New Delhi: Warming of seas due to climate change coupled with overfishing could lead to an increase in methylmercury, a poisonous form of mercury, in certain fishes consumed by humans, found a new research.
More than three billion people rely on seafood for nutrition and the findings have raised serious concern over impact on human health, especially those in the coastal areas.
A potent neurotoxic substance, methylmercury (MeHg), is associated with long-term neuro-cognitive deficit in children that persist into adulthood, with global costs to society in excess of $20 billion.
The study carried out by researchers from Harvard University, Indian Institute of Technology (IIT), Hyderabad, was published in the latest issue of journal Nature.
The study showed that despite a decrease in global mercury emissions since late 1990s, MeHg concentration in fishes like Atlantic cod, Atlantic bluefin tuna, which are widely consumed by humans, has actually increased. In Atlantic cod, they found an increase of about 23% of MeHg concentration between the 1970s and 2000s.
According to researchers, this increase was caused by shifts in diets due to over-fishing. The exploitation of fisheries in the Atlantic Ocean has led to large fluctuations in herring, lobster and cod stocks, which has altered the structure of food webs and the availability of prey for remaining species.
In 2000s, cod had a greater reliance on prey such as larger herring and lobster, which have higher concentrations of MeHg, than other prey fish that it consumed in the 1970s, when herring used to be overfished.
The team developed a model to study the impact of increasing temperatures and overfishing on MeHg accumulation in fishes and used more than 30 years of data on ecosystem, sediment and sea-water MeHg concentrations.
The model also forecast an estimated 56% increase in the mercury concentrations in Atlantic bluefin tuna due to increases in sea-water temperature between a low point in 1969 and recent peak levels.
"There are three factors that affect mercury accumulation in fish– overfishing, which leads to dietary changes among marine animals, sea-temperature changes, which leads to changes in fish metabolism that gears towards survival rather than growth, and mercury pollution in seas," said Asif Qureshi, Associate Professor from IIT Hyderabad, who wrote the first versions of the model code.
The team chose Gulf of Maine, a commercially exploited, marginal sea in the north-western Atlantic Ocean for the study , however, they highlighted that mercury levels in fish in other seas and oceans are likely to have a similar relationship with sea temperature, fishing practices and mercury pollution levels. This region has also seen unprecedented sea-water warming in last few decades.
There have been global efforts to reduce the amount of mercury entering the ocean to reduce the mercury accumulation in fishes and other marine animals. The Minamata Convention of Mercury enforced in 2017 includes a ban on new mercury mines, phase-out of existing ones and strict control measures on emissions.
“Regulatory efforts must not only control the release of mercury into the atmosphere, but also reduce greenhouse gas emissions that lead to sea water warming. Only by tackling both, can we reduce levels of mercury in marine animals and our exposure to mercury in seafood," said Qureshi.