Are the fish we eat toxic?
MARTIN LASALLE
The amount of mercury extracted from the sea by industrial fishing has grown steadily since the 1950s, potentially increasing mercury exposure among the populations of several coastal and island nations to levels that are unsafe for foetal development.
These are the findings
of a study carried out by researchers from Université de Montréal's Department
of Biological Sciences and published this
week in Scientific Reports.
The study combined
data on the amount of mercury fished out of oceans and seas from 1950 to 2014
and the weekly consumption of fish and seafood by the populations of 175
countries between 1961 and 2011.
By comparing this
data, which was published by the Food and Agriculture Organization of the
United Nations (FAO), postdoctoral fellow Raphaël Lavoie was able to estimate
these populations’ per capita intake of methylmercury (MeHg), a highly toxic
form of mercury.
Working under the direction of Professor Marc Amyot, Lavoie estimated that the people of 38 per cent (66 of 175) of the countries examined by the study might be exposed to methylmercury levels higher than the maximum deemed safe for foetal development. The highest-risk countries include the Maldives, Iceland, Malaysia, Lithuania, Japan, Barbados and South Korea.
When humans ingest
excessively high levels of methylmercury, the toxin’s molecules can penetrate
the blood-brain barrier and impact cerebral development, especially in children
and foetuses.
Demand for seafood has
skyrocketed
Industrialization has
released vast quantities of mercury into the atmosphere, which have settled in
oceans and waterways. This mercury is absorbed by sea creatures, many of which
are consumed by humans.
Since 1950, demand for
seafood has skyrocketed while technological breakthroughs have enabled more
intensive forms of industrial fishing. Since the 1990s, when overfishing
drastically reduced stocks, industrial fishing has gradually migrated to
deep-sea and international waters.
“The global marine
catch totals 80 million tonnes of fish per year, which means that we are also
pulling out increasingly large amounts of mercury,” said Amyot.
Of the industrial
fishing areas listed by the FAO, the Northwest
Pacific currently exports the most fish – and the most methylmercury. The
Western Central Pacific holds second place, while the Indian Ocean ranks third.
“Together, these three
fishing areas exported 60 per cent of the mercury resulting from global seafood
production in 2014,” said Lavoie.
The people in these
regions are some of the world’s top seafood consumers. Species high up on the
food chain contain the highest concentrations of mercury. From 1950 to 2014,
large fish represented approximately 60 per cent of the global catch (by
weight) and nearly 90 per cent of the mercury ingested by consumers from fish.
High risk of exposure
To be safe for foetal
development, the threshold for methylmercury consumption is 1.6 micrograms for
each kilogram of a person's body weight per week (1.6 μg/kg/week).
“By comparing FAO data
on global seafood consumption, we observed that from 2001 to 2011 the
populations of 38 per cent of the 175 countries we analyzed would have been
exposed to weekly doses of methylmercury far above the maximum safe level of
consumption for foetal development,” said Lavoie. “Many of these populations
are in coastal and island nations, especially developing countries.”
For instance, during
that 10-year period, people in the Maldives would have consumed an average of
23 micrograms of methylmercury per kilogram of body weight each week, or
more than 14 times what's deemed safe. The next highest-ranking were people in
Kiribati (8 μg/kg/week), Iceland (7.5 μg/kg/week), Malaysia and Samoa
(6.4 μg/kg/week), French Polynesia (5 μg/kg/week), Lithuania, Japan
and Barbados (4.8 μg/kg/week) and South Korea (4.7 μg/kg/week).
By contrast, the
global average for mercury exposure over the same 2001-2011 period was
estimated at 1.7 μg/kg/week. In Canada, exposure totalled
1 μg/kg/week.
Lavoie and Amyot said
their estimates are conservative. The global catch by the fishing industry,
including artisanal and illegal fishing, is probably 50 per cent higher than
the FAO data indicates, they said.
Ways to reduce the
risk?
Both researchers
believe these estimates could help authorities find ways to reduce the risk of
mercury exposure, especially among high-risk populations such as children and
pregnant women.
Some methods of
preparing and consuming fish seem to reduce the risk of methylmercury
contamination, they pointed out. In a recent study, they found that cooking fish
or consuming it in combination with certain polyphenols contained in foodstuffs
like tea could reduce the bioavailability of methylmercury in the human body.
Which is good news,
because contrary to prevailing opinion, the methylmercury we consume may not be
fully absorbed
About this study
“Mercury transport and
human exposure from global marine fisheries,” by Raphael A. Lavoie, Ariane
Bouffard, Roxane Maranger and Marc Amyot, was published April
30 in Scientific Reports. doi:10.1038/s41598-018-24938-3