By Richard Black
Environment correspondent, BBC News website
There is a kind of theft that happens every day in a majority of the world's poor countries - and in many of the richer ones too.
Illegal fishing deprives local catchers of their harvest and their income
It usually happens out of sight, and most perpetrators get away with it.
The monetary value of this theft is about $15bn per year; the ecological cost can only be guessed at.
Yet many people would turn their noses up if they chanced upon a trove of this treasure.
Because these jewels are fish.
"Those that are fishing illegally, they are paying nothing, so we are losing something from our country", says Mamadou Diallo, programme manager for the environmental group WWF's West Africa office, and a former fisheries officer.
One of the biggest problems is identifying the owners, because many of these ships fly flags of convenience
Environmental Justice Foundation
The amount that Africa is losing, if new figures from David Agnew of Imperial College London are right, is about $1bn per year - the cost of licences that illegal fishers should have paid to catch what they are catching.
The ecological cost may, in the long run, be much higher.
"The immediate ecological impact is damage to habitat, because they are using trawls, and trawls are not always good for the ecosystems - they damage habitat for fish," says Dr Diallo.
"The second thing is pollution, because they are discharging at sea, and they can do anything they want."
Precisely how much fish is removed illegally from West African waters is not known - apart from anything else, there is little good data on the state of stocks before the plunder began.
Elsewhere, where ecosystems and commercial fish numbers have been studied for longer, it is clear that illegal fishing can help wreak major damage.
In the Mediterranean Sea, where scientists estimate that illegal catches of bluefin tuna in recent years have almost matched legal catches in weight, changes are afoot.
"Some stocks are on the brink of collapse; but also, something remarkable has happened in the Mediterranean in the last couple of decades," says Ricardo Aguilar, research director for the conservation group Oceana.
"Most of the vertebrates are over-exploited, so populations of invertebrates are growing - that's one of the reasons why we have more jelllyfish."
The Med, like the North Sea, has seen politicians repeatedly raise legal catch quotas well above the levels that scientists recommend - but it is equally clear that illegal fishing contributes to the decline in these stocks.
It is hardly likely to be otherwise elsewhere.
Crossing the line
Fishermen have a whole raft of dodges and evasions that fall on the wrong side of the law.
They may fish where they do not have a licence. Or they may have a licence, but flout its terms by going to forbidden areas, or fishing at the wrong time of year, or targeting prohibited species.
Another class of infraction concerns equipment. Some fishing methods are widely banned, notably driftnets, the subject of a 1992 UN prohibition on the high seas.
Many authorities regulate the size of mesh allowable in fishing nets; and many fishermen use a mesh size below that legal limit, in order to prevent the young ones escaping.
Marine creatures such as turtles can be caught in fishing nets
In the Mediterranean, using planes to spot swarms of tuna is banned; but evidence from several sources suggests the practice continues.
In some jurisdictions, regulations prohibit trans-shipment - transfer of catch from one vessel to another - because when a box of fish has been passed around between ships and perhaps re-labelled in the process, tracing its origin is next to impossible.
Which is precisely why some dodgy operations love trans-shipment.
And there is more. As well as illegal fishing, there is unreported and unregulated fishing - "unreported" when fleets do not report their catches to the appropriate national or trans-national authorities, and "unregulated" meaning operations that may just be legal, but which damage fish stocks and the wider marine ecosystem.
When regulators put it all together, they use the term IUU fishing; and they talk of banning it from the oceans.
So far, it does not seem to be working.
"We found IUU fishing was rampant in West African waters," says Duncan Copeland of the Environmental Justice Foundation (EJF), which last year published results of an investigation into how much IUU fish ends up on British dinner plates.
"There are lots of Asian nations involved - Chinese vessels, Korean vessels - but one of the biggest problems is identifying the owners, because many of these ships fly flags of convenience."
Report after report echoes EJF's conclusions - that flags of convenience, where a country such as Honduras or Panama registers ships for nominal sums and administers little regulation in return, are a major problem for regulators trying to keep fishing legal.
The 2001 UN Plan of Action on IUU fishing, for example, identifies a phenomenon it calls "flag hopping - the practice of repeated and rapid changes of a vessel's flag for the purposes of circumventing conservation and management measures".
Getting a grip
There are some signs, at last, that governments are fed up with the theft.
The European Union recently approved a set of measures recognising that it "has a specific responsibility in making sure that fisheries products imported into its territory do not originate from IUU fishing".
Many African countries reserve coastal zones for traditional artisanal boats
Measures include mandating that imported fish is certified as legal, restricting trans-shipments, black-listing ships known to indulge in illegality and imposing trade sanctions on countries that do not regulate the vessels they flag.
"It's fantastic step," says Duncan Copeland, "but it has to be implemented and then enforced - if it is, it'll make a tremendous difference."
Some countries go to extraordinary length to tackle illegal fishers.
In 2003, Australian coastguards chased a Uruguayan ship they suspected of poaching Patagonian toothfish (also known as Chilean sea bass) for 7,000km across the oceans.
Most African countries could not contemplate such an adventure.
"We don't realy have the resources for surveillance," says Mamadou Diallo.
"Our coastlines are very long; and if you want to patrol the coast, you have to be at sea all the time."
Another part of the EU plan is to help coastal African nations acquire the resources they need.
But even where the resources exist, the lesson of Europe is that they are not always deployed.
Ricardo Aguilar says that while EU countries may be hot on preventing IUU fishing by foreigners, they may actually aid the trade when their own nationals are involved.
"We find illegal trawlers almost everywhere in Spanish waters," he says.
"In Italy they use driftnets, which are banned; while the French routinely exceed their quotas."
The experts agree that combatting illegal fishing - preserving the resources for legal exploitation by fishermen who may need the money more - is a question of two things - resources and political will.
This week, ministers from southern African nations are meeting in Namibia to discuss what they can do, and what assistance they need.
There are many examples of what can happen when regions are rapaciously fished - the Grand Banks off Newfoundland, where cod stocks may have collapsed permanently, is perhaps the starkest.
That collapse was predictable, because fishing was legal and therefore quantified.
The point about IUU fishing is that it is a wrecking ball for your ecosystem and your fishers' livelihoods whose impact you cannot anticipate.
It is vital that African nations, and everyone else, find a way to stop the wrecking soon.