NMFS CONFIRMS YOUNGER BLUEFIN TUNA SPAWNING IN THE SLOPE SEA

 

In 2016, a group of biologists, many but not all of whom were employed by the National Marine Fisheries Service, published a paper in the Proceedings of the National Academy of Sciences, in which they announced,

“We present unequivocal evidence that Atlantic bluefin tuna spawn in the Slope Sea, counter to the current assumption that the Gulf of Mexico and Mediterranean Sea are the exclusive spawning grounds.  We also demonstrate that the age at maturity of western bluefin tuna is currently overestimated, that this stock exhibits size-structured spawning migrations, and that migratory connections exist between western and eastern Atlantic spawning grounds…The implications of our work are most pronounced for western Atlantic bluefin tuna, which have a life history less vulnerable to overexploitation and extinction than is currently estimated.”

The researchers’ conclusions were based on the presence of bluefin tuna larvae in a region known as the “Slope Sea,” an area at the edge of the continental shelf east of the southern New England and upper mid-Atlantic states, as such larvae were too small and too young to have drifted from known Gulf of Mexico spawning grounds.

The presence of the larvae led scientists to question the estimated age at maturity for bluefin tuna. 

For many years, no bluefin less than nine years of age were found to be spawning in the Gulf of Mexico, and electronic tagging data confirmed that such large bluefin migrated from maritime Canada and New England down to the Gulf in order to spawn.  Biologists thus assumed that western stock bluefin did not mature until they were at least nine years old, which was a stark contrast to eastern stock fish, where 50% of the population was believed to be mature when just four years of age.

The discovery of bluefin tuna larvae in the Slope Sea, combined with electronic tagging data and three different lines of biological evidence relating to bluefin reproduction, led the researchers to conclude that tuna as young as five years of age were spawning there.  They calculated that, based on conditions that existed when the research was done, more than half of all western Atlantic bluefin tuna reproduction took place in the Slope Sea, and not in the Gulf of Mexico.

Those findings had implications for the western Atlantic bluefin tuna fishery, for if correct, they would allow biologists to use a lower average age at maturity in their calculations, which would likely increase estimates of both spawning stock biomass and the level of landings that such increased biomass could sustainably support.

The 2016 paper seemed to represent a real breakthrough in bluefin tuna science, but it initially led to substantial controversy.  Dr. Barbara Block, a biologist who has done substantial research on bluefin tuna, called the research “interesting,” but said that she needed more evidence, including seeing sexually mature bluefin in the Slope Sea, before she could agree that younger, but still mature, bluefin were spawning there.

Others went further.

Amanda Nickson, who was the director of global tuna conservation at the Pew Charitable Trust, warned against placing too much reliance on the new findings, saying that

“New science and new information is good.  What one has to be careful of is attempting to manage the Atlantic bluefin population from a single study.  The situation is always complex.”

Dr. Andre Boustany, a bluefin expert at Duke University, also advised that the study’s findings be used cautiously, noting that if bluefin really do mature at younger ages, that the historical spawning stock biomass was larger than previously believed; thus, the SSB target for a rebuilt stock would also be higher.

Contrary to those who believed that a younger age at maturity would allow a larger annual harvest, Dr. Boustany noted that

“If we’re trying to rebuild the population during a certain time frame, than we might need to actually reduce the amount of fish we’re catching now.”

Thus, it would be safe to say that, when the paper was first released, people had a lot of                questions, and many were unwilling to accept its conclusions.  The debate went on for nearly a decade, but over time, after additional research was published, the claim that bluefin tuna spawned in the Slope Sea became more and more accepted.

In 2021, a paper submitted to the Canadian Journal of Fisheries and Aquatic Sciences described research conducted on bluefin larvae collected in the Slope Sea in 2016, three years after the collection of the larvae described in the original study.  The authors of that study found that

“The collections of Atlantic bluefin tuna larvae in the Slope Sea in 2016, together with the otolith analysis and particle tracking analysis that they enabled, support the conclusion that the conditions in the Slope Sea are suitable for their growth and retention, and that they originated from spawning within the Slope Sea.”

