Longline Sampling Confirms Young Bluefin Tuna Spawn in the Slope Sea

Atlantic bluefin tuna are prized by both recreational and commercial fishermen. In 2022 alone, U.S. commercial fishermen generated an estimated $12.4 million in revenue from harvesting bluefin tuna. However, they are also among the most challenging species to study, as they migrate long distances and live in the open ocean for much of the year.

NOAA Fisheries recently embarked on a cooperative research project to understand Atlantic bluefin tuna reproduction (also known as spawning) in the Slope Sea. Scientists infrequently sample this area of the ocean between the Gulf Stream and the continental shelf of the Northeast United States. It may contain a crucial missing piece for understanding the overall population structure of bluefin tuna in the Atlantic.

Historically, this species has been managed as having two primary stocks:

• Eastern stock that spawns in the Mediterranean Sea
• Western stock that spawns at older ages in the Gulf of America

Dr. Molly Lutcavage's lab at the Large Pelagics Research Center in Gloucester, Massachusetts is conducting research on reproduction and electronic tagging. The lab is affiliated with UMass Dartmouth's School for Marine Science & Technology. Their research suggested that spawning might be occurring in other areas of the western Atlantic as well, and by younger, smaller fish. Lutcavage's lab predicted that smaller fish would spawn closer to feeding grounds. Biological evidence from NOAA Fisheries larval fish sampling in the Slope Sea supported this hypothesis. Recent genetic research shows the stocks are interconnected. However, there was little data available on the reproductive condition of adult bluefin tuna, including smaller adults, in the Slope Sea to corroborate these studies-until now.

Collecting Critical Data in the Slope Sea

NOAA Fisheries' scientists traveled to the Slope Sea on two exploratory research expeditions this past summer. The cooperative longline sampling trip looked for spawning adult fish. The second expedition collected the results of that spawning -larval, or baby, bluefin tuna. Their goal was to examine how Atlantic bluefin tuna use this area for spawning. Samples collected during both expeditions will support ongoing genetic studies to better estimate the overall population (stock) size for bluefin tuna. The data will provide critical information to improve how this valuable species is managed.

The longline sampling trip left Fairhaven, Massachusetts, on June 14, 2026 with Chief Scientist Dr. Molly Lutcavage as well as NOAA Fisheries' scientist Dr. Michelle Passerotti and Simon Gulak of Sea Leucas LLC onboard. Land-based collaborators included co-principal investigators Dr. Gilad Heinisch and Dr. Tim Lam from the Large Pelagics Research Center. For 2 weeks, scientists worked alongside the crew of the F/V Eagle Eye II to collect samples from commercial bluefin catch. These samples likely confirm that younger, smaller fish are spawning in the Slope Sea, providing a clearer understanding of bluefin reproductive ecology.

Biological Samples to Determine Reproductive Condition

The science crew collected a range of biological samples and data from commercially caught bluefin during the trip, including:

• Reproductive tissues (gonads)
• Otoliths, or fish ear bones, to determine the fish's age
• Fin clips for DNA profiling

Scientists removed the heads of the fish they sampled. They labeled and froze them for otolith sampling by the University of Maine's Pelagic Fisheries Lab. They collected multiple fin clips to distribute to several collaborators for genetic research. They collected, weighed, and visually assessed gonads and sampled them for histological, energetics, and physiological studies. These studies will identify the reproductive stage of each fish and spawning potential for fish of both sexes.

Spawning Confirmed in the Slope Sea

After many years and attempts to collect this data, this expedition delivered the samples needed to likely confirm spawning by younger, smaller fish in the Slope Sea. Scientists sampled 90 bluefin tuna over the course of the trip-42 females and 48 males. Their body lengths ranged from 38 to 110 inches, and an average of 72 inches. Scientists visually assessed the gonads of the sampled fish and observed multiple reproductive statuses associated with spawning. Researchers will confirm this by examining the tissues under a microscope, as well as studying other biophysical features of reproductive status. Scientists will compare their results to a large data archive on the reproductive status of bluefin tuna sampled in nearby coastal foraging grounds. In addition to the 90 bluefin tuna they sampled, scientists tagged and released more than 25 bluefin tuna and nine sharks during this trip, including:

• Shortfin mako
• Porbeagle
• Common thresher
• Dusky sharks

These samples provide insight into the reproductive status of bluefin tuna in the Slope Sea, as well as corresponding age and genetic information. When combined with the samples collected during the larval survey, they could improve our understanding of bluefin tuna stock structure and population size. In addition, scientists are using the samples to study key aspects of life history, including bioenergetics of reproduction and migration. Scientists will examine genetic relatedness between fish using the new close-kin mark-recapture method. This information can then be used in a mathematical model to estimate the size of the population.

In addition to the scientific collections, researchers used the opportunity to harvest the scientific quota of bluefin catch to donate to local food banks. They provided about 2,000 pounds of nutritious fish to the local community and kept it from going to waste.

Industry Collaboration Key to Successful Data Collection

Researchers knew the trip was successful even before they finished processing the samples. The cooperative aspect of the survey was key to its success. Scientists relied upon the expertise and historical knowledge of Captain Scott Drabinowicz to find and catch bluefin in the size ranges needed for the study. Drabinowicz and the Eagle Eye II owners had been involved with bluefin tuna research since 1994. They were part of the first effort to document spawning bluefin tuna in the Northwest Atlantic offshore region in 2002. Drabinowicz played a key role in successfully deploying electronic tags on bluefin, which helped the Large Pelagics Research Center predict when and where they were likely spawning.

"It is always a pleasure to work with Captain Scotty and the Eagle Eye II," said NOAA Northeast Fisheries Science Center Research Fish Biologist Dr. Michelle Passerotti. "He is a strong believer in the science behind fisheries management, with an incredible wealth of knowledge about highly migratory species in the North Atlantic. It takes real skill to successfully target and land these impressive fish. The science crew could not have achieved the goals of this study without industry expertise."

Science Supports Better Management

These new studies will help clarify current management hypotheses and assumptions about bluefin tuna in the Slope Sea, and their contribution to the Atlantic population's productivity. "With the current management strategy evaluation scheduled to undergo review to incorporate new information starting in 2026, this study coupled with the larval survey will play an important role in the process, providing data that have never been available before," said Dr. John Walter, NOAA Southeast Fisheries Science Center Deputy Director for Science and Council Services.

"After all these years, it's a thrill to have gone out to the Slope Sea to confirm what's going on with bluefin there, especially with renowned Captain Scott Drabinowicz and crew, who helped make it a success," said Lutcavage. "The two separate high seas bluefin tuna efforts this year-one focused on adults and the other on larvae-delivered everything we'd hoped for, and that's a scientific dream come true."

Research Partners

This effort involved contributions from:

• Accuracy Integrity Service, Inc.
• Azura Consulting LLC
• Harvard Medical School
• NOAA Knauss Fellowship Program
• NOAA Northeast Fisheries Science Center
• NOAA Southeast Fisheries Science Center
• NOAA Teacher at Sea
• Oxford University
• University of Bari, Italy's School of Veterinary Science
• University of Maine
• University of Miami