Divers who've spent time at a manta cleaning station know these animals as creatures of habit — same channels, same currents, same slow loops overhead. A new study out of southeastern Papua New Guinea backs that up in a way nobody quite expected: when the monsoons hit and conditions change, the region's reef mantas don't leave. They just dive deeper.
75% of tracked reef mantas stayed within 10 kilometres of home — even through monsoon season.
PhD student Anna Knochel and a team from the University of the Sunshine Coast satellite-tagged and tracked reef manta rays (Mobula alfredi) across Milne Bay and the Samarai Islands for three years, spanning two distinct monsoon seasons. The team expected to see long-distance travel as animals searched out better feeding grounds. Instead, 75 percent of the tracked group stayed within 10 kilometres of their tagging site, and the single farthest swim recorded across the whole study was just 87 kilometres.
What the Depth Data Means for Divers
What changed instead was depth. During the NW monsoon, the mantas averaged dives of around 63 metres (207 feet), chasing the zooplankton they feed on as it shifted deeper in the water column; during the SE monsoon months that average dropped to about 49 metres (161 feet). Reef mantas can weigh up to 850 kilograms with a wingspan of 4.5 metres, and the species is currently listed as vulnerable to extinction, which makes any data on how reliably it behaves worth paying attention to.
Knochel noted that this kind of behavioral flexibility "may also help protect them from threats other than climate, such as overfishing and bycatch" — and it has a practical upside for the dive operators built around these animals. If PNG's reef mantas are staying tight to a 10-kilometre core range year-round rather than roaming with the seasons, that's both a stronger case for protecting those specific zones and better odds for anyone booking a Milne Bay trip hoping to see one.
Image credit: Reef manta ray swimming underwater