NZ scientist’s research offers ray of hope for elusive ocean megafauna

Veronika Meduna

Science & environment writer·New Zealand Listener·

4 Jul, 2026 07:04 PM4 mins to read

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Oceanic manta rays are the largest species, with a fin span of up to 7m and weighing in at two tonnes. Photo / Mark Erdmann

With diamond-shaped bodies, huge wing-like fins and the biggest brains of any fish, manta rays are part of the ocean’s charismatic megafauna.

Oceanic manta rays are the largest species, with a fin span of up to 7m and weighing in at two tonnes. But these giant, stingless plankton feeders
are in decline globally, classified as endangered on the International Union for Conservation of Nature’s red list.

New Zealand has its own population of oceanic manta rays, coming into waters off the north of the country during summer. Lydia Green, a marine biologist and founder of Manta Watch NZ, has been studying this local population for the past six years using satellite tags, drone surveys and citizen science. She’s shown that mantas migrate twice a year between New Zealand and the wider southwestern Pacific around Fiji, Tonga, Tuvalu and the Cook Islands.

But the satellite tagging also pinpointed an area south of Tonga where several mantas appear to have died. This winter, Green and her team are sailing along the rays’ migration corridor to investigate what might be happening.

When Green first began studying New Zealand’s oceanic mantas, very little was known about the local population. They are elusive. Unlike the smaller reef mantas, they cruise in deeper water further offshore. But during the first research season, the team “happened upon a huge feeding aggregation that spanned several nautical miles”.

Our ocean-going mantas are the only population globally that’s being studied entirely in the open ocean. Green’s team has now observed both feeding and courtship behaviour – when a pair swims closely together – and encountered many heavily pregnant females in our waters.

One of the reasons for the manta rays’ global decline is their slow life cycle. They are long-lived and reach sexual maturity only in their teens. Pregnancy lasts a year, and they give birth to a single pup (rarely twins) only every two to five years.

But mantas are also caught deliberately for their meat and gill rakers in some parts of the world and as accidental bycatch in some fisheries. “Any form of exploitation and the population can crash; we’ve seen that worldwide,” she says.

Oceanic mantas are known to take occasional deep dives, often down to about a kilometre. Studies suggest this behaviour helps them navigate across the open ocean and Green’s project has confirmed they often change direction after a deep dive. But these deep dives are usually short – unlike those recorded by satellite tags for some rays off Tonga. Four of the tags that had been deployed on migrating rays to track their horizontal and vertical movements came off prematurely, after a very deep dive that lasted more than three days, and showed very different depth profiles. The most likely explanation is that these mantas interacted with fisheries, Green says. “They were caught and their bodies were discarded and sank to the bottom.”

The team wants to understand the threats this population faces, not only while they are in New Zealand waters, but throughout their entire migration range. In our waters, manta rays are a protected species, but their migration corridor takes them “through multi-jurisdictional and high-seas fisheries”.

The team is now overlaying fisheries data with manta tracks and hopes to identify the types of fisheries they are exposed to and the level of impact from different fishing methods to assess their risks along the migration route.

When the project started, the local population was considered “data deficient” – there was not enough information for conservation recommendations. But at the end of last year, that classification was upgraded to nationally vulnerable. Green is calling on sailors to help the project by logging any manta sightings on the Manta Watch NZ app. “If we protect manta rays, we can protect a lot of other endangered species that follow the same migration path but don’t have the same profile and public appeal.”

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