Sorry, Charlie: Condition of Bluefin Tuna Declining
By Beth Potier, Media Relations
September 5, 2007
The quality of giant bluefin tuna caught in the Gulf of Maine has declined
significantly since the early 1990s, researchers at UNH have found by analyzing
detailed logbooks from a commercial tuna grader at the Yankee Fisherman’s
Co-op. The findings, published this week in Fishery Bulletin, indicate
potential changes in food sources, shifts in reproductive or migratory
patterns, or the impact of fishing may be the cause of this decline.
Walter Golet, a Ph.D. candidate in UNH’s Large Pelagics Research
Lab, along with research assistant professor Andy Cooper and Large Pelagics
Lab director Molly Lutcavage, partnered with veteran tuna grader Robert
Campbell at the Yankee Fisherman’s Co-op in Seakbrook to analyze
the quality of 3,082 Atlantic bluefin tuna (Thunnus thynnus).
“In a drawer, he had two or three notebooks with every fish he graded
in the last 14 years, from 1991 – 2004,” says Golet.
Golet’s findings corroborated observations by fishermen, brokers
and cooperative managers: Not only is the number of giant bluefin in the
Gulf of Maine declining, the condition of those fish caught is of much
lower quality. Specifically, Golet and co-authors analyzed the fat and
oil content and shape of the tuna.
“Fat content is in high demand for the market, because that’s
what makes the meat taste good,” he says, noting that fish with well-marbled
tail meat, fat in their mid-section muscle and belly, and a rotund shape
can command upwards of $50 per pound on the sushi market.
Beyond the tekkamaki, however, fat content is a valuable indicator of
the overall health condition of the bluefin. Highly migratory, traveling
from their spawning grounds to the Gulf of Maine and possibly across the
Atlantic, Atlantic bluefin have high metabolisms and energetic needs. Not
surprisingly, bluefin caught in June, shortly after arriving in the Gulf
of Maine after a swim of more than 1,000 miles, are lean and of lower quality.
Yet Golet and co-authors found that the quality of bluefin caught in August
and September, after several months at the Gulf of Maine’s buffet
table, is declining.
“They look lean,” Golet says.
In 1991, he found, the probability of landing a C+ fish (A being the highest
grade) was 16 percent and 9 percent for August and September, respectively.
By 2004, the probability increased to 68 percent and 76 percent in the
C+ category for August and September, respectively. He also found that
the bluefin are leaner on arrival to the Gulf of Maine; the probability
of catching a poor quality fish (grade C or worse) in June 1991 was 30%
compared with 70% in 2004. Good quality fish, such as B or better, now
comprise less than one percent of the commercial catch at this New Hampshire
cooperative.
For scientists like Golet and his collaborators, this research is less
about tasty tuna than about understanding and promoting this overexploited
fishery.
“One of the big consequences of not fattening as much is the potential
impact it could have on reproduction,” says Golet. “Reduced
energy stores can often force a fish to skip spawning in a particular year.”
Jennifer Goldstein, also a Ph.D. student in the Large Pelagics Research
Lab, looked at this link between bluefin body composition and reproductive
status in an article published in the July 2007 Marine Biology journal
(http://www.springerlink.com/content/q21417091803m56u/fulltext.pdf).
The researchers hypothesize this change in energy – or fat – acquisition
could also shift the bluefin tuna’s migration patterns.
Now that the researchers have documented the decline of bluefin quality,
they’re looking into the reasons behind it. One obvious cause would
be a decline in their food supply; bluefin are voracious predators with
a high metabolism.
“Bluefin will eat just about anything – sponges, seahorses,
dogfish – but according to recent studies, up to 60 percent of their
prey is comprised of Atlantic herring,” says Golet, noting that herring,
along with mackerel and bluefish, is tuna health food, providing maximum
energy to these long-distance swimmers.
Since stock assessments indicate that herring abundance is at historically
high levels, however, the researchers wonder if perhaps the herring themselves
have experienced a decline in quality, or if they have dispersed into smaller
schools, requiring greater energy output: Bluefin have to swim farther
for each meal.
The “junk food hypothesis” probes whether changes in diet
from high-energy food like herring and mackerel to less energetic species
like haddock or sand lance results in lower caloric intake. And evidence
is mounting to support the hypothesis that bluefin migrations are far more
complex than once thought and that bluefin may be traveling farther to
the Gulf of Maine; some Atlantic bluefin in the Gulf of Maine have traveled
from the central and eastern Atlantic, utilizing far more stored energy
swimming greater distances against major currents.
Golet doesn’t expect to find a single smoking gun, or harpoon.
“I’m very convinced that it’s multiple factors working
with each other,” says Golet.
Golet’s research was supported by a grant from the National Oceanic
and Atmospheric Administration to Molly Lutcavage. For more information
on tuna research and the Large Pelagics Research Laboratory, go to www.tunalab.unh.edu.
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