After nearly six decades of absence, Atlantic bluefin tuna have returned to the waters between Sweden, Denmark, and Norway, and a new study reveals what's bringing them back: an abundance of energy-rich schooling fish, particularly garfish, herring, and mackerel.
The groundbreaking research, published in Fisheries Research, provides the first modern insights into the feeding ecology of Atlantic bluefin tuna (ABFT) in the Skagerrak-Kattegat-Sound region.
An international team of researchers led by Swedish University of Agricultural Sciences and DTU Aqua analyzed the stomach contents of 44 giant bluefin tuna caught between 2020 and 2022, revealing a diverse diet dominated by locally abundant prey species.
According to lead author Dr. Gustav Hellström from the Swedish University of Agricultural Sciences (SLU) what's particularly striking is the prevalence of garfish in the diet:
"This species is neither commercially targeted nor managed in the region, yet it appears to be a critical food source for these returning tuna,” says Gustav Hellström.
A Conservation Success Story
Atlantic bluefin tuna populations crashed dramatically in the mid-20th century due to overfishing, and their seasonal migrations to Scandinavian waters came to a near-complete halt by the early 1960s.
Following the implementation of a recovery plan in 2008, including catch restrictions and increased minimum landing sizes, bluefin tuna biomass in the Northeast Atlantic and Mediterranean has increased substantially.
The species began reappearing in Danish and Swedish waters around 2015-2016, commonly observed between August and October. The return may be augmented by the recovery of migratory prey fish stocks in the region, such as herring and mackerel, driven by improved European Union fisheries management policies.
The researchers emphasize the need for continued monitoring of both bluefin tuna diets and the populations of key prey species, including non-commercial species like garfish.
The findings underscore the importance of ecosystem-based management approaches that consider the complex interactions between predators and prey.
Ecosystem Implications
As top predators capable of consuming 3-6% of their body weight daily, the researchers estimate that a single 278-kg bluefin tuna can consume between 750 to 1,500 kg of prey over its typical 90-day stay in the region.
If populations continue to recover to historical levels, bluefin tuna could become a significant mortality factor for local prey populations.
"These fish are not just indicators of ecosystem health — they're active players in shaping the food web," says co-author Dr. Kim Birnie-Gauvin from DTU Aqua and adds:
. "Understanding what they eat helps us better manage the entire ecosystem, not just individual species."
Effective conservation measures
Gustav Hellström from SLU emphasizes that the return of bluefin tuna to Nordic waters is a testament to what effective conservation measures can achieve." But also that maintaining healthy populations of their prey — including species we don't traditionally manage — will be crucial for ensuring they continue to thrive here.