Charles Clover: This is the blue whale of our time

The Atlantic bluefin tuna is a dark, steel-blue teardrop of a fish which migrates across whole oceans and can swim at speeds of up to 45mph. Its acceleration, as it locks on to a ball of bait-fish, has been likened to a supercar.

The bluefin can turn on this electric display because it is one of the most highly developed fish, and warms its blood through a heat-exchanger so more energy can be released on a whim.

This top-level predator's only problem is that its flesh is one of the most delicious things on earth, eaten raw as sushi or sashimi. It is only marginally less delicious the Mediterranean way, seared with a dressing of oil and vinegar.

Bluefin flesh was eaten by Roman legionaries before battle. Its entrails, treated with herbs, were a delicacy known as garum, pots of which have been found throughout the former Roman Empire and as far north as the garrison town of York.

The British until recently called the bluefin "tunny" (from the Latin, Thunnus thynnus). Tunny hunted shoals of herring off Scarborough, where well-heeled anglers sought but never caught a 1,000lb specimen – the world rod-caught record is 1,496lb, off Nova Scotia in 1979. The run of North Sea tunny had mysteriously disappeared by the 1950s.

There are two remaining populations of bluefin; a western one, which spawns in the Gulf of Mexico, and an eastern one that spawns in the Mediterranean. The western stock was hammered close to extinction before the United States and Canada protected it and allowed only anglers to catch it, though the skippers make a fortune by exporting the carcasses to Japan.

The eastern stock is now close to collapse, mainly because of rampant overfishing for tuna "farms" that supply the Japanese market.

Scientists discovered only recently though that some of the eastern and the western stocks mix and interbreed, swimming the whole Atlantic ocean to do so. So the collapse of the bluefin now being predicted is a crisis of Atlantic proportions.

For conservationists, it is the front line. The bluefin is the blue whale for our time.

The End of the Line, the film based on Charles Clover's book, has its national premiere on 8 June, World Oceans Day