Bluefin Tuna and Young Tope Off the Belgian Coast

This autumn, a maintenance technician on the Thorntonbank wind farm, about 30 kilometres off Zeebrugge, watched three large fish break the surface. Each was roughly one and a half to two metres long. They shot out of the water at speed and vanished again, moving "like torpedoes," as the VRT NWS headline put it. He has eight years of experience watching marine life such as porpoises, dolphins and seals, and the Flanders Marine Institute (VLIZ) considers the sighting reliable: Atlantic bluefin tuna, hunting in Belgian waters for the first time since the 1960s. In the same week, a VLIZ survey about 23 kilometres offshore caught three young tope, a shark whose juveniles had not turned up in those nets since 1985.

This piece explains what these two fish are, why they left this part of the North Sea, and how researchers follow them once they are back.

What is a bluefin tuna doing in the North Sea?

Bluefin tuna are built for distance. An adult can grow past three metres and weigh several hundred kilograms. Most fish run at the temperature of the water around them. A bluefin keeps its swimming muscles warmer, using a mesh of blood vessels that hands heat from outgoing to incoming blood before it can escape at the gills. That warmth lets it hunt in cold northern water. The eastern Atlantic population spawns in the Mediterranean in early summer, then adults travel north to feed on schooling fish such as herring and mackerel before heading south again.

The North Sea used to be one of those feeding grounds. From the 1950s, Norwegian purse seiners, boats that close a long net around a whole school, landed thousands of tonnes of large bluefin a year in the North Sea and the Norwegian Sea. In 1963 that fishery collapsed within a season, and the fish all but disappeared from northern Europe for half a century.

Why did the tuna disappear?

There is no single proven cause. The leading explanations combine pressures that peaked in the same years. Catches across the tuna's whole range were heavy, including intensive fishing along the Spanish coast and in the Bay of Biscay as the fish migrated past. North Sea herring, the tuna's main prey this far north, collapsed in the same decades, also from overfishing. And when a migrating population shrinks, the far ends of its route tend to be the first places it stops reaching. Researchers who have worked through the old catch records argue that fishing, food and migration routes acted together.

Why are they coming back now?

From about 2014, anglers and fishers in Denmark and Sweden began reporting bluefin again, first a handful, then hundreds of sightings a year in the Skagerrak and Kattegat. Sightings off the Netherlands and the UK followed, and now Belgium. The most likely reason is management. A recovery plan agreed through the international tuna commission, ICCAT, in the late 2000s cut catches and raised the minimum size at which a tuna may be landed, giving young fish time to grow and spawn. With more adults in the eastern Atlantic, the old northern feeding routes are in use again.

Whether the turbines themselves play a part is an open question. The rock and concrete around their foundations quickly grow a cover of mussels and other life, and studies in Belgian wind farms have found fish such as cod and pouting gathering around them. A field of foundations can hold prey in one place, and a hunting tuna goes where the food is.

What is a tope?

Tope is a slender, grey shark of the open continental shelf that can reach about two metres. In Dutch it is called ruwe haai. It feeds on fish and squid and travels long distances along the coasts of the northeast Atlantic.

What makes tope fragile is its pace of life. A female matures at around ten years old, carries her young for about a year, and may breed only every third year. A litter can hold anywhere from six to more than fifty pups, with larger mothers carrying more. A population built on that rhythm needs decades to replace heavy losses. Tope has been listed as Critically Endangered worldwide since 2020, and Belgian fishers must return any tope they catch to the sea alive.

The three caught by VLIZ were young fish, and that is the interesting part. VLIZ does not yet know what role the North Sea plays for tope: whether young sharks stay along the Belgian coast for a while, or only pass through. Tagging is how that question gets answered.

How does a tag follow a fish?

Two kinds of tag do most of the work.

An acoustic tag is a small transmitter that sends a coded ping at regular intervals. Receivers anchored on the seabed hear it when the fish swims within a few hundred metres, and log the code and the time. Belgium has a network of these receivers along its coast and offshore, including in wind farms, so a tagged fish leaves a trail of check-ins as it moves. All three young tope now carry a tag.

A pop-up satellite tag works without receivers. It rides on the outside of a large fish such as a tuna and records depth, water temperature and light for months. On a set date it detaches, floats to the surface and sends its record to a satellite. From the daily rise and fall of light, researchers estimate where the fish was each day. Scandinavian teams have fitted bluefin in the Skagerrak with these tags in recent years, as part of ICCAT's research programme, to learn where the northern visitors come from and where they go to spawn.

What would a return mean for the Belgian part of the North Sea?

Three tuna and three sharks are a small sample. Both observations still point the same way: large predators that recover slowly are turning up again in a stretch of sea that lost them decades ago. If the tags show young tope staying, the Belgian coast becomes part of the species' life cycle, which shapes how it should be protected. If tuna around the Thorntonbank become a regular autumn sight, the next question is whether they follow herring and mackerel into Belgian waters each year, the way they once did further north.

For anyone on the water, a tuna breaking the surface or a tope in the net is worth noting with a time, a place and, if possible, a photo. Records like that are how a return gets confirmed.