Tagging Blue Sharks to Uncover a Hidden Hotspot in the Bay of Biscay

Tagging Blue Sharks to Uncover a Hidden Hotspot in the Bay of Biscay

New tagging project explores blue shark movements in one of Europe's last telemetry blind spots
est. reading time: 6 minutes

The Most Fished Shark in the World

The blue shark is the most fished shark in the world, yet much of its behavior remains poorly understood. For decades, the Bay of Biscay has been a passageway for commercial fishing fleets and migratory species alike, but not a priority for shark science. The blue shark is thought to play a key ecological role in this dynamic marine system, yet data gaps persist. While fisheries commissions have begun including blue sharks in stock assessments, researchers still lack crucial movement and life history data needed to inform sustainable management.

That is starting to change. A growing research effort led by Dr. Maite Erauskin from the Basque research institute AZTI is now focusing on this overlooked part of the Northeast Atlantic, combining acoustic tagging, satellite telemetry, and catch data to investigate whether these migratory animals are in fact residents, and what that might mean for conservation and management.

Tracking Sharks of All Sizes

On a calm morning off the Basque coast, researchers from AZTI leaned over the side of a small vessel far offshore to release a blue shark freshly fitted with both internal acoustic tags and fin-mounted satellite tags. The animal slipped into the waters of the deep-sea trench below, its acoustic signal now traceable in one of Europe’s last telemetry gaps.

Over weeks of fieldwork, the team observed something unexpected: several females bore clear mating scars, and both adult males and young-of-the-year appeared in the same slope habitats.

“We saw individuals of all sizes, not only juveniles, but also large males and females. Some females had visible mating scars, and we even observed courtship behavior. It was surprising, and it suggests they may be reproducing here. That’s why we’re tagging them to understand what’s really going on in this area.”

🔍 From Migration Corridor to Year-Round Habitat?

The project uses a combination of internal acoustic transmitters and fin-mounted satellite tags, providing both large-scale movement patterns and fine-scale detection via a growing receiver network. Over the course of three years, the team plans to tag 75 blue sharks of different sizes and sexes, with 13 already carrying transmitters.

For years, the prevailing view was that blue sharks simply passed through the Bay of Biscay during the summer months on their way across the Atlantic. Scientists assumed the area served as a seasonal corridor rather than a place of prolonged residence. The hypothesis was that sharks entered from the north, lingered briefly, and then continued south toward Galicia or beyond. Without acoustic data, the Bay remained a blank spot on the map, presumed to be little more than a stopover site on a much larger migration.

“We were thinking that they just came here during the summer and then left,” said Dr. Maite Erauskin. “Now we are seeing that they are there all year round, and we have seen sharks of all sizes, not only the juveniles.”

Fieldwork over the past year has already begun to challenge this assumption. Sharks of different size classes have been tagged and observed in the Bay even in winter, and acoustic detections are now helping to build a clearer picture. While the team is still waiting for more tagging data to confirm patterns, early results suggest that some sharks remain in the area for extended periods while others migrate south, with several detections tracing individuals as far as the Azores.

🌍 Filling a Blind Spot

The blue shark is not the only commercially relevant species for which scientists lack detailed movement data, and the Bay of Biscay is more than just a shark highway. It’s a productive and diverse ecosystem supporting bluefin tuna, albacore, mako, porbeagle, thresher sharks, hammerheads, and even migratory species such as eels and salmon. Yet, in Europe’s vast network of acoustic receivers, this busy stretch of ocean has long been a blank spot — a gap that this project is beginning to close.

Starting this work meant adding new skills and infrastructure. For AZTI, it marked a leap into acoustic telemetry, including the design and deployment of an entirely new receiver array to monitor pelagic species.

“The idea is to have the array here to fill a gap in a very big network of receivers we already have in Europe and in the American side,” said Dr. Maite Erauskin. “We want to see if tagged animals are coming here and if the sharks we tag are going to other places. It’s a way to connect our region to a global picture.”

👩‍🔬 Follow Researcher's Work

Dr. Maite Erauskin Extramiana is a marine scientist at AZTI specializing in pelagic fish ecology, fisheries oceanography, and climate impacts. She leads the Bay of Biscay pelagic shark project, developing its first acoustic receiver array, and contributes to international research networks, conservation initiatives, and science outreach through publications, campaigns, and community engagement.

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