The bluefin tuna, which has been endangered for several years and has the misfortune to be prized by Japanese sushi lovers, has suffered a catastrophic decline in stocks in the Northern Pacific Ocean, of more than 96%, according to research published on Wednesday.

Equally concerning is the fact that about 90% of specimens currently fished are young fish that have not yet reproduced.

Last week, one fish sold in Japan for more than £1m, reflecting the rarity of the bluefin tuna and the continued demand for its fatty flesh, which is sold for high prices across Asia and in some high-end western restaurants.

Bluefin tuna is one of nature's most successful ocean inhabitants, the biggest of the tuna and a top-of-the-food-chain fish with few natural predators. But the advent of industrial fishing methods and a taste for the species among rich sushi devotees have led to its being hunted to the brink of extinction.

If current trends continue, the species will soon be functionally extinct in the Pacific, and the frozen bodies held in a few high-security Asian warehouses will be the last gasp the species.

More than nine out of 10 of the species recently caught were too young to have reproduced, meaning they may have been the last generation of the bluefin tuna.

Amanda Nickson, of the Pew Environment Group, which produced the latest report, said: "There is no logical way a fishery can have such a high level of fishing on juveniles and continue."

She said that urgent measures needed to be taken in order to preserve stocks and allow them to recover. "The population of Pacific bluefin is a small fraction of what it used to be and is in danger of all but disappearing," she said. "It's a highly valuable natural commodity and people naturally want to fish something that gives them such a high return."

She called for fishing of the species to be halted as a matter of urgency. Although there are measures to manage the exploitation of bluefin tuna in the Atlantic, and some measures in the eastern Pacific, the main spawning ground for Pacific bluefin tuna in the western part of that ocean is not managed. The main fishing fleets exploiting the stocks are from Japan, Mexico, South Korea and the US, and the high value of the few remaining fish is a further encouragement to fishermen to hunt down the last of the species. A single specimen could make the catchers rich for life, and without catch limits and rigorous enforcement, there is nothing to stop fishermen pursuing them.

Nickson said: "This assessment shows just how bad the situation really is for this top predator. This highly valuable fish is being exploited at almost every stage of its life cycle. Fishing continues on the spawning grounds of this heavily overfished tuna species."

About two-thirds of the world's tuna comes from the Pacific, but bluefin tuna accounts for only about 1% of this. For years, the species was neglected in fisheries management, being lumped in with other more prolific species. But in recent years it has become clear that it was in danger, from overfishing and its own biology - being bigger than other tuna, it takes longer to come to sexual maturity, which scientists estimate takes between four and eight years, which limits its reproductive ability and makes it more vulnerable to the predations of modern industrialised fishing techniques.