International military chiefs signalled a new, more aggressive strategy against piracy, as swarms of Somalis moved into the waters off East Africa.
Commander John Harbour, spokesman for the EU Naval Force, said the military was "taking the fight to the pirates" after four shootouts showed that high-seas attacks are intensifying with the end of the monsoon season.
Nearly half the 47 ships hijacked off Somalia last year were taken in March and April - the most dangerous months of the year for ships in the Gulf of Aden and Indian Ocean.
In the most serious skirmish yesterday, six pirates attacked a vessel before breaking off and chasing the French fishing boat Torre Giulia, said Cmdr Harbour.
A French military detachment on board a nearby ship fired warning shots at the pirates. The ship then approached the skiff and collided with it, sinking the skiff and throwing the pirates into the water. Four were rescued, but two others were missing, Cmdr Harbour said.
A surge in attacks was likely in the coming weeks, he said. This season, though, ship owners and sailors are more prepared to evade pirates, fight back, or they have armed security on board, raising the likelihood of violence.
"We know the monsoon is over. We know they're coming. We're taking the fight to the pirates," said Cmdr Harbour.
Crews are successfully repelling more attacks, making it harder for pirates to capture ships and earn multi-million ransoms.
In turn, the Somali gangs are increasingly turning violent. The International Maritime Bureau says only seven ships were fired on worldwide in 2004 but 114 ships were fired on last year off the Somali coast alone.
That compares with 39 incidents off Somalia and in the Gulf of Aden in 2008.
In a second incident yesterday, the EU Naval Force intercepted a pirate group of one mother ship and two skiffs that had attacked a separate French vessel. That attack was also repelled by military personnel on board.
An EU Naval Force helicopter tracked the pirates and watched them throw a rocket launcher, grappling hooks and fuel barrels into the ocean. The naval force said it destroyed the mother ship and one skiff and took 11 pirates into custody.
In the third and fourth attacks, pirates attacked two Spanish tuna fishing boats off the coast of Kenya, Spain's Ministry of Defence said.
A spokesman said the boats had contacted Spanish navy forces in the area, which dispatched a plane.
Between the air support and the private guards on the boats, they repelled the attack.
Yesterday's clashes followed a firefight on Thursday between pirates and private security contractors on board a Spanish fishing vessel. The pirates hit the ship with a rocket-propelled grenade and the guards returned fire.
No-one was hurt, but the International Maritime Bureau has expressed fears that the increased use of armed contractors could spark an arms race between fishermen and pirates, who are firing at ships with increasing frequency.
"The EU Navfor agrees with that recommendation because we don't want an escalation of firepower," Cmdr Harbour said. "Also, there are lots of gas and oil tankers in the Gulf of Aden that wouldn't benefit from grenades and bullets flying around."
Pirate attacks off East Africa have increased dramatically over the past three years. Somali pirates attacked ships 217 times in 2009, according to the International Maritime Bureau. That was up from 111 attacks in 2008.
Many ship owners are investing in physical defences like stringing razor wire and adding fire hoses that can hit attackers with streams of high-pressure water. Some ships are even having electric fence-style systems installed.
Crews have thrown everything from oil drums to wooden planks at would-be hijackers clambering up ladders. Last month a crew played the sound of dogs barking over an amplifier to frighten off attackers.
Mohamed Omar Dalha, the deputy speaker of Somalia's parliament, said Somali communities inside and outside the chaos-wracked country were working to negotiate the "unconditional release" of the Chandlers. Mr Dalha was hopeful they would be released within two weeks without payment.
Somalia has not had a stable government since warlords overthrew long-time dictator Mohamed Siad Barre in 1991.