Climate change could cause an increase in toxic mercury in seafood such as cod and tuna as warmer waters force them to eat more to keep going, scientists have warned.

Around four-fifths of the mercury put into the atmosphere from natural and human causes, such as burning coal, ends up in the ocean where some is converted by tiny organisms to a particularly dangerous form known as methylmercury.

This methylmercury, which can affect brain function, works its way up the food chain and accumulates in top predators such as cod and tuna in high concentrations.

Cod

As the seas warm, these fish are using more energy to swim which requires more calories – so they are eating more and storing up more of the toxin.

This means that while regulation to curb emissions of mercury are leading to decreases in the concentrations of the toxin in fish, rising ocean temperatures due to climate change are predicted to push it up again, the researchers from Harvard warn.

Changes in the diet of species including cod and spiny dogfish as a result of overfishing of their food sources such as herring can also affect how much methylmercury they are consuming and storing in their bodies.

The researchers from Harvard John A Paulson School of Engineering and Applied Sciences and the Harvard T H Chan School of Public Health modelled the impacts of reductions in mercury emissions.

They also looked at the impacts of overfishing which changes what top predators eat, such as reducing how many large herring cod are eating.

Warming

Their study, based on three decades of data from fish and seawater from the Gulf of Maine and published in the journal Nature, also looked at what temperature increases would do.

Concentrations of the toxin in cod increased by up to 23% between the 1970s and 2000s as a result of dietary shifts initiated by overfishing and then a recovery of herring populations, they suggested.

The researchers’ computer modelling predicts an increase of 1C in seawater temperatures compared to how warm it was in 2000 would lead to a 32 percent increase in methylmercury levels in cod and a 70% increase in spiny dogfish.

Even with a 20 percent decrease in methylmercury in sea water as a consequence of reductions in emissions, a 1C temperature rise would lead to increases of 10% in cod and 20% in spiny dogfish, the researchers said.

They also analysed the effects of recent ocean warming from a low in 1969 on concentrations of the mercury in Atlantic bluefin tuna and found it could contribute to an estimated 56 percent increase in levels in the species.

Exposure

Elsie Sunderland, a senior author of the paper, said: “We have shown that the benefits of reducing mercury emissions holds, irrespective of what else is happening in the ecosystem.

“But if we want to continue the trend of reducing methylmercury exposure in the future, we need a two-pronged approach.

“Climate change is going to exacerbate human exposure to methylmercury through seafood, so to protect ecosystems and human health we need to regulate both mercury emissions and greenhouse gases.”

This Author

Emily Beament is the PA environment correspondent.