More than twice as many tonnes of Atlantic bluefin tuna were sold last year compared with official catch records for this threatened species, according to a report released on Tuesday.

This "bluefin gap" occurred despite enhanced reporting and enforcement measures introduced in 2008 by the 48-member International Commission for the Conservation of Atlantic Tunas (ICCAT), which sets annual quotas by country, it said.

Trade figures showed that real catches of bluefin in 2009 and 2010 totaled more than 70,500 tonnes, twice ICCAT's tally for those two years, according to the report compiled by Washington-based Pew Environment Group.

"The current paper-based catch documentation system is plagued with fraud, misinformation and delays in reporting," said Roberto Mielgo, a former industry insider and author of the report. "Much more needs to be done."

Before 2010, ICCAT systematically set fishing quotas substantially higher than the recommendations of its own scientific committee, which had warned repeatedly that stocks were in danger of crashing.

In 2010, the target quota -- 12,900 tonnes for fish caught in the Mediterranean and Northeastern Atlantic -- fell for the first time within the panel's recommended range.

But the new report, released ahead of a November 11-19 ICCAT meeting in Istanbul, implies the industry has circumvented the catch limits and tougher compliance measures.

"This (catch) gap exists mainly because of loopholes in the ranching industry," Mielgo told AFP.

He referred to the practice of netting young wild tuna in the Mediterranean and then corralling them for fattening, a system he helped to pioneer.

"Essentially, more bluefin tuna are harvested from the ranches than initially reported when they are first transferred there."

About 70 percent of the bluefin caught in the Mediterranean are netted by industrial purse-seine vessels with vast, sack-like nets that encircle tuna as they gather to spawn.

The Pew study says that reported catch for bluefin tuna from 1998 to 2010 was 395,554 tonnes.

By comparison, market figures for this period show 491,265 tonnes, leaving a gap of nearly 100,000 tonnes worth some two billion euros (2.7 billion dollars) at wholesale prices.

These figures do not include what experts say is a sizeable black market in bluefin, along with fish that has been mislabeled as another species.

Japan consumes 80 percent of the gleaming fatty fish, with American appetites accounting for another nine percent.

Pew and other environmental groups have called for the swift implementation of electronic tagging of fish to replace paper-based records, which they say are vulnerable to fraud.

A new electronic system "should include a real-time reporting requirement and central database, which would allow information from ranches and vessels to be cross-checked instantly," said Lee Crockett, head of Pew's bluefin task force.

ICCAT has committed in principle to such measures, but the timetable for implementation and funding have yet to be settled.

The Pew report is based on official ICCAT statistics, as well as trade figures from Japanese customs, the US Department of Agriculture, Eurostat and the Croation Chamber of Economy.

The analysis avoided double counting tuna raised over two years in off-shore ranches in the Mediterranean, as well as frozen stockpiles in Japan.

This year, the International Union for Conservation of Nature (IUCN), the reference agency for species conservation, declared Atlantic bluefin tuna to be "endangered" and "susceptible to collapse under continued excessive fishing pressure".