 |
 |
NEWS RELEASE
8/21/01
Mark Shwartz, News Service (650) 723-9296; e-mail: Bluefin tuna -- one of the most sought-after fish in the sea -- migrate
thousands of miles across the Atlantic, according to a new study by
Stanford biologist Barbara A. Block and a team of marine scientists.
Using electronic data recording tags placed on wild tuna, Block and
her fellow researchers discovered that some Western Atlantic bluefin
mingle with their counterparts from the Eastern Mediterranean -- a finding
that could have a profound impact on international efforts to conserve
the magnificent animal, whose numbers have sharply declined as a result
of intense commercial fishing in recent decades.
The study, published in the Aug. 17 issue of the journal Science,
emerged after five winters of electronic tagging by Block and colleagues
from the Tuna Research and Conservation Center (TRCC) --- a joint project
of Stanford's Hopkins Marine Station and the Monterey Bay Aquarium.
Block, lead author of the Science study, is the Charles and Elizabeth
Prothro Professor in Marine Science at Hopkins and director of TRCC.
Scientists from the National Marine Fisheries Service also participated
in the study.
The majestic bluefin -- one of the few warm-blooded species of fish
-- can grow to 10 feet long and weigh 1,500 pounds. The animal is highly
prized by sushi and sashimi lovers, particularly in Japan where a single
fish weighing 444 pounds recently sold at auction for $175,000. Atlantic
bluefin in the Tokyo seafood market routinely sell for $8 to $45 per
pound.
Commercial harvesting of bluefin and other Atlantic tuna is managed
through catch quotas established by the International Commission for
the Conservation of Atlantic Tunas (ICCAT) based in Madrid, Spain.
Electronic tags
In the Science study, Block and her fellow researchers analyzed data
recovered from electronic tags placed on bluefin off the East Coast
of North America starting in 1996. Of the original 377 electronic tags
deployed, 137 delivered data back to the researchers.
Most of the original tags (279) were "archival" --- electronic devices
that scientists surgically implanted inside the fish with the help of
commercial fishers off the coast of North Carolina. Forty-nine (49)
archival tags eventually were returned by fishers from throughout the
North Atlantic and the Mediterranean Sea.
The research team also deployed 98 "pop-up" satellite tags -- devices
externally fastened to the fish that detach and float to the surface
at a pre-programmed date. Data on depth, temperature and location were
stored on the pop-up tags during the mission and transmitted back to
the lab via satellite.
Tagging data revealed that Atlantic bluefin -- considered overexploited
in the Western Atlantic since 1982 -- often travel throughout the entire
North Atlantic and, in some cases, into the Mediterranean. Tuna tagged
in the Western Atlantic in most cases resided for a year or more on
Western North Atlantic feeding grounds. While many fish remained in
the west, some of the recaptured fish had migrated to the Eastern Atlantic
or Mediterranean Sea. Individuals also migrated from the Western Atlantic
to the east and back again in the same year.
Most fish showed at least one year of western residency traveling
between the Carolinas and New England and back again to the original
release location. The fidelity to these two locations and a third site
near the Flemish Cap off Newfoundland represents bluefin feeding aggregations.
Researchers also reported that the western-tagged bluefin travel to
distinct spawning grounds in the Gulf of Mexico or the Eastern Mediterranean.
Vulnerability
"The results indicate western-tagged bluefin are vulnerable to fishing
mortality from all Atlantic bluefin tuna fisheries," the authors write
in Science, noting that the tagging data emphasize "the need to protect
both major eastern and western spawning regions, as they directly influence
the western fishery."
This is an important consideration since the 2001 catch quota for
Western Atlantic fisheries (5.5 million pounds) is one-twelfth the quota
for the Eastern Atlantic fisheries (65 million pounds). Current management
of the fishery by ICCAT assumes that catches involve only a low level
of mixing between tunas from the Western and Eastern Atlantic.
ICCAT imposes strict quotas on fish caught in the Western Atlantic
and much more liberal quotas on bluefin landed in the Eastern Atlantic
and Mediterranean Sea. The Western Atlantic management is aimed at recovering
the breeding population to levels that produce "maximum sustainable
yield." The Eastern Atlantic breeding population is considered to be
in decline and overfished, although it is of greater overall abundance
than the western breeding population.
According to the Science study, "The electronic tagging data indicate
that mixing between the two management units exists at a higher level
than ICCAT has incorporated into . . . . stock assessments. However,
movements to distinct breeding grounds are apparent, suggesting a mixing
of stocks on feeding grounds and a separation to distinct breeding localities.
Future assessment of stock status should evaluate the new information
and reassess the management strategies applied to Atlantic bluefin tuna."
Block says that, "From the data, it's clear there are two breeding
populations of bluefin tuna that spend considerable time together on
the rich Western North Atlantic feeding grounds . . . . Our results
demonstrate that bluefin tuna are capable of ranging widely throughout
the North Atlantic without regard to the stock boundary in the mid-Atlantic.
That means efforts to bring about a recovery of bluefin tuna populations
will require increased cooperation among all nations fishing for bluefin
tuna."
Tagging data revealed that bluefin dive to remarkable depths, at times
exceeding the 3,281-foot pressure sensors in the tags, although they
spend most of their time closer to the surface. The animals also displayed
a remarkable range of temperature preferences -- from near-freezing
waters when feeding (37 F) to very warm temperatures when breeding (86
F) -- while always maintaining a high internal temperature (around 77
F to 80 F) even in the most frigid waters.
The researchers reported the first descriptions of what they believe
to be breeding behavior in giant tunas. Most surprising is that the
spawning period occupies a short interval in the yearly activities ---
less than one month. A unique behavioral repertoire recorded by the
electronic tags suggests that the giant tunas breed at night in very
warm waters, swimming up and down in the water column for hours.
"Importantly," notes Block, "our data indicate that bluefin are sorting
to distinct breeding grounds where efforts to protect the breeding aggregations
should be increased."
-30-
By Mark Shwartz/Ken Peterson
Electronic tags reveal
transatlantic migrations and breeding grounds of Atlantic bluefin tuna