2024-03-01 07:30:26
Combating mercury contamination presents persistent challenges, according to a new study revealing that mercury levels in tuna remain unchanged since 1971, despite efforts to reduce emissions of this toxic contaminant.
Mercury is a widespread contaminant that can naturally convert to methylmercury in marine ecosystems. It is associated with neurocognitive deficits in fetuses and children, as well as cardiovascular effects in adults. Human exposure to methylmercury occurs mainly through the consumption of seafood. In the dock: tuna, the most consumed marine fish in the world. They can accumulate high levels of methylmercury by feeding on contaminated prey, such as smaller fish or shellfish. The global impacts of exposure to methylmercury are estimated at $117 billion per year.
Despite efforts to reduce mercury emissions into the environment, particularly through Convention de Minamata signed in 2013 and entered into force in 2017, French researchers, in collaboration with an international team, report in Environmental Science & Technology Letters that the levels of mercury in tuna seem unchanged since 1971. They are established over the period at around one microgram per gram of muscle. Researchers therefore argue that it is necessary to set more drastic mercury emission reduction targets in order to reduce these levels in tuna.
Tuna contaminated with mercury
The researchers compiled previously published data and their own data on mercury levels in tuna muscle in nearly 3,000 samples caught in the Pacific, Atlantic and Indian Oceans between 1971 and 2022. “As they do not undergo transoceanic migrations, any contamination found in the animals’ muscles likely reflects the waters in which they swim and feed”underlines the study.
The researchers targeted tropical tunas, namely yellowfin tuna, bigeye tuna and skipjack tuna. These tuna are top predators, widely distributed and heavily exploited, constituting 94% of the global tuna catch.
After standardizing mercury concentrations in tuna according to their size, the study found generally stable mercury concentrations globally, with the exception of the northwest Pacific. There, mercury concentrations in tuna increased significantly in the late 1990s. Near Asia, mercury concentrations in skipjack tuna reached concentrations up to four times higher than those observed elsewhere in the world ocean. This suggests increased contamination along Asian coasts, close to mercury emission hotspots.
With the exception of the Pacific Northwest, the study shows stable long-term mercury concentrations in tuna, but annual variability. The latter probably reflects natural biogeochemical and/or ecological processes.
An “inherited” mercury rising from the depths
While tuna contamination remains stable in most parts of the world, atmospheric mercury has declined worldwide over the same period. The researchers therefore hypothesize that this stability in the mercury level in tuna could be caused by “legacy” mercury, coming up from deeper waters (50 – 1,500 meters). This legacy mercury would have been emitted decades ago, and therefore does not yet reflect the effects of reduced air emissions. It continues to feed ocean food webs at depth and on the surface.
Thus, the results suggest that the reduction of contamination in tuna may take decades following emission reduction measures. “Due to the upward mixing of this legacy mercury, the surface ocean reservoir (0 to 50 meters) takes years or even decades to respond to changes in primary emissions”suggest the researchers.
And for tomorrow?
The researchers also wanted to simulate the impact of three increasingly restrictive environmental policies on ocean mercury and tuna levels in the future. Their mathematical models suggest that even the most restrictive emissions policy would take 10 to 25 years to influence ocean mercury concentrations. Then declines in tuna would follow decades later.
Their results indicate the need for a global effort to more aggressively reduce mercury emissions and a commitment to continued, long-term monitoring of mercury in ocean life to assess the effectiveness of policy decisions and reduction efforts compared to human health.
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