However, the 2021 paper limited its conclusions to the origins, distribution, and mean abundance of bluefin tuna larvae collected in the Slope Sea.  It did not address questions such as the size and/or origin of the adults spawning in that location, or the relative contribution that the Slope Sea spawners make to the overall western Atlantic bluefin stock, compared to fish spawning in the Gulf of Mexico and elsewhere, but only noted that the presence of a Slope Sea spawning ground “may” contribute to the resilience of the stock. 

The authors acknowledged that

“An important open question is the abundance, distribution, and identity of the spawning adults in the Slope Sea.  How many adults are spawning there, and do they consistently utilize the suitable habitat…Are they western individuals that mature earlier than previously understood, or is there significant stock mixing occurring between eastern and western individuals?...”

Some of those questions might have been answered in a paper published in 2023.  Titled “Evidence of bluefin tuna (Thunnus thynnus) spawning in the Slope Sea region of the Northwest Atlantic from electronic tags,” and appearing in the February 16, 2023 edition of ICES Journal of Marine Science, the paper said that, based on the results of electronic tagging,

“Our findings complement the larval analyses of [the papers published in 2016 and 2021], providing detailed behavioural data for the adult spawners that these [papers] inferred were present.  However, most of the spawners we identified were >10 years old…and, therefore, these results to not support [the 2016 paper’s] hypothesis that the [Slope Sea] is primarily a spawning ground for smaller individuals, nor do they support the argument for a lower age at maturity…Although our tagging is biased toward larger individuals, between 1996 and 2020 we added tracks for 123 fish tagged in the west at <10 years old, of which 73 extended into June the following year…A total of seven of the 73 were classified as [Slope Sea] spawners, the same as the proportion across the entire dataset.  Furthermore with the identified [Slope Sea spawners comprising only [about] 12% of [North Carolina]-tagged fish (and [about] 6% of those tagged in the [Gulf of St. Lawrence]), we do not find support for the contention that the [Slope Sea] spawning ground may be responsible for a more significant portion of western spawning than the [Gulf of Mexico].”

So, by 2023, biologists were beginning to build a consensus that the Slope Sea was definitely a spawning ground for bluefin tuna, but had not come to agreement on its importance, nor on the size or origin of the tuna spawning there.

Finally, on December 4, 2025, NMFS issued a press release announcing that

“Longline Sampling Confirms Young Bluefin Tuna Spawn in the Slope Sea.”

The release went on to describe NMFS researchers making two trips to the Slope Sea in the summer of 2025.  The purpose of the first trip was to sample bluefin caught on commercial longlines.  The samples taken appear to confirm that younger bluefin tuna are, in fact, spawning in the Slope Sea.

Scientists sampled 90 bluefin tuna—42 females and 48 males—ranging in size from 38 to 110 inches, with an average length of 72 inches, and

“observed multiple reproductive statuses associated with spawning,”

although those visual observations will be confirmed through further laboratory study.

The second trip to the Slope Sea collected thousands of bluefin tuna larvae.  As NMFS explained,

“The team is using genetic information across life history stages—from larvae to adults—to examine kinship, or the relationships between different fish.  They are using a technique called close-kin mark-recapture.  They’re also looking at how much mixing is occurring between the eastern and western Atlantic stocks.

“Analyzing these relationships will help us better estimate population size and resolve the 40-year-old mystery of tuna stock mixing in the West Atlantic.”  

Some of the initial results of the new NMFS research were presented and discussed at the 2025 meeting of the International Commission for the Conservation of Atlantic Tunas, which occurred in late November, where a modest increase in the western Atlantic bluefin tuna harvest was approved.

The recent NMFS research largely confirms the conclusions reached in the original 2016 paper which announced the discovery of a new bluefin tuna spawning ground in the Slope Sea, which is utilized by bluefin notably smaller than those which spawn in the Gulf of Mexico.

There are still some questions that need to be answered, but the history of Slope Sea bluefin research, from the initial paper published in 2016 to the latest NMFS research, provides a good example of how fisheries science is developed, how it progresses and, most recently, how such science is used.