Species: MANTA RAYS
A DEEP DIVE INTO THE SOCIAL AND SEXUAL HABITS OF THE OCEANS’ LARGEST AND SMARTEST FISH
One of the first nations in the world to protect mantas, the island republic of Maldives is swarmed twice a year during the monsoon seasons by hundreds of the giant rays, which swim into Hanifaru Bay to feed on plankton. With wingspans as wide as seven meters across, oceanic manta rays (mobula birostris) are some of the largest animals in the ocean. They are also some of the smartest — they have the largest brain-to-body ratio of all fish, which might go some way in explaining their social and curious natures.
As filter feeders, they swallow thousands of cubic meters of plankton-filled water every day – and inevitably, sadly, thousands of pieces of microplastic too. Over decades, these toxic substances can accumulate and alter the hormones that regulate an animal’s metabolism and reproductive functions. But in this interview with Tam Sawers, who heads up the Maldivian Manta Ray Project (MMRP), we learn that ‘ghost’ fishing nets and lines also present just as urgent a threat.
Founded in 2005, the Maldivian Manta Ray Project is a country-wide network of dive instructors, biologists, communities, tourism operators and a dozen MMRP staff working to improve our understanding of mantas in order to ensure their on-going management and protection – often collaborating with Parley to remove the plastic they encounter on live-aboard trips to remote islands. Since the project’s inception, the MMRP has identified over 5,000 different reef manta rays (Mobula alfredi) – which reach up to 4.5 meters across – making the Maldives reef manta ray population one of the largest, and one of the most intensively studied in the world. Here Tam describes how it feels to swim with mantas, getting sucked into a cyclone during their feeding frenzies, and their very unique courtship ritual – the manta mating train.
Reef manta rays, chain feeding at Hanifaru Bay © Guy Stevens | Manta Trust
Reef manta rays, mass feeding event at Hanifaru Bay © Guy Stevens | Manta Trust
DO YOU REMEMBER YOUR FIRST ENCOUNTER WITH A MANTA RAY?
I was very fortunate that my first encounter was out in the Maldives, in Hanifaru Bay. I went there for my master's degree, and I was based with the Manta Trust. My first day out on the research boat started out as a very glorious, beautiful, picture-perfect day with the sun beaming down, and perfect, idyllic islands around me. And we cruised, meandering around the islands to go to the hotspots where the team regularly encountered manta rays, and we reach Hanifaru Bay after about half an hour, an hour. The captain moored up the boat while the team readied themselves and got into the water. I was still a bit clumsy – I've only had the cold seas of South Africa, where I’m from, to swim in, and it's never been with the snorkel and mask. So, we got in the water, and lo and behold there was this graceful, beautiful manta coming in line with me, glided past me, and I was just absolutely overwhelmed. I remember at that moment, everything just froze – I was completely mesmerized. I felt like I was probably the luckiest person on the planet at the time. And then it started to rain, and I saw another manta cruise past, and I was like, ‘There's more. And they're huge. They're massive.’ I wasn't expecting that. It was something I will remember forever – it changed the course of my career.
WHAT WERE YOU RESEARCHING FOR YOUR MASTERS DEGREE?
I did more of a social science project. I was interviewing the fishermen on the local islands because we have a lot of information about the mantas from 2005 onwards, but we really don't know what the situation was like in the Maldives prior to 2005. I used that information to see how things might have changed, and how the balance has changed in the past couple of decades. Also, as Hanifaru Bay is a protected area, we wanted to see how they felt about the designation and whether it had affected them negatively or positively, because now it's a tourist destination. I was interviewing fishermen of 70, 80 years-old and they were telling me all their stories about mantas and whale sharks.
Tam Sawers identifying a reef manta ray © Simon Hillbourne | Manta Trust
“There was this graceful, beautiful manta gliding past me, and I was just absolutely overwhelmed. I remember at that moment, everything just froze – I was completely mesmerized.”
HAVE MANTAS EVER BEEN FISHED IN THE MALDIVES?
Manta rays were never really fished in the Maldives, but they used to fish whale sharks for their liver oil to oil their boats [as fellow filter-feeders, whale sharks are often found alongside mantas]. Manta rays in the Maldives are locally known as enmadi: ‘madi’ refers to the ray, and ‘en’ is a word used for baitfish. So, for fishermen they associated the presence of manta rays with the availability of baitfish which they used for fishing. They saw mantas as something that they really want to protect and look after, because they were essentially helping them in their work.
WHAT MAKES MANTA RAYS SO UNIQUE?
Well, they are probably the largest and most intelligent animals in the sea. They have a very large brain to body ratio, and very curious, which makes any encounter with the manta quite a magnificent experience. They also have very social, complex behaviors. When you swim with a manta, you'll probably see that they come up to you – they're inquisitive. They also have well-developed feeding strategies, and they have very intricate and elaborate cleaning processes and courtship behavior. So, these are some of the things that I really feel make these animals unique.
WHAT DOES THE MANTA TRUST DO?
We coordinate global research and conservation efforts. We have a team that consists of marine biologists, scientists, experts in media, educators, and we work together to share and promote the knowledge and expertise we have. We conduct long-term and robust scientific studies to help inform management decisions for NGOs and governments to ensure they have good data to work with when making marine management decisions. We also have projects in about 25 countries now, affiliate projects, and we specialize in collaboration and driving conservation as a collective. Another very big part of our organization is education and spreading awareness – this is where a lot of our efforts go as well.
Reef manta ray, Rasfari North, North Malé Atoll © Guy Stevens | Manta Trust
CAN YOU TELL US ABOUT SOME OF THE KEY FINDINGS FROM YOUR RESEARCH IN THE MALDIVES?
