From Sargasso to Lost City Hotspots that need prioritising under high seas treaty

From next week, the UN’s high seas treaty will at last be open for signatures – an important moment that starts the process for nations to ratify it into their laws. At least 60 countries must do so for the treaty to come into force. Scientists hope it will finally allow marine protected areas (MPAs) in the high seas to be established.

Conservationists are urging governments to act quickly. Fishing hours on the high seas rose by about 8.5% between 2018 and 2022, according to new estimates published this week by Greenpeace using data from Global Fishing Watch. The high seas are areas that lie beyond national jurisdictions and so have no legal protections.

They cover nearly 50% of the planet and house many unique ecosystems. Yet many high seas areas are under threat not just from overfishing, but also from pollution, climate change, shipping damage and deep-sea mining.

“The high seas make up twothirds of the world’s ocean, so it’s absolutely critical that we start establishing MPAs,” said Rebecca Hubbard, director of the High Seas Alliance. But where to begin?

The Costa Rica thermal dome

This diversity hotspot forms in the eastern Pacific when warm waters from the coast meet cooler waters carried by ocean currents, causing an upswell. This brings nutrientrich waters from the sea’s depths to its surface, perfect conditions for a blue green algae to grow.

The phytoplankton that feed on the algae are the start of a rich marine food web, supporting species such as marlins, sea turtles and blue whales. The area is under threat from overfishing, plastic pollution and ships on their way to the Panama Canal.

White Shark Cafe

In the Pacific Ocean, about halfway between Hawaii and the American mainland, is the only known meeting spot for north Pacific white sharks, known as the White Shark Cafe. Estimates suggest about 75% of great white sharks migrate here.

They make a “winter pilgrimage” to the site from breeding grounds along the North American coast. Scientists observe tagged sharks in the area, including some doing rapid dives up to 450 metres deep.

Sargasso Sea

This area, known for its clumps and mats of sargassum seaweed, is 2,000 miles long and 700 miles wide. It is nicknamed the “golden

rainforest of the high seas” thanks to its rich biodiversity and because it acts as a carbon store.

The seaweed is home to more than 127 species of fish, 145 invertebrates, 26 types of seabird and other unique animals such as the sargassum frog fish. Marlins and dolphinfish spawn here; white sharks breed here; endangered turtles such as the hawksbill and loggerhead live here; and all American and European eels start life here.

But the area is at risk from overfishing, pollution, shipping traffic and a garbage patch. “The Sargasso Sea is also experiencing significant effects from the climate crisis, with a 5,000-mile-long belt of free-floating sargassum seaweed – dubbed The Great Atlantic Sargassum Belt – which risks impacting the Sargasso Sea and wreaking havoc on coastal ecosystems and tourism,” said Dr Simon Walmsley, chief marine adviser at WWF-UK.

The Lost City hydrothermal field

This “is a remarkable geobiological feature … in the deep sea (700-800 metre water depth) that is unlike any other ecosystem yet known on Earth”, according to a 2016 report by Unesco. But it is under threat, particularly from deep-sea mining.

The site was discovered in 2000 during a dive in the MidAtlantic Ridge. There are 30 huge hydrothermal vent chimneys rising from the seabed. The chimneys are formed when seawater reacts with rock in the Earth’s mantle and is heated by magma. As the water warms and the pressure rises, minerals begin to dissolve in it. The water then rises and exits into the ocean. When the hot water meets cold sea water, it cools forming mineral deposits that build the chimney-like structures.

There are 30 chimneys, and the largest, known as “Poseidon”, is 60 metres high. Although a harsh environment, the area is teeming with everything from microorganisms and snails to crabs and jellyfish. The site has been recognised as an ecologically or biologically significant marine area under the convention on biological diversity, a Mission Blue “hope spot” (scientifically identified as critical to the health of the ocean), and a high seas gem by the Marine Conservation Institute.

Salas y Gómez and Nazca ridges

These chains of mountain ridges sit deep in the clear waters of the south east Pacific. They contain many species of ecological importance – from rare turtles to commercial food sources such as swordfish and jack mackerel – and are migration corridors for at least 82 endangered species. Marine biodiversity here is threatened by everything from deep-water trawling to plastic debris in the South Pacific gyre and possible future mining exploration.

Creating MPAs needs careful thought, experts warn. Their establishment, governance and management should be “inclusive, equitable and human rights-based… underpinned by broad community and stakeholder support”, said Walmsley. This included balancing ecological benefits and local social and economic circumstances.

Alexander Killion, managing director of Yale University’s Centre for Biodiversity and Global Change, said: “It is important new protected areas be prioritised to serve the species that we know are most in need of protection,” while also helping to meet carbon and climate goals, to prevent further habitat loss. There was also significant work to do to map the distributions of underrepresented marine species to prevent extinctions.

Walmsley favours a network of MPAs – an idea noted in the Greenpeace report – that protect a representative range of marine habitats, species and ecological processes, including migration corridors. “Marine species know no borders. So a network of MPAs that aid ecological connectivity and protect transboundary and migratory species – such as whales, turtles, sharks and tuna – is going to be essential for effective long-term protection, especially when trying to adapt to the climate crisis.”