NAIROBI, Kenya — Swarms of Somali pirates are moving into the waters off East Africa, triggering four shootouts Friday including a skirmish with French military personnel that sunk a pirate skiff, officials said.

The end of the monsoon season and the resulting calmer waters signal the beginning of the most dangerous period for ships traveling the Gulf of Aden and Indian Ocean. Nearly half the 47 ships hijacked off Somalia last year were taken in March and April.

Cmdr. John Harbour of the European Union Naval Force said a spike in attacks was very likely in coming weeks. But this season, ship owners and sailors are more prepared to try to evade pirates, fight back, or have armed security onboard, raising the likelihood of violence.

“We know the monsoon is over. We know they’re coming,” Harbour said. “We’re taking the fight to the pirates.”

In the most serious skirmish Friday, six pirates attacked a vessel before breaking off and chasing the French fishing boat Torre Giulia, Harbour said. Two other French fishing vessels nearby — the Jalenduic and the Trevignon — aided the Torre Giula.

A French military detachment onboard the Trevignon fired warning shots at the pirates, but failed to stop the attack. The Trevignon approached the skiff and collided with it, said Harbour, sinking the skiff and throwing the pirates into the water. Four were rescued and a military aircraft was searching for the other two, he said.

In a second incident Friday, the EU Naval Force intercepted a pirate group of one mothership and two skiffs that had attacked a separate French vessel. That attack was also repelled by military personnel onboard.

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 mothership and one skiff and took 11 pirates into custody.

In the third and fourth attacks, pirates assaulted two Spanish tuna fishing boats off the coast of Kenya, Spain’s Ministry of Defense said. A spokesman said the boats had contacted Spanish navy forces in the area, who dispatched a plane. Between the air support and the private guards on the boats, they were able to repel the attack. The spokesman spoke on condition of anonymity because government rules don’t allow him to be identified but the clashes were confirmed by deputy Prime Minister Maria Teresa Fernandez de la Vega.

These incidents followed a firefight Thursday between private security contractors onboard a Spanish fishing vessel and pirates. The pirates set the ship on fire with a rocket-propelled grenade and the security 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,” said Harbour. “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 dramatically increased 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.

Last year, the average ransom was around $2 million, according to piracy expert Roger Middleton of the British think tank Chatham House. This year, two ransoms paid were around $3 million and $7 million, he said.

The original Somali pirates were fishermen aggrieved over the huge foreign trawlers depleting their seas — a complaint the international community has yet to address despite pouring hundreds of millions of dollars into anti-piracy patrols. Huge ransoms lured criminal gangs into piracy, though, and ransom inflation has made it more expensive to buy the freedom of the more than 130 hostages still being held.

Among those hostages are a retired British couple snatched last year from their sailboat, who a Somali official said Friday could be freed within weeks. Paul and Rachel Chandler were seized from their 38-foot yacht last October.

Mohamed Omar Dalha, the deputy speaker of Somalia’s parliament, told The Associated Press that Somali communities inside and outside the chaos-wracked country are working to negotiate the “unconditional release” of the Chandlers. Dalha was hopeful they would be released within two weeks without payment.

Somalia has not had a stable government since warlords overthrew longtime dictator Mohamed Siad Barre in 1991.