For millions of years the giant bluefin tuna has been one of the most successful predators in the ocean.
But serious overfishing is threatening to wipe out the species.
Stocks in the Mediterranean have fallen close to a level at which the tuna will be unable to breed.
Scientists are beginning a tagging process to find out if anything can be done to save it.
Al Jazeera’s Nick Clark reports from the Catalan coast in Spain.