I had been gazing at the surface of the sea for more than an hour from the deck of a fishing boat, just off Zahara de los Atunes on Andalusia’s Costa de la Luz. Our boat, the Tierra, was one of four making up the sides of a square, the men on board gradually raising the net that was slung between them. We were nearing the climax of the almadraba, a technique used to catch bluefin tuna on this stretch of coast since the Phoenicians settled 3,000 years ago.

With seven others, I had set off from the harbour soon after dawn on a sunny Saturday morning in May. We were led by Ignacio Soto, who runs Nature Tarifa, the only company that takes people to witness the almadraba from the proximity of the trap itself. Recognised by the European Parliament as a sustainable fishing method in a 2015 study, it uses a maze of nets, anchored to the seabed, to catch tuna as they follow their migratory route from the cold waters of the Atlantic to the Mediterranean to reproduce. As they swim close to the coast in search of the entrance to the Mediterranean through the Strait of Gibraltar, some of the fish are diverted into the nets, and funnelled through them until they reach a final chamber, the copo. Once a day from the end of April to the beginning of June, the floor of the copo is lifted to the surface — the moment we were waiting to see.

There are four almadrabas along this stretch of coast, set up off Conil, Barbate, Zahara de los Atunes and Tarifa, but together they catch a tiny percentage of the tuna that pass through these waters. The size of the mesh means that only mature fish are caught, usually more than 10 years old, which will have already reproduced several times. The fishermen adhere to a strict quota, which is increasing every year as stock levels rise. “Until about five years ago, nearly all the catch from this Zahara almadraba went to Japan, but now more than half stays here,” said Soto. “This is the best bluefin tuna you can get and people in Spain and the rest of Europe now want to eat it, too.”