Image caption The vast majority of bluefin flesh ends up in sushi and sashimi restaurants

Bluefin tuna boats in the Mediterranean Sea continue to catch many more fish than they report, a study concludes.

Commissioned by the Pew Environment Group, it finds that last year 140% more bluefin meat from the Med entered the market than was reported as caught.

The fishery's regulator, Iccat, put new measures in place two years ago aimed at stopping over-fishing, but Pew found there were still holes in the system.

The Atlantic bluefin is so depleted as to qualify as a threatened species.

In 2008, member governments of Iccat - the International Commission for the Conservation of Atlantic Tunas - agreed to implement a system of paper-based catch records, in principle allowing fish to be tracked from the sea to their final destination.

This was intended to remedy severe flaws in the system that had been identified in a number of reports, including one by the International Consortium of Investigative Journalists (ICIJ).

But the Pew investigation found several ways in which the numbers can still be fiddled.

"The ICIJ report covered the year to 2008, and in response Iccat said 'well that was then' - so we thought, 'we'll see'," said Lee Crockett, who directs Pew's Atlantic tuna work.

"And as you can see, they clearly haven't fixed the problem - in fact, the gap [between the reported and total catches] has increased, which is a pretty clear indication that they need to do a much better job of making sure that the catch reports track the quota," he told BBC News.

Missing reports

How are these vessels over-catching when they're supposed to have an independent observer on board? Roberto Mielgo Bregazzi, Pew consultant

The research involved scanning through available trade data - exports from EU nations, Japanese customs documents, the US Department of Agriculture's Foreign Agricultural Service - and comparing these with catch reports from Iccat's member governments.

As such, Pew argues that it probably under-estimates the scale of the problem as it does not include catches by straightforward illegal fishing operations, for which there are by definition no records.

In 2008, they calculate, just over 38,000 tonnes from the Med was traded internationally, against a Total Allowable Catch (TAC) quota of 29,082 tonnes.

In 2010, the quota had been slashed to 13,525 tonnes; but Pew's estimate for the actual catch was just over 35,000 tonnes.

Iccat's scientific advisers recently reported that in their eyes, there had probably been a marked drop in the actual catch in the last few years.

"Declared catches in 2010 were significantly below the 2010 TAC of 13,500 tonnes," they wrote.

"However, some [Iccat member countries] did not report their 2010 catch."

Caged beasts

After they are caught, many of the tuna are towed in cages to "farms" or "ranches" in Spain, Croatia and Malta.

There, they are fattened before being killed and exported - the vast majority to Japan, where the flesh is highly prized in sashimi.

Image caption Purse seine boats usually target the bluefin as they come together to reproduce

Roberto Mielgo Bregazzi, who researched the data for the Pew report, said this was where many of the problems lay.

"Most of the over-catch being produced in the Med in my opinion comes not from illegal fishing boats but from the legal purse seine fleet," he said.

"And the question is, how are these vessels over-catching when they're supposed to have an independent observer on board, when they're taken to a farm where another observer is waiting to watch the transfer to the farm?

"What really happens is there is no effective way to actually calculate and certify first of all the amount in kilos that's being transferred into a cage, and second the number of fish."

Divers are sent down into the purse seine nets to estimate the number of fish and the total mass caught - but Dr Mielgo, a former tuna diver himself, said this was a job that could only be done accurately by an experienced operative.

Size does matter

The root cause of the problem, he said - and he is far from the first to make this point - is that boats have far more capacity to catch tuna than they are legally allowed to.

Many Mediterranean fleets acquired new, more powerful purse seine boats in 2005-6; and having invested in them, owners now need to catch above quota to break even, he said.

"Let's say your vessel was conceived to catch 300 tonnes per year and break even at that point.

"If your quota has been cut and you are not able to catch 300 and sell at a given price, then overheads start eating your budget and you cannot make a profit.

"So you are bound to overfish. The best way is to catch 200 tonnes and declare 50 - then you can continue fishing."

This is a point picked up by Iccat's scientific advisers, who write: "While current controls appear sufficient to constrain the fleet to harvests at or below TAC, the Committee remains concerned that substantial excess capacity remains which could easily harvest catch volumes well in excess of the rebuilding strategy adopted by the Commission."

Pew's main recommendation is to change from a paper-based monitoring system to an electronic one.

That way, it believes, data will have to be properly entered (which is not always the case now), and algorithms can be established that will flag up potentially dodgy entries - such as a consignment that has apparently been fattened beyond feasible limits in ranches.

Iccat's member governments meet next in Turkey in mid-November.