Deep beneath the high seas, researchers find rich coral oases

Corals on the Debussy Seamount, a submerged peak more than 2000 meters below the surface in the North Pacific Ocean 

Aiming to bolster conservation on the high seas, a team of marine researchers today released the first comprehensive survey of coral reefs in the high seas–the roughly two-thirds of the ocean outside of national jurisdictions.

After combing through more than half a million observations of reef-building corals, the team identified 116 reefs located in the high seas. Most of these corals live between 200 and 1200 meters beneath the surface, the researchers found. But a handful are found more than 2 kilometers deep. And there are likely many more high seas corals still to be found, the authors note, as surveys have typically prioritized corals close to shore.

The study coincides with the launch of the Coral Reefs on the High Seas Coalition, a group of scientists and nonprofits that aims to support research cruises to survey the steep, deep-water slopes where many of the reefs sit. Eventually, the coalition hopes the data will help persuade policymakers to give these poorly understood ecosystems greater protection in global agreements currently under negotiation.

“Some of the first marine protected areas were specifically designed around coral reefs. … So much literature suggests these are the rainforests of the seas,” says co-author Daniel Wagner, the coalition’s coordinator and an ocean technical adviser at Conservation International. The coalition of nonprofits hopes to influence implementation of a United Nations pact, the Intergovernmental Conference on Marine Biodiversity of Areas Beyond National Jurisdiction, which is expected to set rules for establishing marine preserves on the high seas. (A final meeting of the negotiators set for earlier this year was postponed because of the COVID-19 pandemic.)

The deep reefs “are some of the most under surveyed of all ocean ecosystems,” the coalition notes, “and because they are not protected by the laws of any country, they are among the most vulnerable and potentially overexploited reefs on Earth.”

The study suggests scientists have much more to learn about corals beyond the coasts. All records of high seas corals were scleractinian, a common family of hard, reef-building coral, and a few were observed much deeper than where they are typically found. Most reefs were found on seamounts, escarpments, and submarine ridges in the Pacific and Atlantic oceans, with a small minority in the Indian Ocean.

The study also draws attention to a need to coordinate protections from human activities in the ocean. It found that just one-fifth of the known deep-sea reefs are protected from bottom fishing, for example, and none is protected from various impacts of shipping. One known reef already exists in an area protected by the seabed mining regulator International Seabed Authority, and two are near active mining exploration contracts.

Expedition challenges

The coalition has been racing to gather more information on high seas reefs, but pandemic restrictions delayed its first two expeditions until early 2022. The first expedition was scheduled to set sail later this year for seamounts near Rapa Nui, or Easter Island, off the coast of Chile. The seamounts fall within Chile’s national waters, and are already included in a national marine reserve. But expedition leader Richard Pyle, an ichthyologist at the Bishop Museum known for his work on dimly lit mesophotic reefs, expects the conditions to be representative of high seas coral reefs, and the marine reserve may support a legal argument for protection.

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