The International Maritime Bureau says the number of hostages killed or injured by Somali pirates increased significantly last year.
A report by the bureau says 35 people who were held hostage by Somali pirates died in 2011.
Levels of violence against seafarers in coastal waters around east Africa and the Gulf of Aden also remained high.
the report said that while the number of successful hijackings decreased, hostages are being held for longer.
Most of the hostages who died were killed while trying to escape during rescue operations.
Other deaths were directly at the hands of the pirates or due to malnutrition or disease during captivity.
A BBC analyst says it used to be rare for hostages to die in captivity.
At least 1206 hostages were held by Somali pirates in 2011, including 555 seafarers attacked and taken hostage during the year, and 645 captured in 2010 who remained in pirate hands.