Illegal tuna fishing in the Pacific has reached a "staggering" value of up to $740 million a year, a study released on Tuesday found.
The Pacific supplies about 60 percent of the world's tuna, an economic mainstay for some small island nations, but the Forum Fisheries Agency (FFA) study is the first attempt to quantify the impact of banned activities.
After two years of research, the European Union-funded study concluded 276,000 - 338,000 tonnes of Pacific tuna were taken illegally every year.
It estimated the value of the black market catch at $616 million, but said it could range anywhere from $520 million to $740 million.
"The results seem confronting when you hear them up front - the thought of $616 million dollars’ worth of illegal fish is staggering," FFA director general James Movick said.
He said the study would help the FFA - a Honiara-based organisation that helps regulate fishing in the waters of 17 Pacific nations - to combat the problem of illegal fishing.
Island nations typically have huge territorial waters but limited resources with which to monitor fishing activity.
Palau has only one long-range patrol boat to police an area of 500,000 square kilometres (193,000 square miles), roughly the size of Spain.
The report found the bulk of illegal fishing was carried out by trawlers which are licenced to operate in Pacific waters.
It said they either under-report their catch or transfer it to another vessel out of sight of monitors.
An Australian think-tank this week urged Canberra to play a more active role in protecting the Pacific fishery, which it said was approaching "tipping point."
The Australian Strategic Policy Institute (ASPI) said the tuna catch from the central and western Pacific was worth $5.8 billion in 2014.
But in a report released Monday, it said the fishery was under pressure from factors such as overexploitation, population growth and climate change.
The think-tank said it was in Australia's interests to protect the resource, describing it as a "game-changer" for some island nations.
"If regional fisheries were to become seriously depleted, [Australia] would be under considerable political pressure to provide greater economic support for most of our island neighbours, with possible long-term implications for political stability," it said.
Much of the over-fishing in the Pacific is blamed on so-called "distant water" fleets originating in Europe, the US and Asia.
ASPI said that in addition to helping island states with fisheries management and enforcement, Canberra should mount a diplomatic initiative to make conservation a priority for the distant water fleets.
It suggested the creation of an "Ambassador of Fisheries," with attachés in countries with large distant-water fleets such as Japan and Indonesia.
"If our fisheries engagement is done well. It will facilitate stronger relations to support our broad regional political, economic, social, environmental and security objectives," it said.