Ransoms of up to $400,000 (£273,000) were paid to gangs which hijacked ships in the Gulf of Guinea in 2015, the BBC reported on Tuesday, citing a maritime document.
A 2009 amnesty for militants in Nigeria stemmed kidnappings for several years
The region was said to be the most dangerous in the world for seafarers, with pirates becoming more violent.
A total of 32 seafarers had been kidnapped so far this year compared with 15 in 2015, the report said.
Kidnapping for ransom took place mainly in the oil-producing areas off Nigeria's coast, it added.
The spike in kidnappings appeared to be linked to political developments in Nigeria, the report by the US-based group Oceans Beyond Piracy group said.
There had also been a sharp drop in oil theft last year, which the report put down to improved patrolling of Nigeria's waters, and the fall in oil prices making it less profitable.
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According to the report, one of the most high-profile cases was that of the Malta-flagged MT Kalamos, an oil super tanker, which was attacked in February 2015.
The tanker's abducted crew was freed after the $400,000 ransom was paid, the State of Maritime Piracy 2015 report said.
