This just in from the American Museum of Natural History:
While
most of us would never willingly consume a highly endangered species,
doing so might be as easy as plucking sushi from a bento box. New
genetic detective work from the Sackler Institute for Comparative
Genomics at the American Museum of Natural History shows that bluefin
tuna is routinely plated in sushi bars sampled in New York and
Colorado. A quarter of what was labeled as tuna on sushi menus was
bluefin, and some was even escolar, a
waxy, buttery fish often labeled “white tuna” that is banned for sale
in Japan and Italy because it can cause gastrointestinal distress. The
new research is published in PLoS ONE.
“When
you eat sushi, you can unknowingly get a critically endangered species
on your plate,” says Jacob Lowenstein, a graduate student affiliated
with the Museum and Columbia University. “But with an increasingly
popular technique, DNA barcoding, it is a simple process for
researchers to see just what species are eaten at a sushi bar.”
DNA
barcoding efficiently identifies the species from which a chunk of
meat—or even a leather handbag—came from. Using a short sequence of
mitochondrial DNA from the cytochrome c oxidase subunit 1 gene, or “cox1,” to identify a species, this tool has,
among other results, identified the presence of endangered whales in
Asian markets, looked at the species of ungulates appearing in African
bushmeat markets, and documented fraud in the labeling of caviar and
red snapper.
Lowenstein
and colleagues used DNA barcoding to identify the kind of fishes
labeled “tuna” in one Denver and 30 New York City restaurants. Almost
half of the restaurants did not accurately label the kind of tuna sold,
and only 14 of the samples used for this study were listed on the menu
by a specific name like bigeye tuna, albacore, or bluefin.
(Image: mconnors)
Bluefin are three species of large, fast-moving,
high-energy tuna that can cover enormous distances in the ocean. All
three—Northern, Southern, and Pacific bluefin tuna—are highly sought by
fishermen because a single fish can garner tens of thousands of
dollars. But over-fishing has brought populations into sharp decline,
and western stocks of Northern bluefin tuna are estimated as 10% of
pre-exploitation numbers, and eastern stocks are following suit because
of rampant illegal fishing. Just a month ago, on October 16, the
country of Monaco nominated Northern bluefin tuna for Appendix I listing (a complete international trade ban) to be considered when the Convention on International Trade in Endangered Species (CITES) meets in March.
All eight species within the Thunnus genus
diverged from their common ancestor recently and are even more closely
related to each other than humans are to chimpanzees and bonobos.
Because of this, previous attempts to identify tunas with DNA barcodes
have not worked well, and Lowenstein and colleagues chose to use this
group of animals as a case study to compare DNA barcoding techniques.
In contrast with the more widely used genetic distance method, the team
was able to reliably identify closely-related species with a genetic
key based on 14 nucleotide positions within the cox-1 gene.
“Because
we compared different methods of analysis, we could show that indirect
estimates like genetic distances do not distinguish among tuna species.
But our genetic key and direct sequence matching does work,” says
Sergios-Orestis Kolokotronis, the coordinator of the DNA Barcoding
Initiative for Conservation at the Museum. “The standard method for
identifying species relies on morphological characters, so using
genetic characters allows us to enhance taxonomic identification.”
The
results of the detective work are clear: while the most prevalent tuna
found in sushi is bigeye (30, or almost half, of the 68 samples
collected for this research), nearly a third of the tuna was bluefin (3
Pacific, 12 Northern, and 7 Southern bluefin tuna). Only eight of the
22 bluefin samples were labeled “bluefin” on menus, and nine of the
restaurants that sold the bluefin did not label it as such on the menu,
although restaurants that did advertise bluefin were accurate in their
representation and charged more for the sushi. Five of the nine samples
labeled in restaurants as “white tuna” were not albacore but rather
escolar.
“It
is very difficult to get reliable information about the species you are
eating, especially since the FDA’s approved market name for all eight
species of Thunnus is simply
‘tuna’,” says Lowenstein. New requirements that would market each
species under its own name would help to clarify cases of economic
fraud and allow conservation-minded consumers to avoid bluefin.
“The
ability to identify species in trade is a prerequisite for CITES
listing, and this research confirms that this requirement can be met,”
says George Amato, Director of the Sackler Institute for
Comparative Genomics. “The long-term application of barcoding is the
development of a hand-held identification device that anyone can use,
so that wildlife management teams can seamlessly and efficiently
identify a species and maybe the geographic origin of specific
samples.”
Lowenstein,
Kolokotronis, and Amato authored this paper. Funding from the Alfred P.
Sloan Foundation and the Richard Lounsbery Foundation is greatly
appreciated. The research article: http://dx.plos.org/10.1371/journal.pone.0007866.
