Climate change, overfishing could expose more people to toxic mercury through seafood
Climate change will worsen human exposure to highly toxic mercury through seafood, scientists have warned. Warming oceans are leading to an increase in the harmful neurotoxin methylmercury in popular seafood, which includes Atlantic bluefin tuna, cod, and swordfish, according to research led by the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Harvard T. H. Chan School of Public Health (HSPH).
The researchers developed a comprehensive model that simulates how environmental factors, including increasing sea temperatures and overfishing, impact levels of methylmercury in fish. They also collected and analyzed 30 years of ecosystem data from the Gulf of Maine, including an extensive analysis of the stomach contents of two marine predators, Atlantic cod and spiny dogfish from the 1970s to 2000s.
The Gulf of Maine is one of the fastest-warming bodies of water in the world. The researchers found that between 2012 and 2017, methylmercury levels in Atlantic bluefin tuna increased by 3.5% per year despite decreasing emissions of mercury. The reason: as waters get warmer, fish use more energy to swim, which requires more calories, and they end up consuming more toxic methylmercury.
Analysis of data and ecosystem modeling further shows that an increase of 1 degree Celsius in seawater temperature as compared to the year 2000 leads to a 32% increase in methylmercury levels in cod and a 70% increase in spiny dogfish.
“This research is a major advance in understanding how and why ocean predators, such as tuna and swordfish, are accumulating mercury. Climate change is going to aggravate human exposure to methylmercury through seafood. So, to protect ecosystems and human health, we need to regulate both mercury emissions and greenhouse gases. It is important also to remember that fish are very healthy food overall, and when people switch away from fish in their diet, they generally pick less healthy alternatives. We can all agree less methylmercury in these fish in the future would be a good thing,” say the researchers in their findings, published in Nature.
The researchers explain that while the regulation of mercury emissions have reduced methylmercury levels in fish, spiking temperatures are driving those levels back up and will play a significant role in the methylmercury levels of marine life in the future. For example, the computer modeling predicted that methylmercury concentrations in Atlantic bluefin tuna (Thunnus thynnus) is estimated to have increased by 56% (between a low point in 1969 and recent peak levels) due to increases in seawater temperature.
The researchers simulated different scenarios simultaneously.
According to one of the scenarios, even with a 20% decrease in mercury emissions, a 1-degree increase in seawater temperature can result in increases in methylmercury levels of 10% in cod and 20% in spiny dogfish.
However, a 20% decrease in emissions, with no change in seawater temperatures, decreases methylmercury levels in both cod and spiny dogfish by 20%.
The research team says that human exposure to methylmercury has been associated with long-term neurocognitive deficits in children that continues into adulthood, with global costs to society that exceed $20 billion.
“More than three billion people rely on seafood for nutrition. However, fish are the predominant source of human exposure to methylmercury, a potent neurotoxin substance. In the US, 82% of population-wide exposure to methylmercury is from the consumption of marine seafood and almost 40% is from fresh and canned tuna alone,” says the study.
According to the World Health Organization (WHO), mercury can have toxic effects on the nervous, digestive and immune systems, and lungs, kidneys, skin, and eyes.
Methylmercury is a form of mercury and is toxic to the central and peripheral nervous systems. Children are at particular risk.
“Methylmercury exposure in the womb can result from a mother’s consumption of fish and shellfish. It can adversely affect a baby’s growing brain and nervous system. The primary health effect of methylmercury is impaired neurological development. Therefore, cognitive thinking, memory, attention, language, and fine motor and visual-spatial skills may be affected in children who were exposed to methylmercury as fetuses,” says the WHO.
Human activities (such as the burning of fossil fuels, especially coal) and natural activities such as volcanic eruptions release mercury into the atmosphere, which then enters the ocean, where it gets converted into the dangerous methylmercury, and it is passed up the food chain.
“Around 80% of the inorganic mercury that is emitted to the atmosphere from natural and human sources is deposited in the ocean, where some are converted by microorganisms to methylmercury. In predatory fish, environmental methylmercury concentrations are amplified by a million times or more,” says the study.
However, despite the severe health impacts of methylmercury, the research team says that the effects of ongoing changes in marine ecosystems on bioaccumulation of methylmercury in fish that are frequently consumed by humans (for example, tuna, cod, and swordfish) have not been considered when setting global policy targets.
The findings also show that methylmercury concentrations in Atlantic cod (Gadus morhua) have increased by up to 23% between the 1970s and 2000s as a result of dietary shifts initiated by overfishing.
This is because due to overfishing of sardines (small herring), which are low in methylmercury, the cod was forced to substitute herring with food higher in methylmercury.
“Ocean warming and fisheries management programs will be major drivers of future methylmercury concentrations in marine predators,” says the study.