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Global measures to regulate the fishing industry lack the capability to tackle illegal catches, warn researchers.

Writing in the journal Science, they say that up to 26m tonnes of fish, worth an estimated $23bn (£16bn), are landed illegally each year.

They add that a global monitoring and information sharing network is needed to crack down on illegal operators.

Eighty percent of the world's fish stocks are deemed to be fully or overexploited.

"Illegal, unreported and unregulated (IUU) fishing is a global problem and it needs a global answer," said co-author Kristin von Kistowski, a senior adviser to the Pew Environment Group, a US-based think tank.

"By creating this first comprehensive overview of port state performance, we have identified the weaknesses and problem in the system."

Closing the net

Under the UN's Convention on the Law of the Sea, the control of a vessel's activities are the responsibility of the "flag state", the nation where the boat is registered.

But in November 2009, in an effort to strengthen measures to tackle IUU fishing, the UN approved a legally binding Port State Measures Agreement (PSMA) that would require the "port state" to close its ports and ban the landing of fish of any vessel listed as being involved in illegal or unregulated activities.

To date, 15 nations and the EU have adopted the PSMA, but it needs 25 nations (the EU counts as one nation in this case) to ratify the agreement in order for it to take effect.

"All of the problems we encountered as we attempted to compile data - on the vessels, their movements and the actions taken by ports they visited - were telling in themselves as far as the weaknesses of the system are concerned," Dr von Kistowski told ����ý News.

The research highlighted three key concerns:

Chatham House, the London-based international affairs think tank, says IUU fishing has a number of "serious consequences".

It states that it "not only leads to depletion of fishing stocks, which can have serious knock-on effects on ecosystems, but it also deprives often poor communities of their livelihoods and can cost governments millions of dollars in lost revenues".

"it is not enough to have something on paper," Dr von Kistowski told ����ý News.

"We saw that there was inadequate implementation... and compliance was far from perfect.

"Even when the legally binding port state agreement comes into force, it does not mean that it will be ratified by all countries, so there will be loopholes.

"This worldwide problem of IUU fishing will need to have a system with greater transparency, accountability and global reach in order to be effective."

The findings, compiled by an international team of researchers, will be presented at the UN headquarters in New York next week during a review conference of the UN Fish Stocks Agreement.