Is the climate crisis driving rare bluefin tuna to British waters?

Not far off the coast of Britain, great shadows can be seen racing beneath the surface of the sea.

And for those looking in the right place at the right time, occasionally the water is broken, and the rigid silver bodies of muscular bluefin tuna glint momentarily in the light.

These almost forgotten apex predators appear to be back in British waters decades after their populations ebbed away amid major concerns of overfishing.

Amid an apparent rise in numbers, the UK is conducting a catch and release tagging programme which will give scientists a greater level of insight into the numbers of these fish around the British Isles.

According to the government’s Centre for Environment, Fisheries and Aquaculture (Cefas) in recent years Atlantic bluefin tuna, a migratory species, have been recorded arriving in May and staying as late as January. However, peak numbers were recorded between August and October each year.

We examine what’s happening and what it means for the UK.

1. Are bluefin tuna returning to the UK?

   Yes, it’s true, after an absence of more than 60 years, the mighty bluefin tuna are roaring back to British waters in great numbers.

   The first sightings were in 2014, but since then, more of the fish have been sighted increasingly regularly, and have also been recorded in larger group sizes.

   According to the government, “scientific surveys and sightings by members of the public suggest an increased number of them in UK waters”.

2. Where are they? What are they doing?

   In recent days bluefin tuna have been filmed zooming through waters just off St Ives in Cornwall to eat scraps thrown for them, and large numbers were seen off Start Bay, south of Kingsbridge in Devon.

   In south Cornwall, off the small village of Portmellon, close to Mevagissey, a kayaker reported seeing as many as 20 bluefin tuna, alongside dolphins and porpoises.

   But the species has also been seen as far north as Scotland where they were also a previously common sight, but have not been seen in decades. This week a large dead tuna was found washed up on the shore of a highland loch.

   In October last year, a bluefin tuna weighing an estimated 300kg - 47-stone – was caught in a fish farm off Great Bernera in Loch Roag, Lewis, according to Scottish newspaper The Herald.

   This large specimen, some 50 times larger than the salmon in the pens, was caught by farm workers and released back into the sea.

   And they have also been turning up in Irish waters in recent years. In September three fisherman caught an 8-foot-long specimen reportedly worth £3m to the restaurant trade. However, the fish was caught as part of a tag-and-release programme and was then let go.

3. Could they attack humans?

   No! While they are big animals which can move fast, they are not interested in attacking humans.

   According to marine biologists, they tend to target swarms of fish such as herrings, anchovies and sardines, in order to maximise feeding potential.

4. Ok, what’s so great about them?

   There aren’t plenty more fish in the sea anymore. We’ve hoiked a devastatingly large number of them out, especially tuna, leaving a limited supply which not only makes them a rarity, but also raises their market price – thereby increasing the desire to catch them.

   This is a shame, as they are astonishing predators and play a vital part in maintaining functional ocean ecosystems.

   Bluefin tuna have been recorded at up to 680kg (107 stone) in weight, and rival the marlin and swordfish as the largest perciformes – a group containing about 41 per cent of all bony fish, including over 10,000 species.

   They can grow up to 3.7m (12 feet) in length, and swim at speeds of 43mph (70kmh). The WWF describe them as the “Ferraris of the ocean – sleek, powerful, and made for speed”.

   Bluefin typically hunt small fish such as sardines, herring, and mackerel, and invertebrates such as squid and crustaceans.

5. What caused their previous decline?

   Overfishing was part of the problem, though not so much around the UK, as by the time they’d reached their greatest popularity, they had largely left British waters.

   At the peak of their overfishing in the mid 1990s, an estimated 50,000 to 61,000 tonnes of bluefin tuna caught annually in the East Atlantic and Mediterranean Sea. Industry regulations were subsequently tightened.

   In 2021, the International Union for the Conservation of Nature (IUCN) changed their entry for Eastern Atlantic bluefin tuna from “endangered” to “least concern”.

   The UK government said: “This reflects the improving state of the stock. The only recent labelling of Eastern Atlantic bluefin tuna as “least concern” underlines a need for a continued cautious approach to its management.”

   However, it is now thought changing water temperatures could have played a role in the decline of tuna in UK waters.

6. So why are they back?

   According to researchers, the key driver for the return of the bluefin is largely unrelated to fishing, and is much more about the ranges they inhabit due to warming and cooling waters.

   While the climate crisis, and the warming surface waters is likely to play a role, scientists have also identified another contributing factor – the influence of a 60-120 year cycle which brings warmer waters further north.

   A 2019 study said there was strong evidence the phenomenon, known as the Atlantic Multidecadal Oscillation (AMO) was bringing bluefin tuna further north, as has been seen in the past.

   The AMO affects a complex range of atmospheric and oceanic processes in the northern hemisphere, including the strength and direction of ocean currents, drought on land, and even the frequency and intensity of Atlantic hurricanes.

   Around every 60 to 120 years the AMO switches between positive and negative phases which shifts the distribution of Atlantic bluefin tuna.

   During a warm AMO phase, which has been the case since the mid-1990s, bluefin tuna have been sighted as far north as Greenland, Iceland and Norway, while almost disappearing from the central and south Atlantic, according to the researchers from the University of Lille in France.

   During the AMO’s last warm phase in the middle of the 20th century, the North Sea had a bluefin tuna fishery in Scarborough that rivaled those in the Mediterranean.

   The UK’s catch, tag and release programme is designed to shed greater light on the return of these magnificent fish.