Throughout most of my life as an offshore angler, it seemed
that scientists had a very firm grasp on where Atlantic bluefin tuna
spawned. There was a large eastern stock
that spawned in the Mediterranean Sea, often at relatively young ages and
smaller sizes, and a much smaller western stock that spawned in the northern
Gulf of Mexico, but generally not until they were about nine years old and
somewhere around 80 inches (fork length) long.
The stocks mixed on the feeding grounds, with eastern stock
fish crossing over to North America and western stock fish crossing over to
Europe from time to time, but when it came to spawning, the Mediterranean and
Gulf were the only games in town.
That picture began to change about a decade ago, after a group of
biologists published a paper, titled "Discovery of a spawning ground reveals
diverse migration strategies in Atlantic bluefin tuna (Thunnus thynnus),” which
argued that western stock bluefin were also spawning, and spawning at younger
ages, perhaps as young as five years old, in the Slope Sea, an area at the edge
of the continental shelf off southern New England and the upper mid-Atlantic.
The
paper created substantial controversy when it came out, but biologists
have now generally accepted the evidence that bluefin do, in fact, also spawn
in the Slope Sea.
Now, there is some evidence that the western Atlantic stock
of bluefin tuna may spawn over a much broader area—perhaps even in a continuous
swath of water that ranges from the Yucatan Channel, in the southern Gulf of
Mexico, all the way up to the Slope Sea.
Just two weeks ago, the National Marine Fisheries Service issued a release
announcing that
“New
Research Reveals Broad Spawning Distribution for Bluefin Tuna,”
which said that
“Working with partners, NOAA Fisheries scientists did a deep
dive into bluefin tuna spawning patterns.
They compiled a large data set from fisheries surveys, archive and
museum specimens, and research cruise reports going back to the 1950s. Their analysis included more than 35,000
plankton tows, and they examined nearly 5,000 individual tuna larvae. The results…indicate that bluefin tuna have a
much broader spawning distribution than previously recognized…
“The results suggest that bluefin spawn in a continuous area
during a prolonged spawning season.
Spawning starts in April in the southernmost areas—the northwest
Caribbean and southern Gulf of America—and ends in early August in the Slope
Sea.”
The NMFS release was referring to the
paper “A re-evaluation of Atlantic bluefin tuna (Thunnus thynnus) spawning
distribution in the western Atlantic Ocean,” which appeared in the February
2026 issue of the journal, Progress in Oceanography. In that paper, the researchers stated that
“Our work documented persistent spawning by bluefin in the
Yucatan Channel, southern and northern Gulf [of Mexico], Blake Plateau [located
80-200 miles off the southeastern United States], an area off the Carolinas
shoreward of the Florida Current, and the western Slope Sea, despite changing temperatures
in each of these regions over the past decades.
This work also revealed how uneven the allocation of sampling effort has
been in space and time, leaving open the possibility of additional spawning
grounds. One hypothesis, consistent with
the available data, is that bluefin have a continuous spawning distribution in
the western Atlantic from the southern Gulf in early-spring to the western
Slope Sea in early-summer that is only interrupted by narrow areas of the
fast-moving western boundary current (i.e., Florida Current; Gulf Stream) or
shallow bathymetry. An alternate hypothesis,
also consistent with the data, is that the western Slope Sea and Gulf are
discrete spawning grounds of near equal magnitude with some limited spawning
elsewhere. While sampling throughout the
western north Atlantic would be useful, we consider a larval survey to evaluate
spawning in June in the waters south of the Gulf Stream and east of the Florida
Current to be the most critical to testing these alternate hypotheses.”
The researchers came to their conclusion not by conducting extensive
new surveys, but by conducting a comprehensive analysis of data that was
already available, having been collected by research cruises dating back to
1956. While most of those cruises were
not directly investigating bluefin tuna, they nonetheless collected plankton
samples, which were preserved for future use, as well as data on water
temperature, etc. Electronic tagging
data and information generated by longline tuna fisheries was also included in
the data set.
