The cost of Somali piracy to the global economy fell by almost half last year as attacks slumped, but piracy in West Africa is increasing and more than 50 hostages remain in captivity.

An annual maritime security report, published on Wednesday by the group Oceans Beyond Piracy, put the total cost of Somali piracy, the largest single threat to international shipping in recent years, at only $3.2bn in 2013.

"The efforts of the international community and the shipping industry have considerably reduced the threat of Somali piracy," said Jens Madsen, one of the report's authors, according to Reuters news agency. "But we have yet to achieve the goal of ... zero vessels captured and zero hostages held."

There were still at least 50 hostages in Somali captivity in desperate conditions, held on average for around three years each, the report said.

Lack of cooperation

Somali pirates were increasingly attacking local fishermen and smaller crafts, often hoping to use their vessels to attack larger ships.

In West Africa, tackling the attacks was complicated by the lack of regional cooperation and information sharing, the report said.

"Piracy in the Gulf of Guinea is fundamentally different [from] events taking place in the Indian Ocean," said Madsen.

"We observe... a high degree of violence in this region," he said. "The constantly evolving tactics of West African piracy make it extremely difficult to isolate it from other elements of organised crime."

At the height of Somali pirate attacks in 2011, up to a dozen or more merchant ships were being held captive at any one time, often for multimillion dollar ransoms.

Since then, growing use of private security details and the presence of international warships have largely foiled attacks. No large vessels were seized in 2013.

Source: Agencies