ‘two pronged approach’ of lowering greenhouse gases and mercury emissions to protect both ecosystems and human health.
Dr Amina Schartup, said: ‘Being able to predict the future of mercury levels in fish is the holy grail of mercury research.
‘That question has been so difficult to answer because, until now, we didn’t have a good understanding of why methylmercury levels were so high in big fish.’
It’s long been understood creatures at the top of the food chain have higher levels as they eat species with mercury inside them.
The researchers at Harvard T. H. Chan School of Public Health analysed 30 years of ecosystem data from the Gulf of Maine, off the east coast of the US.
They analysed stomach contents two marine predators – Atlantic cod and spiny dogfish, from the 1970s to 2000s.
The findings, published in the journal Nature, showed methylmercury levels in cod were six to 20 per cent lower in 1970 than they were in 2000.
The spiny dogfish levels were 33 to 61 per cent higher despite being on a similar part of the same food web.
Both species turned to different substitutes to survive when the Gulf of Maine experienced a dramatic loss in herring due to over-fishing in the 1970s.
The cod started eating other small fish like shads and sardines, which are low in methylmercury, while spiny dogfish went food with higher levels such as squid.
When the herring population bounced back in 2002, cod reverted back to their high mercury diet while spiny dogfish cut back.
As Dr Schartup struggled to figure out why mercury levels were so high in tuna, the penny dropped as she watched US swimmer Michael Phelps compete in the Olympics.
TV commentators talked about how Helps consumed 12,000 calories a day during the competition.
Dr Schartup said: ‘I thought, that’s six times more calories than I consume. If we were fish, he would be exposed to six times more methylmercury than me.’
It turned out that high-speed hunters and migratory fish use much more energy and consume more calories than scavengers and other varieties of fish.
Because the ‘Michael Phelps-style fish’ swim so much, they do not have the compensatory growth which dilutes their overall mercury levels.
As waters get warmer, fish use more energy to swim and require more calories, Dr Schartup added.
Between 2012 and 2017, methylmercury levels in Atlantic bluefin tuna increased by 3.5 per cent per year – despite decreasing emissions of mercury.
Based on their model, the researchers predict a one degree Celsius increase in seawater relative to the year 2000 would trigger a 32 per cent increase in methylmercury levels in cod.
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