The available information, along with unprocessed plankton
samples in which bluefin tuna larvae might be found, was analyzed as scientists
searched for patterns that might suggest the presence of spawning bluefin tuna
in areas outside of the Gulf of Mexico and the Slope Sea. The researchers were careful to avoid
assuming that the presence of larval bluefin was a definite indicator of
spawning activity, as larvae could also be transported by currents from a known
spawning area; instead, only larvae that were too young to have been transported
from such a known area were considered likely indicators of spawning activity.
After analyzing the data, the researchers came to a conclusion
that arguably resurrected, if not actual knowledge, then at least hypotheses
that existed decades before. Their paper
reminded readers that research conducted as early as 1974 suggested that
bluefin also spawned, not only in the Slope Sea, but also between Cuba and
Haiti, in the so-called Windward Passage, as well as north and east of the
Bahamas, and that another paper, published in 1995, proposed that western
Atlantic bluefin spawning migrations varied, depending on the size of the fish
involved.
The new paper appears to validate at least some of those old
proposals.
As noted in the new paper, more work will have to be done to
determine whether the spawning grounds for western Atlantic bluefin form a
continuous band between the Gulf of Mexico and the Slope Sea, or whether those
two areas account for most of the spawning, with only sporadic episodes
in-between.
And it is still too early to tell whether the new paper will
attract the same sort of negative attention the paper discussing the Slope Sea
spawning ground drew ten years ago, or whether it will serve as a jumping off
place for additional research that will confirm, refine, or rebut its
conclusions at some point down the road.
Challenging, testing, and confirmation are all part of the
scientific process, and are all valid responses to the new work. And it’s important to get the science right,
because as
one of the authors, Trika Gerard, a deputy director of NMFS’ Southeast
Fisheries Science Center, observed,
“These new data tell us where bluefin tuna are spawning and
when, which gives us an idea of where they are migrating and how much they are
contributing to future generations.
This, combined with information about genetics and population structure,
helps managers decide the most appropriate way to sustainably fish for this
prized species.”
The conclusions reached in the recent paper aren’t merely of
academic interest. They can have a real
impact on the health and the future of the western Atlantic bluefin stock.
Then, too, there is another lesson taught by this new paper
that its authors certainly did not intend, but is vitally important,
nonetheless.
All of the analysis leading up to its findings were based on
prior research, much of it performed by NMFS in the course of surveying aquatic
life on various portions of the continental shelf, surveying protected species,
or performing other ongoing studies. The
data developed in the course of that research was maintained and curated by
NMFS and by other institutions, just as the processed and unprocessed ichthyoplankton
samples, which contained the bluefin tuna larvae, were.
Those earlier surveys could not have been performed without
an adequately funded National Marine Fisheries Service, and the data and
plankton samples could not have been maintained and readily accessed unless
there was enough funding to both archive the survey records and maintain people
on staff who can locate and utilize the information. In addition, eight of the sixteen authors of
the paper, including the lead author, are NMFS scientists employed at one of
the agency’s regional fisheries science centers.
Good fisheries science can’t exist without adequate
funding. As
I noted in last Sunday’s blog post, and in
some earlier essays, right now NMFS’s fisheries science is under assault,
with some members of Congress, and some members of the current Administration,
seeking to cut needed funding. Already, cuts
in scientific personnel are hampering NMFS’ ability to conduct surveys, perform
stock assessments, and execute the other tasks necessary to its role as the steward
of the nation’s marine resources.
It’s very possible that, if NMFS had been in similar financial
straits over the last 50 or 75 years, the recent paper would never have been
written, as the data that underlies it would never have been compiled.
If the researchers are correct, and western Atlantic bluefin
tuna are spawning over a greater expanse of ocean than previously believed, it
is possible that the stock is larger and more resilient than biologists had
thought, and might be able to withstand a modestly higher level of harvest, to
the benefit of both the commercial and the recreational fishing industries of
the United States.
But unless NMFS is funded at levels that will allow
meaningful research to continue, and hopefully expand, and for fisheries to be
monitored closely enough to ensure that higher harvest levels don’t negatively
impact the health of fish stocks, there is a very good chance that such benefits
to the nation, whether accruing from bluefin or from other fish stocks, will
never be realized.