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 18km southeast of Bali, suggesting the area may be a nursery for this threatened species.

Reef mantas (Mobula alfredi), which grow up to 5m, 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 Marine Protected Area (MPA) a snorkeling and diving hotspot.

One of the popular dive sites (Manta Bay) described in this new study is predominantly frequented by juvenile males looking for food. Some are as small as 1.5m in disc width on first sighting, providing strong evidence that the site forms part of a nursery habitat and foraging ground for the local reef manta population. Most individuals have been seen repeatedly across multiple years.

Image: Simon Pierce (www.simonpierce.com)

“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”, said lead author Elitza Germanov. “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.”

Interestingly, mature males and females seem to prefer Manta Point, a second dive site a mere 12 km away. Here, the manta rays are mostly observed visiting cleaning stations and engaging in social activity as well as courtship display during the mating season (peaking in May).

Nusa Penida attracts more than 200,000 tourists each year. 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 Indonesia², 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% since 2012 and tourism is highest during the mating season.

Germanov commented: “Large diving groups and boat engine noise can cause chronic stress to these vulnerable animals. Tourists may disrupt the foraging, cleaning and mating behavior 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 snorkeling with manta rays mandatory are proposed as a way to minimize the impact from tourism. Seasonal closure of Manta Point could also be considered to protect the manta rays from disturbance during the mating period.

Artisanal fishing poses a further threat to the local population. During the study, 14% 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 said: “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.”

For more information about the work of the Marine Megafauna Foundation visit their website by clicking here.