Kiribati's deputy secretary of fisheries, Takuia Uakeia, says the new marine reserve may have an impact on revenue from commercial fishing licences.
The President of Kiribati Anote Tong announced the creation of the world's third-largest marine reserve at a UN meeting in Brazil.
The Phoenix Islands Protected Area bans commercial fishing to protect more than 120 species of coral and 520 species of fish inside its 184,700 square kilometres.
It is also the world's first marine park with deep-sea habitat, including underwater mountains.
Mr Uakeia says although it's likely that fewer commercial fishing licenses will then be granted, conservation is important.
"As I said this is only the plan first where we are trying also to contribute to the good concept of trying to preserve a certain part of our marine resources as well as our environment."
Takuia Uakeia says the New England Aquarium in the United States and Conservation International are helping Kiribati set up the reserve and will pay costs for its management.