Report Reveals French Polynesia’s Mountainous Underwater Landscape

The waters around French Polynesia, a French overseas territory in the South Pacific Ocean, make up the world’s largest contiguous exclusive economic zone (EEZ). Beneath these waters lies a vast range of seamounts—mountains that rise, often dramatically, from the ocean floor. For centuries these seamounts have held cultural significance for the people of French Polynesia, but many of them remained largely unknown to scientists—until now.

With the support of the Pew Bertarelli Ocean Legacy Project, four students from École Polytechnique, a university in France, have created the first detailed overview of these geographical formations and their biological significance. The report synthesizes existing knowledge on the seamounts from fishermen, local experts, research organizations, and international scientists, and offers critical context for the future protection of these undersea formations.

The study identified over 500 seamounts across French Polynesia’s EEZ and included all with summits between the ocean surface and a depth of 3,000 meters (9,843 feet). Formed by volcanic activity, the peaks and slopes of these mountains are home to a wide variety of marine life, including deep-sea species of snappers and groupers, and attract large marine mammals and pelagic predators such as tuna and sharks. The unique geography of the seamounts promotes biological diversity: Currents draw nutrients from deeper waters to create high concentrations of plankton above the seamounts, while the mountain ranges attract highly mobile species such as tuna, which form larger schools around the peaks than in the open ocean. These species may use these geological formations as feeding grounds, nurseries, “rest stops,” or as refuge from predators.

Protecting French Polynesian waters

The richness of the seamounts, both biologically and in terms of minerals, has attracted offshore fishing, industrial activities, and, more recently, conservation efforts. In an effort to preserve these natural wonders, the French Polynesian government has committed to protect the seamounts by 2021 or 2022, and states in its proposed management plan that “These particular ecosystems must be protected. All mining activities should be prohibited and fishing restricted.”

The negotiations on the plan will involve a variety of stakeholders—including fishermen, local associations, and the people of French Polynesia—to define the zoning of protected areas. In June, French Polynesia’s Council of Ministers also announced plans to seek a UNESCO biosphere reserve designation for the Austral Islands by 2023. If successful, these conservation initiatives could preserve the biological richness and cultural significance of French Polynesian waters for generations to come.

Closing the scientific knowledge gap

Even today scientific knowledge of French Polynesia’s seamounts remains fragmented. Each one fosters a unique ecosystem due to its topography, depth, latitude, and the hydrodynamics surrounding each underwater formation. Although satellite data and advances in digital processing have helped scientists better understand what happens on and around seamounts near the ocean’s surface, additional research is needed to fully explore their ecological significance throughout the water column. 

The Pew Bertarelli Ocean Legacy Project is committed to furthering scientific research of French Polynesia’s rich and diverse waters, and is working closely with public officials, scientists, and local communities to develop marine protection solutions that align with traditional Polynesian culture and community values.

Jérôme Petit is a senior manager and Donatien Tanret is an officer with the Pew Bertarelli Ocean Legacy Project in French Polynesia.