The tuna boat seized over the weekend and used to harass commercial ships in the Gulf of Guinea this week remains a threat to vessels and their crews.

Diaplous Maritime Services' intelligence division said on Wednesday that the Gabon-flagged fishing vessel Lianpengyu 809 is still being used as a pirate mothership in the waters off Sao Tome and Principe.

"It is assessed that the vessel is being used as a [pirate action group] mother vessel and therefore remains a threat to merchant shipping in...