"Catastrophic" levels of illegal fishing in west Africa are costing the region millions in lost revenue and hundreds of thousands of jobs, a development think tank said Wednesday.

Countries such as Senegal, Sierra Leone and Mauritania are missing out on vital income because of the masses of fish taken from their waters by trawlers from as far afield as South Korea, according to research by Britain's Overseas Development Institute (ODI).

Senegal lost $300 million, or 2.0 percent of its GDP, to the practice in 2012, while Sierra Leone -- one of the region's poorest nations -- missed out on $29 million, said the report, entitled "Western Africa's Missing Fish".

A lack of government transparency in the region, limited capacity to patrol the seas and legal loopholes once west Africa's fish arrive in Europe, its biggest market, were all contributing to the situation, report author Alfonso Daniels told AFP.

"It's a huge problem and it's only getting worse," he said.

West Africa's "illegal" fish are transported in giant refrigerated containers mixed with other cargo to escape scrutiny at port, Daniels explained.

"Four-fifths are coming through container ships and (they) are not considered at all by the anti-illegal fishing legislation of the European Union, which is the largest market for fish in the world," Daniels said.

Europe is the destination for 44 percent of all west Africa's fish exports.

Despite this loophole, the EU remains a leader in sanctioning producers of illegal fish.

Meanwhile, a series of opaque agreements between West African nations and the likes of Russia, China and South Korea mean that the scale of the region's "missing fish" could be much larger.

"Fisheries agreements... should be public and openly available as they are with the European Union," Daniels said.

And if Africa fished its own waters rather than striking shady deals with other countries, more money would flow into the governments' coffers.

The sale of fishing rights to foreign operatives netted Africa $400 million in 2014, according to the UN's Food and Agriculture Organization, but could in theory generate $3.3 billion if the continents own fleets caught and exported the fish.

Another practical step towards combating the problem would be creating a blacklist of illegal vessels.

The ODI report estimated more than 300,000 new jobs could be created if measures such as a global tracking system for fishing vessels were instituted, loopholes were closed and a blacklist created.

"Further development benefits would derive from increased export revenue. Sustainable management of fisheries resources would also strengthen food security," the report said.

Achieving "marine environmental sustainability" would stop permanently depleting endangered stocks and would provide a long-term income, it said.