Bluefin tuna dominates across entire Atlantic Ocean, with tagging study revealing vast migrations, stock mixing and pressure on Mediterranean fisheries.
Landmark Tagging Study Tracks Tuna Across Entire Atlantic
A major international study has confirmed that Atlantic bluefin tuna move extensively across the entire North Atlantic, ignoring stock boundaries that have long underpinned fisheries management.
The research, published in Proceedings of the National Academy of Sciences, analysed electronic tagging data from more than 1,700 fish collected over 30 years in waters off the United States, Canada, Norway, Ireland and the Canary Islands.
Leif Nøttestad, a co-author of the study, said the species operates on a fully international scale. He stated that “although Atlantic bluefin tuna is mainly fished in the Mediterranean during the spawning period, it migrates and feeds across the entire North Atlantic for most of the year and its life”.
Younger Tuna Cross Atlantic Between East And West Stocks
The study identifies previously undocumented behaviour, particularly among younger fish.
Tagged tuna show that some individuals cross the Atlantic Ocean, moving between the eastern and western stocks. This challenges long-standing assumptions about separation between the two management units.
Nøttestad explained that stricter size limits and lower fishing pressure in North American waters may provide a form of indirect protection for eastern-origin fish entering those areas.
Researchers suggest that this dynamic may have contributed to the recovery of the eastern stock in recent years, although the study stops short of establishing direct causation.
Larger Spawning Fish Remain Linked to Eastern Atlantic
While younger tuna display wide-ranging movement, larger fish appear more consistent in their migration patterns.
Keno Ferter, who leads Norway’s tagging programme, said that fish weighing between 200 and 500 kilograms return annually to Norwegian waters after spawning in the Mediterranean.
He stated that “we see few signs that these fish cross to the western Atlantic, although they may have done so earlier in life”.
Data shows that these fish typically feed along the Norwegian coast during summer before moving towards the central Atlantic or Canary Islands during winter. By May, they gather in the Mediterranean to spawn, before migrating north again from June.
Mediterranean Fishery Remains Central to Stock Distribution
The study underlines the significance of fishing activity in the Mediterranean, where most harvesting of eastern Atlantic bluefin tuna takes place.
Nøttestad said that fishing on spawning aggregations has a disproportionate impact, stating that “fishing there is almost like going to a supermarket compared to fishing in Norway”.
He warned that overfishing of large, high-value fish in the Mediterranean would first affect the component of the stock that migrates to northern waters.
This reinforces the importance of maintaining strict management measures to ensure long-term sustainability.
Tagging Data Reveals Interaction With Global Fisheries
Tagging work has also provided insight into how tuna interact with fishing operations beyond Norwegian waters.
Fish tagged in Norway have been recorded in Mediterranean holding cages and, more recently, on a Japanese longline vessel operating west of Ireland.
Ferter said the project continues to deliver new information, stating that “we are learning not only about the fish, but also about the fisheries”.
Future phases of the programme will deploy tags capable of remaining attached for two years, allowing researchers to determine whether observed migration patterns are consistent over time.
