While Somali pirates top headlines with brazen ship hijackings for ransom, many smaller-scale attacks in the world's oceans - maritime muggings, essentially - go unreported, depriving mariners of information about possible threats to their safety and vessels.

Industry analysts say some owners and masters of commercial boats prefer not to report relatively small losses from piracy, or attempted boardings, because they worry about clean records, costly delays in the event of an investigation in the nearest port, jittery clients who might take business elsewhere, and the likelihood of higher insurance rates if they log an attack with authorities.

The International Maritime Bureau, a leading monitor of piracy, warns that a failure to report even a minor or failed assault at sea makes it harder to raise public awareness and pressure governments to take robust measures against pirates.

"You have to look at the criminal intent. They come at least armed with a knife or a pistol," said Noel Choong, head of the maritime watchdog's piracy reporting center in Kuala Lumpur. "They may not steal anything valuable, but people may be killed for nothing."

Choong estimated that more than 50 percent of pirate attacks are not reported, but some experts speculate that the figure is even higher. The maritime bureau recorded 239 pirate attacks as of July 1 this year, well over half carried out by Somali pirates; it recorded 116 attacks in the same period last year.

Reporting is especially erratic in the piracy hotspot of Nigeria, where militants in speedboats have staged attacks in their campaign against the alleged exploitation of oil wealth by multinational firms. Some armed groups are believed to use that cause as political cover for thievery and extortion, targeting oil vessels and off-shore installations for economic gain.

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