Giant oceanic manta rays, the largest species of ray, glide thousands of kilometres across vast, featureless stretches of open ocean between the tropical and subtropical habitats they call home.

Now, an international collaboration of researchers in Peru, Indonesia and New Zealand have revealed they might be diving to extreme depths to better navigate after moving away from the coast.

These extreme dives might help the species – Mobula birostris – collect information such as changes in oxygen, temperature, light levels and even the Earth’s magnetic field strength and gradient.

“Our study highlights how dependent migratory species are on both coastal and offshore habitats, stressing the need for international cooperation in their conservation,” says first author and marine ecologist Dr Calvin Beale.

“It also reminds us that the deep ocean – which regulates Earth’s climate and underpins global fisheries – remains poorly understood but vitally important.”

Beale and collaborators tagged 24 oceanic manta rays at Raja Ampat in eastern Indonesia, near Tumbes off the coast of northern Peru, and near Whangoroa in northern New Zealand. 

They observed the animals’ behaviour over a total of 46,945 dives and found that they dove to extreme depths, defined as deeper than 500m and reaching a maximum of 1,250m, on 79 occasions.

Of these, 71 occurred in the waters off New Zealand. The manta rays remained in shallower coastal habitats in Peru and Indonesia.

Giant oceanic manta rays, the largest species of ray, glide thousands of kilometres across vast, featureless stretches of open ocean between the tropical and subtropical habitats they call home. Now, an international collaboration of researchers in Peru,

“The paper examines these dives in detail and concludes that the likely reason the mantas braved the cold, dark, deep waters was not in search of food, nor to avoid predators – but likely for navigational purposes,” says study coauthor and Re:wild Shark Conservation Director, Dr Mark Erdmann.

“We now know that many, if not all, of the oceanic mantas that feed off New Zealand’s North Island in the summer months eventually migrate northwards towards warmer waters (including around Fiji and Tonga) as the seas around New Zealand cool off in the autumn and early winter.

“The deep dives of New Zealand oceanic mantas all occur as they leave the continental shelf and began their northward migration, and it seems likely the dives are conducted in order to ‘take a better bearing’ of the magnetic lines on the bottom of the ocean.

“The study shows that immediately after deep dives, New Zealand oceanic mantas spend a fair bit of time warming up on the surface (likely recovering from the thermal stress of these very cold deep waters), but then start moving ‘with a purpose’ – covering up to 200km per day in a highly directional manner for several days in a row.

“Importantly, the research team in New Zealand has continued to satellite tag oceanic mantas every summer since 2019 and expects to publish additional exciting insights into the movements and behaviours of these majestic creatures over the coming few years.”

The research is published in the journal Frontiers in Marine Science.