Local divers in Papua New Guinea’s Samarai Islands kept seeing the same reef manta rays at the same cleaning stations and channels. Nobody had tracked them long enough to know why.
Researchers fitted ten of these rays with satellite tags and tracked them through two monsoon seasons. What the data showed was unlike almost anything recorded for the species.
The rays almost never left. When surface food thinned, they dove deeper instead of swimming away.
Reef manta rays at home
The study was led by Anna M. Knochel of the University of the Sunshine Coast and the conservation group Manta Trust. Her team ran the first close look at reef manta rays in this part of Papua New Guinea.
Between 2016 and 2018, the team fitted ten adult rays with pop-off satellite tags off the Samarai Islands. Each tag logged a ray’s depth and movements, then floated free and beamed the data to satellites.
Some tags shed within days. Others held on for months, one lasting the full 181 days. Three tagged rays even reappeared at the same sites up to two years later, a sign of how rooted they are.
Staying close to home
Three of four positions sat within 6 miles (10 kilometers) of where a ray was tagged. The biggest excursion reached 54 miles (87 kilometers) before looping back, and none crossed into deep offshore water.
That stay-put habit stands out because the species is built for range. Other populations dive past 2,000 feet (610 meters) for food and swim hundreds of miles in near-straight lines.
The farthest-flung manta on record turned up 3,700 miles (5,950 kilometers) from the nearest known group. These rays carry that range but use almost none of it, living more like residents than nomads.
Why they stay anchored
A few plain features keep the rays anchored here. Most of their positions clustered over a broad, shallow underwater plateau, and they rarely crossed into the open depths just offshore.
Water depth was the strongest predictor of where rays turned up, ahead of food and temperature. They favored warm water near 85°F (29°C) and patches rich in the drifting plant life that feeds their prey.
Narrow channels between the islands were hotspots. Currents funnel food into these gaps on outgoing tides. There the rays feed and visit cleaning stations, where small fish pick parasites off their skin.
Diving deeper for food
The most surprising result had never been documented in this population. When surface food thinned, the rays didn’t head elsewhere for better grounds. They went down after it – straight down.
The depth records told the story. Through one monsoon, the rays stayed mostly in the upper 165 feet (50 meters). In the next, they went much deeper.
The deepest descents passed 1,300 feet (400 meters), a depth earlier research links to feeding below the reef. Those dives matched months when the mixing layer thinned and surface productivity fell.
The rays seemed to follow food downward in those months, though the tags recorded only depth, not diet.
When the surface thins
The seasons here swing between two monsoons, and wind drives the difference. Strong southeasterly winds churn the sea, pulling cool, nutrient-rich water up from below.
That upwelling feeds the drifting creatures these rays strain from the water. When the winds slacken and swing around in the other season, the mixing weakens.
Less food reaches the surface, and the rays that rely on it must work harder for a meal. Most answered by diving deeper, though not all.
A couple stayed up in the northern channels through the lean months, a hint that those sheltered waters held enough food to ride it out.
A case for protection
Because these rays keep to a small range, local action alone could safeguard them. That’s hopeful for a species listed as vulnerable to extinction, with no national protections in Papua New Guinea.
The team sees room to build careful ecotourism around the animals. Manta watching already generates substantial revenue worldwide, with one paper putting the industry’s global value above $100 million a year.
The catch is that the channels drawing visitors are also where rays face the most danger, from boat strikes and fishing gear to the rare but real threat of hunting.
With limits on boats and divers, tourism could fund protection rather than erode it.
For the first time, there’s a clear picture of where this population lives and how it weathers a changing ocean.
These reef manta rays are homebodies with a small range, riding out lean seasons by changing their depth, not their address.
That flexibility shifts how scientists think the rays survive in one place. Rather than chase food across open water, they work the vertical space beneath them, rising and sinking with the seasons.
The practical payoff is concrete. A government weighing how to protect a vulnerable species now has evidence that guarding a few islands and channels could be enough.
That same evidence points to a path: turning these rays into a draw that funds their own protection.
The study is published in the journal PLOS ONE.
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