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.