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Bluefin tuna reveal patterns of ocean mercury pollution

The long-lived, migratory fish are great indicators for the risk that mercury poses to ocean and human life.

(CN) — New research shows that by studying bluefin tuna, scientists can detect patterns of mercury pollution across the world's oceans.

Bluefin tuna are among the largest and speediest fish species in the world and can be found in oceanwaters around the globe. Within their muscles are high levels of neurotoxic methylmercury, a form of mercury that increases in concentration as the fish ages.

The increase in mercury levels within the fish is due to their environment, including the water they live in and available food. This can result in neurotoxic concentrations of mercury that make them unsafe for human consumption.

Given their availability throughout the world, researchers set out to study bluefin tuna to establish a standard baseline for comparison of mercury pollution in oceans across the world.

The researchers’ study – published Monday afternoon in the journal Proceedings of the National Academy of Sciences, or PNAS – details findings that drew from a far-reaching examination of previous studies that date back to 1998.

“Our study shows that mercury accumulation rates in bluefin tuna may be used as a global pollution index that can reveal patterns of mercury pollution and bioavailability in the oceans, natural and human caused emissions and regional environmental features,” said John Reinfelder, one of the study’s senior authors and a professor in the Department of Environmental Sciences at Rutgers University-New Brunswick.

The study found that mercury rates in bluefin tuna were the highest in the Mediterranean Sea and were lower in the north Pacific, Indian and north Atlantic oceans. The Mediterranean Sea is of particular interest because it is the most important bluefin fishery in the world.

Mercury rates in the water are subject to several factors, both manmade and natural. The Mediterranean Sea and north Pacific and Indian oceans are affected by natural occurrences such as mercury leaching from rocks. However, these bodies of water are also affected by humans, who contaminate the water with processes like metal mining, smelting and the burning of fossil fuels.

Taking natural and manmade contamination together, the mercury accumulation rates in the bluefin tuna can be used as a global pollution index that encompasses all these factors, including the circulation of deep-ocean currents which are affected by things such as temperature.

This type of research presents some unique challenges. While different types of tuna have previously been suggested as good biological indicators of the changes in mercury pollution of the ocean, direct comparisons are difficult.

The mercury levels within an individual fish can vary according to several factors, including age, size, place in the food chain and type and quantity of prey. Add in differences between types of fish and the comparison of mercury levels can be even more challenging.

To combat these problems, the researchers reviewed studies and mercury analysis in tuna tissue samples from 1998 through 2019. The research encompassed studies of four ocean sub-basins and included the three different types of bluefin tuna.

The study found that the mercury accumulation rates of tuna increased in response to the concentrations of mercury in regional seawater and zooplankton. This connects the mercury levels to how available it is at the base of each ocean sub-basin’s food web, a term that means the totality of all food chains in a specific ecosystem.

“Overall, mercury accumulation rates provide a means to compare mercury bioavailability among geographically distinct populations of upper trophic level marine fish across ocean sub-basins, to investigate trophic dynamics of mercury in marine food webs and to improve public health risk assessments of mercury exposure from seafood,” Reinfelder said.

Rutgers University-New Brunswick was not alone in conducting the research. It was assisted by researchers at the National Taiwan University and the University of California, Santa Cruz.

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Categories / Environment, Science

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