The true cost of unsanctioned fishing in Pacific tuna fisheries - potentially stretching over $600 million - has been condemned as "larceny on the high seas".

The figure has been revealed in a new, 100-page report prepared by marine consultant company MRAG Asia Pacific for a Pacific economic forum.

It is the first attempt to quantify the volume and value of illegal, unreported and unregulated (IUU) fishing in Pacific tuna fisheries.

IUU is a broad definition that covers vessels fishing in state jurisdictions without permission, misreporting or not reporting harvest volumes, or flouting conservation and management measures.


Using estimated volumes between 2010 and 2015, and built on the best information available, the report suggested IUU fishing was stripping between 276,000 and 338,000 tonnes from the fisheries with a value of $517 million to $740 million.

The report further found the volume of IUU product was highest in the purse seine fishery - accounting for around three quarters of it - while the tropical longline and southern longline fisheries accounted for around 19 per cent and 11 per cent respectively.

Among the main targeted tuna species, skipjack was the largest proportion, about 33 per cent of total IUU volumes, and more than 100,000 tonnes. Yellowfin was the next largest volume at 31 per cent and 96,126 tonnes, or an equivalent 15 per cent of the total yellowfin catch in the Western and Central Pacific Fisheries Convention Area in 2014.

Although the report was described by its authors as a "first cut", the estimates were still lower than the $700 million to $1.5 billion estimate for IUU fishing in the western and central Pacific.

It also found the estimates were dominated by licensed fleet vessels, and that relatively high levels of IUU fishing in coastal states on the western Pacific seaboard influenced the overall results.

The authors suggested a range of ways to tackle the problem, such as stronger supply chain monitoring, electronic technology and observers on vessels.

"The reality is that, for Pacific Island nations, this is their exclusive economic resource," said Shane Jones, New Zealand's Ambassador for Pacific Economic Development.

"Every dollar filched from fishing is a dollar denied to our Pacific neighbours. The major challenge for New Zealand and our Pacific neighbours is the larceny that is happening on the high seas."