Tuna catches in the southwestern Indian Ocean fell by as much as 30 percent last year as pirates blocked access to some of the world's richest tuna waters off Somalia, fisheries experts say.

European fleets say the Somali pirates, who are better known for their audacious hijackings of commercial vessels including the Saudi supertanker, Sirius Star, threaten an industry worth up to $6 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, told Reuters.

"Instead they had to look further east and probably caught 50 percent 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. But catches have suffered for two consecutive years as stocks fall.

Fisheries experts say foreign currency earnings will have fallen as a result of the dwindling tuna catch, hurting hopes for an economic recovery in the debt-laden archipelago.

In the Seychelles, tuna and related industries -- re-export of fuel to vessels, port services, electricity and water for vessels -- account for up to 40 percent of foreign earnings.

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.

The Kenya-based Maritime Seafarers Assistance Programme said in 2006 there were hundreds of illegal fishing boats in Somali waters at any one time, mainly chasing tuna.

Some pirates have told Reuters they only turned to hijacking to stop foreign fishing vessels destroying their own small boats and equipment. But the ransoms earned simply increased their appetite for hunting other ships.

From August to November, the waters beyond Somalia's Exclusive Economic Zone (EEZ) hold some of the planet's richest stocks of Yellowfin tuna. Pirates attacked tuna boats at least three times last year, leading to one ransom over $1 million.

"The pirates' impact on the fishing off Somalia has been huge," said one European skipper, on condition of anonymity. "At least half our business is there. Now we cannot go there anymore. The last season was wrecked."

The audacious attack on the fully laden Sirius Star, 450 nautical miles out to sea, alerted the world to the pirates' extended range of activity, using ocean-going "mother-ships" to deploy smaller skiffs for an assault.

Pirates freed the ship this month for a $3 million ransom.

"They are in the south too. We've seen suspect boats on the same longitude as (Tanzania's capital) Dar es Salam. They were there long before the Sirius attack," the skipper said.

"We have seen them in the waters of countries where we hold licences, including Kenya and the Seychelles," he said.

The financial implications for the Seychelles are hard to fathom as the tuna industry is shrouded in secrecy. In Japan, top quality fish can sell for up to $100,000 each.

According to data seen by Reuters, French vessels averaged some 4,000 tonnes each in 2008 compared to about 6,000 tonnes in 2006. But the financial ramifications go beyond the fleets.

The Seychelles is paid per tonne of fish landed for port facilities -- an important source of foreign exchange for the archipelago -- and reduced catches mean fewer calls to port.

"The pirates' biggest impact, however, is reduced supply, driving prices up," said Rondolph Payet head of the Seychelles Fisheries Authority.