European fleets say the Somali pirates, who are better known for their hijackings of commercial vessels including the Saudi supertanker, Sirius Star, threaten an industry worth up to £4.3 billion across the Indian Ocean region.
France and Spain, which both base fleets in the Seychelles, would expect to haul in nearly two-thirds of the year's catch off Somalia between August and November, Alejandro Anganuzzi, head of the Indian Ocean Tuna Commission, said.
"Instead they had to look further east and probably caught 50 per cent of what they would usually catch," he said.
Some 50 trawlers use the capital Victoria's port, through which up to 350,000 tonnes of tuna are handled each year.
Somalia has said piracy was merely a symptom of rampant illegal fishing by vessels from Europe and Asia in its waters after the country's central government collapsed in 1991.
Pirates attacked tuna boats at least three times last year, leading to one ransom of more than £725,000.
