Genomics Help Uncover Mysteries of Atlantic Bluefin Tuna

The Slope Sea may hold a key to better understanding Atlantic bluefin tuna .

This summer, scientists journeyed to the Slope Sea on two exploratory surveys to examine how Atlantic bluefin tuna use this area for reproduction (also known as spawning). The first survey was a cooperative longline survey to look for spawning fish. The second survey followed closely on their heels to look for the results of that spawning-larval, or baby, bluefin tuna.

The objective was to document the potential contributions of spawning between the Slope Sea and other spawning areas like the Gulf of America. Next they will analyze DNA from both larval and adult bluefin tuna to estimate the population (stock) size. Atlantic bluefin tuna has been managed as having two primary stocks-one that spawns in the Mediterranean Sea (the eastern) and the other in the Gulf of America (the western). Recent research suggested the Slope Sea could be a major spawning ground comparable to the Gulf of America. Genetic research has shown the two stocks are interconnected . This year's survey aims to clarify remaining uncertainties about bluefin tuna stock structure and spawning dynamics.

"Prior to this year, sampling of the Slope Sea was piggybacked on other surveys, limiting the number of bluefin tuna larvae we could collect," said Dr. Dave Richardson, lead scientist on the project and a research fishery biologist at the NOAA Northeast Fisheries Science Center. "Our goal this year was to collect as many larvae as possible. We intend to provide these larvae to multiple research groups that are using independent approaches to evaluate linkages among spawning grounds."

Catching Baby Bluefin

The researchers targeted sampling locations the same way anglers do on a fishing trip. They checked ocean conditions like sea surface temperature, currents, and sea surface salinity to determine where to focus their efforts, and shifted their location as conditions changed. This ensured they had the best chance of encountering-and therefore sampling-areas with larval bluefin. If the tuna were there, and spawning, the team should be able to collect larvae close by. And if the bluefin heavily use this area for spawning, the team should see a lot of larvae.

When the scientists arrived onsite, instead of dropping fishing lines, they towed fine-mesh nets and drifting light traps to collect bluefin larvae from the open ocean.

The team conducted 70 net tows, where they pulled a long, cone-shaped, fine-mesh net through the top 20 meters of the water column. This equipment targets plankton near the surface (including fish larvae). As hoped, the nets were successful in collecting bluefin tuna larvae. The scientists found larvae at more than half of the stations-likely numbering in the many thousands! (The team still has a lot of counting to do.) They also deployed the light traps as passive drifters that attract larvae using light at night. This method was run as a pilot study for future use and also did well at catching larvae and a few juveniles. The team is still working on identifying the species collected through this method.

Using two different sampling methods will help maximize our chances of finding and capturing larval bluefin.

"We got a substantial number of larvae, which we are very excited about!" said Kristen Walter, M.S., a NOAA affiliate of the Southeast Fisheries Science Center through the University of Miami Cooperative Institute for Marine and Atmospheric Studies. "Now it's just a matter of going through the samples, identifying the bluefin, examining size ranges, and doing genetic testing to determine our findings."

What These Data Will Tell Us About Bluefin Tuna in the Atlantic

Ultimately, this effort will help us understand how bluefin tuna use the Slope Sea for spawning, and the resulting impacts on population dynamics. That's a huge question for tuna anglers. But, finding larvae is just one piece of the puzzle. The bigger question is: "What is the stock structure of Atlantic bluefin tuna?"

The team is using genetic information across life history stages-from larvae to adults-to examine kinship, or the relationships among 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 bluefin tuna stock mixing in the West Atlantic.

"NOAA Fisheries is exploring the application of modern genomic techniques to improve fisheries data quality and efficiency," stated Dr. Matt Lauretta, one of the leads on the bluefin project. "Ultimately, these data can be used to determine the contribution of the different bluefin stocks to the fisheries and estimate total abundance so we can get a better handle on how many bluefin are out there to catch."

Angler-Driven Science for Bluefin

While the full results from genetics and modeling could take several years, the cruise itself is a significant step. The project also emphasizes the importance of collaboration. NOAA scientists are supporting partners at the University of Maine's Pelagic Fisheries Lab and anglers up and down the Eastern Seaboard who are expanding citizen science efforts for bluefin tuna. Through this collaboration, anglers can request fin clip kits and biopsy punch tools to collect DNA tissue samples to better understand bluefin tuna genetics and stock mixing.

Learn more about collecting bluefin tuna genetic samples

Translating Science to Management

"We hope this work can help answer the questions many fishermen have about what's going on with bluefin," noted Dr. Clay Porch, director of the Southeast Fisheries Science Center. "There's a strong perception among fishermen that bluefin tuna abundance is higher than it has been in a long time, leading to calls for increased quotas."

We will present the initial results of this research to the International Commission for the Conservation of Atlantic Tunas in fall 2025. ICCAT sets a separate Total Allowable Catch for each of the western and the eastern stocks of Atlantic bluefin tuna through a management procedure. A management procedure uses several indices to track the abundance of both stocks, with the total allowable catch for each stock going up or down depending on the indices.The first management procedure was adopted in 2022. It is scheduled to undergo review, beginning in 2026 and ending in 2028, to update it with new information. Also ICCAT will embark upon a status assessment or health check of the stock in 2026 to evaluate whether management is working as intended. We anticipate that the Slope Sea larval survey and the collaborative longline survey will both play an important role in these processes, providing data that have never been available before.

Learn how total allowable catch is set for Atlantic bluefin tuna

Research Partners

This effort involved contributions from:

• Accuracy Integrity Service, Inc.
• Azura Consulting LLC
• NOAA Knauss Fellowship Program
• NOAA Northeast Fisheries Science Center
• NOAA Southeast Fisheries Science Center
• NOAA Teacher at Sea
• Oxford University
• University of Miami
• University of Maine