Western Atlantic Provides Refuge for Bluefin Tuna

An international team of scientists deployed electronic tags on bluefin tunas across three decades to investigate stock movement between the eastern and western Atlantic. They also examined catch data dating back to 1950 to better understand the proportion of removals among regions.

The study was published in the Proceedings of the National Academy of Sciences.

Dr. Barbara Block, lead author from Stanford University summarized, "Our research demonstrates that lower fishing mortality in the West and North Atlantic has provided a refuge for eastern-origin Atlantic bluefin tuna, and highlights the importance of Atlantic waters to bolster the bluefin tuna population as a whole."

Atlantic bluefin tuna is a highly coveted species, managed as two stocks by the International Commission for the Conservation of Atlantic Tunas. The western stock is harvested by the United States, Mexico, Japan, and Canada. The eastern stock is targeted by many countries across the Mediterranean basin and eastern Atlantic. ICCAT divides management of these two stocks at the 45^oW meridian, setting separate catch limits and management measures for each stock.

Eugenio Piñeiro Soler, NOAA's Assistant Administrator of Fisheries stated, "Management of this internationally shared resource requires high quality scientific information-and this work provides one example of that. This research addresses one of the primary sources of uncertainty surrounding the amount of mixing and movement occurring between these two stocks. This is a critical piece of information needed to inform sustainable yield advice."

Tagging Data: Following the Food

The scientists deployed tags on 1,720 bluefin tuna from 1996 to 2025. Tag data provides information on where and when fish migrate, where they feed, ideal environmental conditions, and when fish enter and leave known spawning areas. They also inform fisheries stock estimates by providing measures of natural and fisheries mortality.

Data collected from the tags confirm trans-Atlantic migration and stock mixing-most frequently in the east to west direction. The results show that many bluefin tuna move from eastern Atlantic and Mediterranean waters to the western Atlantic to forage and grow. These fish often remain in the west for several years before returning to the Mediterranean Sea or spawning in the Slope Sea. Tagging also demonstrated that adult fish tagged in U.S. and Canadian waters move back to the Mediterranean Sea to spawn and often return the following year to the western Atlantic to forage along the U.S. eastern seaboard. Importantly, bluefin tuna that originated in the western Atlantic, however, tend to stay west of the 45^oW management line.

Why the great migration? Tuna are likely heading west to find and feed on vital prey such as Atlantic mackerel, Atlantic herring, and menhaden, which are abundant in U.S. and Canadian waters.

A Deep Dive into the Catch Data

The team also analyzed regional catch data dating back to 1950 to better understand the proportion of removals among regions. The data show a dramatic difference in fishing pressure:

• 55% of the total catch was caught in the Mediterranean Sea (BF59 in the figure)
• 17% in the eastern central Atlantic (BF57 and BF58)
• 17% in the northeastern Atlantic (BF53 and BF54)
• 11% in the entire West Atlantic (all regions west of 45°W)

In the past 30 years, the disparity has become even more extreme; the Mediterranean now makes up 72 percent of the catch across the entire Atlantic.

The scientists determined that bluefin tuna from the eastern Atlantic experienced much higher fishing mortality when in the east compared to the west or north. This indicates that migrating to the West and North Atlantic offered eastern Atlantic bluefin tuna safety from high harvest levels .

The majority of Atlantic bluefin tuna are captured by purse seines in the Mediterranean where the fish are more vulnerable to fishing. Historically, these catches targeted age one and two fish, harvesting them before they can reach the size and age when they exit the Mediterranean on feeding migrations. This restricted the ability of the population to access productive Atlantic waters.

In the last 30 years, Mediterranean fisheries have increased pressure on the population-particularly with the rise of capture-based aquaculture that targets more mature bluefin tuna. Catches during the 1990s and 2000s ramped up substantially, peaking at 60,000 tons in 2007-nearly double the scientific recommendations at the time. In 2010, after global concerns for the state of the stock, ICCAT implemented substantial cuts in quotas, as well as binding conservation and management measures.

In contrast, for more than 45 years, western catches have followed binding total allowable catch limits-typically a tenth of the eastern limits. They have had strong conservation measures including prohibitions on targeted fishing in the Gulf of America.

Bottom Line: Bluefin Tuna Conservation Measures Work

The major conservation and management measures taken by ICCAT, combined with shifting catches to larger fish, resulted in a dramatic turnaround of the stock. Reducing fishing mortality to sustainable levels allowed several year classes of fish to survive, grow, and leave the Mediterranean to seek the productive feeding grounds of the North Atlantic. This escapement from the Mediterranean spawning areas to wider Atlantic waters with stricter harvest measures allows bluefin to live longer, grow larger, and contribute more offspring to the population. This emphasizes the importance of managing fishing mortality in areas of high vulnerability such as spawning aggregations.

John Walter, deputy director of science and council services at the NOAA Southeast Fisheries Science Center , and co-author on the study summarized, "It really is an international success story-we have thriving fisheries throughout the Atlantic and a return of the fish to many areas where they had not been observed in many years. The key to this rebound may be surprisingly simple-allow a fish restricted to spawn in a narrow environmental niche but evolved to seek productivity anywhere in the ocean, to do just that."

Whether the current Atlantic bluefin fishery total allowable catch remains sustainable in the face of a rapidly changing ocean and increasing human demand remains uncertain. What is certain is the return on investment for state-of-the-art science.

U.S. anglers have been frustrated in recent years by strict harvest measures for bluefin, but the science confirms these limits are effective. Our conservation efforts are having huge impacts-not just for the United States, but across the Atlantic. Anecdotal evidence supports this, with anglers along the U.S. East Coast catching more and larger bluefin than in the past 30 years. The stock recovery is such that ICCAT adopted a higher total allowable catch for the western stock. NOAA Fisheries is working on rulemaking to implement the corresponding U.S. quota increase in 2026.

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Research Partners

This study was funded by the U.S. Bluefin Tuna Research Program , grants to the Block lab at Stanford University, and by sponsorship of tags at The Ocean Foundation Tag-A-Giant Fund . The study included partners from the following organizations:

• Stanford University
• Acadia University (Canada)
• University of Hawai'i
• Swedish University of Agricultural Sciences (Sweden)
• NOAA Fisheries
• Institute of Marine Research (Norway)
• Marine Institute (Ireland)
• Barcelona Zoo Foundation (Spain)

Read the paper in PNAS