"The problem of illegal fishing is enormously widespread," observes Michael Lodge, an OECD fisheries expert.
Illegal vessels rarely come into port in order to avoid detection
"We have estimated the problem as being as much as 20% of the global catch."
Since 2000, the UN has been warning about the grave consequences of overfishing in the world's seas.
However, the impact of illegal fishing is adding to the strain on the already overexploited oceans.
The skippers of the illegal fishing boats tend to favour the waters of some of the poorest nations, which are often inadequately policed as a result of a lack of resources.
The Atlantic Ocean, off the coast of West Africa, is one of the most fertile fishing grounds in the world.
For centuries, the waters have supported generations of small coastal communities, but as the world's appetite for fish continues to grow, the rich fishing grounds have attracted the attention of illegal vessels.
Many developing nations do not have funds to police their waters
Almost half of the boats in the area are estimated to be operating outside the law.
Marine conservationist Helen Bours, who has been tracking illegal and unlicensed boats for more than 20 years, says that it is a hidden world of which very little is known.
"These vessels are at sea for years," she tells Television Trust for the Environment's (TVE) Earth Report programme on the BBC World News Channel.
"They transfer their fish on to other vessels, they get refuelled at sea; even the crews are changed at sea.
"So nobody sees what's happening, and there's nobody to go there and tell them to respect the rules. It's another world."
Fisheries experts from the UK government have attempted in recent years to assess the scale of the problem.
"In 2005, we commissioned a major study of the impact of illegal fishing on developing nations," said Tim Bostock, a fisheries advisor for the UK's Department for International Development.
"We were able to derive a total figure for the value of fish stolen from the world each year. This figure was of the order of US $9bn (£6.3bn).
Two inspectors from Guinea, during an expedition organised by Greenpeace and the Environmental Justice Foundation, headed out to sea with a list of the vessels authorised to fish the nation's waters.
Local fishermen say illegal fishing is threatening their way of life
From the air, a group of Chinese trawlers was spotted and after a quick check it was found that one of the vessels was not licensed.
"This vessel is under arrest for fishing without a licence in Guinean waters," explained Helen Bours.
"They never expect a surveillance patrol to come that far from shore because they know the Guinean authorities don't normally have the means to come out this far."
During the month-long expedition, about half of the 92 vessels spotted in the region were found to be fishing illegally.
Many of the unlicensed boats use huge weighted nets with a very fine mesh. These are scraped along the sea bed, scooping up everything in their path.
This method catches a very large amount of juvenile fish, wiping out the chance of these creatures reaching sexual maturity and spawning future generations to replenish the fish stocks.
Not only is the problem threatening the long-term economic opportunities for the region, it is depriving the population of a very valuable source of protein.
"It's stealing the fish, killing people and endangering the marine environment and the fish stocks; not just here but all over the world," said Ms Bours.
The precise figure for the number of vessels fishing illegally is unknown, but officials are worried that the size of the unlawful fishermen greatly exceeds most national fishing fleets.
"China is the largest fisher in the world, and the illegal fishers would come second," says Joe Borg, the EU fisheries commissioner.
"So we are speaking of fishing carried out legally by bodies like the EU, Chile and Peru being outranked by illegal fishing.
"We are speaking of a very, very big problem."
The Television Trust for the Environment's (TVE) Earth Report - Stolen Fish - will be broadcast on the BBC World News Channel on 20-25 February 2008. Please check schedules for further details