Fighters against piracy reached a milestone on May 10. A year had passed since the last hijacking, when Somali pirates took a Greek tanker carrying 135,000 tons of fuel oil.

The Smyrni and her crew of 26 were released 10 months later after a record ransom payment. According to the International Maritime Bureau, an organization that fights shipping crime, five ships and 71 sailors are still in pirate hands — a far cry from the count of 33 vessels and 758 hostages in early 2011, when seizures were taking place at a rate of more than 200 a year.

The decline in Somali piracy (which, according to a recent World Bank report, may at its peak have cost the world up to $18 billion a year in extra shipping expenses and lost trade) is partly the result of increasingly sophisticated coordination by international naval task forces.

Shipping companies also are making their vessels harder to attack thanks to a range of defensive measures, such as razor wire around decks, high-pressure hoses and maintaining speeds that make boarding hazardous. Armed security guards on many of the ships transiting pirate-infested waters have helped, too.

The high cost of victory

But the pirates could still make a comeback. The cost of deterring them is high. Shipping companies may lower their guard if they think the threat has passed, and patrolling naval forces could be needed elsewhere.

And although Somali piracy has faded, West Africa has seen a surge in attacks on ships passing through the Gulf of Guinea.

Tom Patterson, a maritime security expert at Control Risks, a consultancy, says the pirates in West Africa, who largely come from militant groups in the Niger Delta, have a different business model than their Somali counterparts. They tend to hold ships for about two to five days, removing as much of their cargo as possible (usually oil) and then auctioning it to the highest bidder.

Hostages are taken if potentially valuable. Last week five Poles and Russians, who had been held since April 25 when pirates attacked the German-operated City of Xiamen container ship off Nigeria's coast, were released, doubtless after a ransom payment.

International naval forces are unlikely to intervene. Nigeria has a decent navy of its own that claims to be upping its efforts to contain piracy. But foreign diplomats believe that some military officials turn a blind eye to thefts in return for a share of the spoils.