New research showing that that mercury levels are higher in some
species of tuna could help consumers minimize their consumption of
the silvery metal in their sushi and provide a powerful new tool
for regulatory organizations. The new research - combining DNA
barcoding at that American Museum of Natural History with analysis
of mercury content at Rutgers University - is published in
Biology Letters early online edition and shows surprisingly
that tuna sushi purchased in supermarkets might be healthier than
that from restaurants. The sushi made for supermarkets tends to be
yellowfin tuna.
"We found that mercury levels are linked to specific species,"
says Jacob Lowenstein, a graduate student affiliated with the
Museum. "So far, the U.S. does not require restaurants and
merchants to clarify what species they are selling or trading, but
species names and clearer labeling would allow consumers to
exercise greater control over the level of mercury they
imbibe."
"People who eat fish frequently have a particular need to know
which species may be high in contaminants," says Michael Gochfeld,
professor at Robert Wood Johnson Medical School. "Some agencies
have been afraid that any mention of contaminants will discourage
people from eating any fish.
Sushi samples for this research project were taken from 54
restaurants and 15 supermarkets in New York, New Jersey, and
Colorado. The results are based on 100 samples, all of which were
identified with DNA barcoding as either bigeye tuna (Thunnus
obesus), yellowfin tuna (Thunnus albacares), or bluefin
tuna species (Thunnus maccoyii, Thunnus orientalis,
and Thunnus thynnus). All samples were tested for relative
mercury content.
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Combining mercury content and species identification in this
study shows that all species exceed or approach levels permissible
by Canada, the E.U., Japan, the U.S., and the World Health
Organization. Mercury levels are significantly higher in bluefin
akami (sushi from lean, dark red tuna) and all bigeye tuna
samples than in bluefin toro (sushi from fatty tuna) and
yellowfin tuna akami samples. This is probably because
mercury accumulates differently in different tissue types: mercury
has an affinity for muscle and not fatty tissue, so the leanest
fish tend to have the highest concentration.
But there also seem to be other factors in play. Although
yellowfin tuna is very lean, this species tends to have lower
accumulation of mercury, likely because yellowfin are typically
smaller than other tuna and are harvested at a younger age.
Furthermore, yellowfin are tropical and do not thermoregulate like
the warm-blooded bigeye tuna and bluefin tuna. Because bigeye and
bluefin species eat three times more than yellowfin to maintain
their energy level, they might bioaccumulate, or slowly increase
the level of toxins over time.
"Although levels are highest in top-level predatory fish, some
fish that are lower on the food chain have high levels," says
Joanna Burger, professor at Rutgers University. "The levels of
mercury in some tuna are sufficiently high to provide a health risk
both to the fish themselves and to the predators that eat them,
including humans, particularly those who eat fish frequently."
"We show how you can use DNA as a tool to uncover patterns of
species-specific bioaccumulation," says Sergios-Orestis
Kolokotronis, a geneticist at the Sackler Institute for Comparative
Genomics at the Museum. "This is one of first applications of DNA
barcodes in a non-academic setting - using this method in any human
health context and not just for determining whether barcodes can
quickly and accurately identify a species."
SOURCE
