Mercury in Amazonian fish: chemical speciation, environmental drivers and implications for food security and human health: a 36-year scientometric review

In the vast, winding waterways of the Amazon, fish are more than just a resource; they are the foundation of life for millions of people. For riverine and Indigenous communities, the river provides the primary source of protein, a daily sustenance that has supported families for generations. However, this vital food source carries a hidden danger that has long concerned scientists and health officials: mercury. This heavy metal, which can enter water systems through natural processes like the weathering of rocks or through human activities such as gold mining and deforestation, does not simply wash away. Instead, it undergoes a transformation in the water, changing into a form that is easily absorbed by living things. Once inside a fish, this toxic substance builds up in the muscle tissue. As smaller fish are eaten by larger ones, the concentration of the toxin increases, a process known as biomagnification. For the people who rely on these fish for their survival, this creates a difficult paradox: the very act of eating to stay healthy could expose them to a poison that harms the brain and nervous system, particularly in young children and pregnant women.

To understand the scale of this problem across the entire region, a team of researchers from Brazil's National Institute of Amazonian Research and the Brazilian Agricultural Research Corporation conducted a massive review of scientific knowledge. They gathered and analyzed nearly one hundred studies published over thirty-six years, from 1988 to 2024. Their goal was to map out where mercury contamination is most severe, which types of fish are most affected, and what this means for the safety of the food supply. By looking at the data as a whole, they could see patterns that individual studies might miss, painting a clear picture of how the metal moves through the ecosystem and where the risks are highest for human consumption.

The researchers found that scientific attention to this issue has grown significantly in recent years, with a notable surge in publications over the last decade. However, the data is heavily skewed toward Brazil, which accounted for eighty percent of all the studies reviewed. While Brazil has provided a wealth of information, the researchers noted that other Amazonian nations, such as Peru, Colombia, and Bolivia, are significantly underrepresented in the literature. This gap means that the full extent of the contamination in those countries is likely underestimated, as there simply are not enough studies to tell the whole story. The review also highlighted a critical limitation in how scientists have been measuring the problem. For most of the past three decades, researchers have focused almost exclusively on measuring total mercury, which counts every form of the metal in a fish's body. While this provides a general idea of contamination, it misses a crucial detail: the specific form of mercury that is most dangerous to humans is methylmercury. This organic form is the one that the body absorbs most easily and holds onto the longest. The review showed that very few studies have measured this specific form, leaving a gap in our understanding of exactly how much of the total mercury is actually the toxic kind that poses the greatest health risk.

When the team looked at the fish themselves, they identified 436 different species that had been tested. As expected, the fish that eat other fish, known as piscivores, and those that are carnivorous generally carried the highest levels of mercury. This confirms the pattern of biomagnification, where the toxin concentrates as it moves up the food chain. Species like the large catfish and the peacock bass, which are popular targets for both sport and food fishing, were frequently found with high concentrations. However, the findings were not limited to just the top predators. The researchers discovered that even fish that eat plants, detritus, or small invertebrates sometimes contained dangerous levels of mercury. In some cases, these non-predatory species exceeded safety limits, suggesting that the contamination is widespread and not confined to the top of the food web. The data revealed that nearly half of all the species analyzed had mercury levels above the safety threshold set by the World Health Organization, which is 0.5 milligrams per kilogram. More than one hundred and forty species exceeded an even stricter limit of 1.0 milligrams per kilogram.

The geographic distribution of these high levels varied across the region. In Brazil, the highest concentrations were found in several states, including Amazonas, Pará, and Rondônia, areas often associated with gold mining and deforestation. Outside of Brazil, the situation was also serious. In French Guiana, some fish species reached concentrations as high as 3.0 milligrams per kilogram, while in Colombia, levels reached 1.5 milligrams per kilogram. These numbers are alarming because they represent the actual meat that people are eating. The researchers emphasized that for many communities in the Amazon, fish consumption is incredibly high, often reaching up to 200 kilograms per person per year. This is ten times the global average. When you combine such high consumption rates with the frequent presence of mercury in the fish, the risk to human health becomes severe. The study pointed out that vulnerable groups, such as pregnant women and children, are at the greatest risk of suffering from neurotoxic effects, which can impact brain development and overall health.

The review also explored the forces driving this contamination. While natural sources of mercury exist in the Amazon, the study reinforced that human activities are the primary drivers of the current crisis. Artisanal gold mining, which uses mercury to extract gold from soil and sediment, releases vast amounts of the metal into the environment. When this mercury enters the water, it settles in the mud and is converted by bacteria into methylmercury. Deforestation and the burning of forests also play a major role. When trees are cut down and the land is cleared, the mercury that was naturally stored in the soil is released and washed into the rivers. Even the construction of large hydroelectric dams can worsen the problem by flooding vast areas of forest, creating conditions that encourage the conversion of mercury into its toxic form. The study made it clear that this is not just a local issue but a complex web of environmental changes that affect the entire basin.

Ultimately, the paper concludes that the current approach to monitoring and managing this risk is insufficient. The heavy reliance on measuring total mercury instead of the more toxic methylmercury means that risk assessments may be inaccurate. Furthermore, the lack of data from many parts of the Amazon leaves large blind spots in our knowledge. The researchers argue that protecting the health of Amazonian populations requires a new strategy that integrates ecology, toxicology, and public health. This means not only reducing the sources of mercury pollution, such as mining and deforestation, but also improving how we monitor the fish and understanding the specific diets of local communities. Without these integrated efforts, the people who depend on the river for their survival will remain exposed to a silent threat that undermines their food security and their health. The path forward requires international cooperation, better data, and policies that recognize the unique relationship between the people of the Amazon and the river that sustains them.