NEWLY RELEASED PAPER LOOKS INTO JUST WHERE WESTERN ATLANTIC BLUEFIN TUNA SPAWN

 

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.