At least 35 hostages died as a result of the Somali piracy crisis last year, although the number of prisoners taken fell to 555, compared with 645 in 2010, according to a report by the One Earth Future foundation and the International Maritime Bureau.
The document has been published to mark the International Maritime Organization’s annual Day of the Seafarer.
The death toll includes eight known to have been killed by their captors either during a hijack attempt or later executed and a further eight died of malnutrition or disease.
The rest were killed either during rescue attempts by military forces or while trying to escape.
Although data on previous years is unreliable, the total of 35 almost certainly represents the highest number of piracy-related fatalities in a single year.
At least 149 hostages had now been held for more than a year, the report said, with 26 held for more than two years.
Many of those released reported abuse that included beatings, removal of fingernails and dumping in the sea. More than 40% of hostages claimed to have been used as human shields at some stage while in captivity.
Most hostages were from developing countries, particularly the Philippines, India and China and from Gulf and African states.
The level of violence is also increasing, the report said. In 2011, more than 3,800 personnel were on board ships that were attacked by pirates with firearms.
Casualties among the pirates also appear to be rising, with reports of at least 111 killed in 2011, some 70% of them in clashes with naval forces.
Nevertheless, there are indications that the number of Somali piracy-related incidents has hit a downward trend.
Just five hijackings have been reported in the year to date.