New Discovery: Manta Ray Nursery
One of Asia’s most popular big animal havens is the focus of a fascinating new study on the movements and behaviours of its iconic residents. Bali’s Nusa Penida is turning heads once again, as new research digs deeper into the manta ray population, revealing little by little the secrets of these ocean giants
Scientists from the Marine Megafauna Foundation and Murdoch University are reporting a large number of juvenile manta rays in the waters of Nusa Penida, a small island 18 kilometres (11 miles) southeast of
Bali, suggesting the area may be a nursery for this threatened species. The finding is exciting for ocean lovers as few studies have uncovered such a large number of juvenile manta rays before, including significant numbers of newborns.
Reef mantas (Mobula alfredi), which grow up to five metres (15 feet) in disc width, tend to reside and feed in shallow, coastal habitats. They also visit “cleaning stations” on coral reefs to have parasites or dead skin picked off by small fish. In Nusa Penida, they are present year-round making the recently appointed marine protected area (MPA) a snorkelling and diving hotspot. But divers have known this for decades. What is truly surprising is how the animals use and organise themselves around the well-known dive sites Manta Point and Manta Bay.
Manta Bay is described in this new study as predominantly frequented by juvenile males looking for food. Some are as small as 1.5 metres in disc width on first sighting, whichstill might sound pretty big, but is in fact within the known range of newborns (1.3–1.6 metres or 4.2–5.2 feet).This is strong evidence that the site forms part of a nursery habitat and foraging ground for the local reef manta population. Interestingly, most individuals have been seen repeatedly across multiple years, in some cases recording individuals right up to maturity.
“Where manta rays are born and grow up still baffles us. Our research in Indonesia suggests there might be a reef manta nursery in the Nusa Penida area and possibly also in the famous Raja Ampat archipelago,” says lead author Elitza Germanov, a manta ray researcher at Marine Megafauna Foundation and PhD student at Murdoch University. “It’s really important for us to know where these nurseries are. They provide a safe space for young, vulnerable manta rays to grow and develop away from the reach of predators.”
The data for this six-year-long study was obtained from citizen scientists and trained observers submitting ID photos to the global manta ray database, Mantamatcher.org. Overall, 624 individual reef manta rays were identified from 5,913 sightings based on their unique ventral colouration patterns and sex and maturity indicators. And here is where it gets really fascinating.
Interestingly, mature males and females seem to prefer Manta Point, a second dive site a mere 12 kilometres away. In contrast to the foraging juveniles down the street, these adults are here for entirely different reasons. The rays at Manta Point are mostly observed visiting cleaning stations and engaging in social activity as well as courtship display during the mating season (peaking in May). And just like the local disco after hours, it’s extremely rare to see a juvenile at Manta Point!
Manta Bay seems to address all the indicators for a nursery: protection from predators, ample food source for young rays, and the animals appear to be using the site over a period of several years at least. Taken together, however, the research points to more than just a possible nursery, but an intricate spatial complexity that may come as a surprise to divers Like all compelling science, the research raises more questions: Why do juveniles avoid Manta Point? Is it a physical risk or are there social barriers in manta culture? And surely there is a pupping ground nearby as well, where females give birth?
The population here also shows a skew towards males (1.4:1) in general, which is the largest yet seen worldwide. It seems there is far more going on at these long-favoured sites than we ever imagined. And yet this also points to the extreme fragility of their world.
With so many mysteries now swirling, the primary focus is on the future survival of a species that has become extremely valuable. The manta ray industry’s total worth to Indonesia has been estimated at USD15 million, making it the secondmost valuable on Earth. Nusa Penida attracts more than 200,000 tourists each year. On the surface, given the range of manta rays as a species, this might not appear to be too bad. However, when you consider that Bali’s mantas are only rarely seen at other dive sites, showing extremely high “site fidelity”, a large proportion of divers are being funnelled through these two small sites.
Of the 11 main dive spots in these waters, Manta Bay and Manta Point are frequented most by tourism operators, putting the local manta ray population under increasing pressure. While manta rays are protected in Indonesia, there are currently few regulations in place to manage the growing tourism industry. The number of boats allowed to enter manta ray habitats is not limited and codes of conduct for manta ray interactions are voluntary. The average annual number of boats present at the two manta ray sites has increased by 60 percent since 2012 and tourism is highest during the mating season.
“Large diving groups and boat engine noise can cause chronic stress to these vulnerable animals,” says Germanov. “Tourists may disrupt the foraging, cleaning and mating behaviour of manta rays if they get too close, which can have serious implications for their growth and fitness.” Limiting the number of tourism vessels allowed at one time and making codes of conduct for diving and snorkelling with manta rays mandatory are proposed as a way to minimise the impact from tourism. Seasonal closure of Manta Point could also be considered to protect the manta rays from disturbance during the mating period.
I. Gede Hedrawan from Udayana University in Bali, another author on this study, says: “Since it was declared a marine protected area in 2014, Nusa Penida has become an attractive area for tourism and even for international scientists. Local authorities have since implemented a code of conduct for zonation and management in this coastal tourism park. This new research will prove useful for MPA managers, informing future reviews and allowing them to assess the appropriate capacity for manta ray watching in this diving hotspot.”
Artisanal fishing poses a further threat to the local population. During the study, 14 percent of manta rays were either trailing hooks and lines or had injuries and even amputations from fishing gear cutting through their skin and cartilage skeleton.
While all fishing activities are officially prohibited in both Manta Bay and Manta Point, the whole west coast of Nusa Penida could be closed off as a precautionary measure.
Dr Andrea Marshall, principal scientist and co-founder of the Marine Megafauna Foundation, says: “This is clearly an important habitat for reef mantas. In fact, the resighting rates in the Nusa Penida MPA are higher than anywhere else in the world.
The two aggregation sites are very close to each other but used for different purposes. We still don’t know much about when and how the mantas move between the feeding, cleaning and mating grounds. Putting the pieces of this puzzle together will help us figure out how to protect these slow-growing animals.”
“Manta Bay is a foraging ground, used primarily by immature individuals, whereas Manta Point is an important site for adult social and reproductive activity” - Germanov et al., 2019