"This recent attack should send a clear message to the oil companies that even with the four Mantra class seaward defence boats and two Augusta helicopters acquired by the navy, the only thing that can stop attacks is justice," a Mend statement said.
"Mend is in touch with the group and will ensure the abducted man is released unharmed at the earliest convenience."
The tanker was believed to be travelling from Lagos to the main oil city of Port Harcourt when it was attacked at the Bonny Fairway Buoy.
Oil industry targeted
The incident is the latest in a series of violent attacks off the Niger delta, home to the country's oil and gas industry.
On Sunday, fighters attacked a loading vessel, a tanker and a tug boat at a crude oil platform operated by the Shell oil company and took eight crew members hostage.
One person was killed in the attack, a military spokesman said.
Pipelines, facilities and personnel form oil companies operating in the region have been frequent targets of violence in the past three years, with kidnappings of both local and foreign workers.
A number of armed groups and criminal gangs operate in the delta.
The unrest has reduced the country's oil output by more than one quarter, down to around two million barrels a day from 2.6 million about three years ago.