What we've learnt about manta behavior out there is that they migrate seasonally to utilize feeding and cleaning areas on the monsoonal down-current edge of the atolls – there's a lot of movement of the mantas throughout the atolls. We've documented individuals from about 21 of the 26 atolls. We know that they can do very, very long-distance migrations: a lot of our individuals tend to migrate seasonally to take advantage of the concentrations of zooplankton that build up on the different sides of the island, of the atolls, based on the monsoons. There's one manta, her name is Ewok, and she used to be a regular sighting in Baa Atoll, in the central northern area of the Maldives. She used to be seen almost every year, every season, coming to feed in Hanifaru Bay. But one year she didn't arrive, and we weren’t too sure where she'd gone – usually, we see the same individuals returning each year, especially the predominant females. Two years passed, and still no sightings of Ewok, so we were very concerned about what had happened to her. Then we received a submission from someone that was on a live-aboard; ID pictures of manta rays that they had encountered down in the south in a place called Addu Atoll, about 650 kilometers south of Baa Atoll. And one of them was of Ewok – she was pregnant – and she’s never actually made a migration back to Baa. So, what we know about their movements, their courtship behavior, feeding strategies, we’ve learnt primarily in the Maldives.
SO, YOU'RE NOT GPS TRACKING THEM THEN?
No. We don't do any tagging in the Maldives. It's not really legal in the country, they don't approve it for research permit purposes – nor are we allowed to send up drones in Hanifaru Bay. Though we are hoping to get something passed in the next couple of years to tag the oceanics down in the south of the Maldives in a place called Fuvahmulah, because there's a great concern that they are more migratory. Oceanic manta rays have the ability to travel great distances, it seems, but we're not sure where they're going or what they're doing, and where they're coming from. We fear that they might be targeted in Sri Lanka, where they are caught for their gill plate. So, we would like to get more of an understanding of where they're going, even if it's not very far outside of the Maldives. It would be nice to know so we can protect them.
Tam Sawers taking photo IDs © Simon Hillbourne | Manta Trust
“Like any society, the females spend more time cleaning, or just preening themselves, than the males.”
CAN YOU TELL US A LITTLE BIT MORE ABOUT THEIR COURTSHIP BEHAVIOR?
This is something I always really enjoy watching in the Maldives. We have some specific cleaning sites that we're aware of in certain atolls, and this is really a nice place for divers who have never encountered manta rays to really get the full view of them in a kind of peaceful setting – the feeding aggregations are always very amazing, but they’re often rather frenzied, and very exciting. Manta rays use these coral blocks that are quite definite features on the reef as their cleaning station, and this is where all the cleaner fish are, so the manta rays typically come to these sites to clean. Predominantly, females are seen more regularly on these sites – like any society, the females spend more time cleaning, or just preening themselves, than the males. The males usually come to seek a partner, and if they encounter a receptive female, they will start to follow her on the courtship dance. This can actually result in about 20, 30 individuals following the female in what we refer to as a mating train. What's actually happening here is the males are following the female in a line, one behind the other, and the female is assessing the fitness of the males that are following her. She's doing tricks and darting between divers – she could even jump out the water in excitement. She races off, and these males have to follow her if they want to mate with her. And eventually, the one that manages to last the longest and to keep up with her is the one that she lets mate with her. This mating process is only a few seconds to maybe a couple of minutes, and then they separate, and there's no further association. So, mantas are polygamous; they connect with many, many individuals throughout their lifetime.
IT SOUNDS LIKE A SATURDAY NIGHT IN LONDON. YOU’D MENTIONED MANTAS FEEDING CAN BE VERY FRENZIED – CAN YOU TELL US MORE?
When there's a thick concentration of zooplankton in the area, usually a large number of individuals come into the region. It starts out relatively small. Manta rays glide in the form of a feeding chain and may start to loop forming a circular motion; some may be at the surface feeding, and others just circling slowly at the bottom on the sandy floor where the plankton may have settled. As more mantas join in, they form this type of vortex or cyclone. What they're doing is using their sheer body force to pull in all the plankton as a kind of cooperative-feeding. In doing so, they create a whirlpool effect, so everything is drawn in – even you, as researchers, are. If you're taking IDs, you'll be on the outside, but you'll get pulled into this mass of feeding mantas. This cyclone means that they can just open their mouths and they have this abundant source of food which they can engulf with very little effort and very little energy use.
© Simon Hillbourne | Manta Trust
DO YOU WITNESS CYCLONE FEEDING OFTEN IN HANIFARU BAY?
Hanifaru Bay lends itself very well to collection of plankton, and all the things that happen there are primarily because of the shape of the reef, the currents that pass through there, and makes an ideal environment for this kind of activity to take place. People come to the Maldives to see cyclone feeding, and you have to really bring them down to Earth and say, ‘This is not a very common occasion, it's maybe once or twice a year that us researchers might encounter it.’ It does happen elsewhere – I was able to witness it two years ago in another atoll – but we just don't really know where.
WHAT ARE SOME OF THE THREATS TO MANTA RAYS?
The biggest threat to mantas, and mobulas in general, is excessive, targeted and incidental catch in fisheries. This is really driven by the international trade of gill plates, which are used in an Asian health tonic that is not based on anything scientific. Mantas are a K-selected species, meaning that they take a long time to reach maturity and they only produce a few offspring during their lifetime. So, if there is excessive, targeted fishing – or even catching mantas as by-catch – this will have devastating implications for the population in any area. We are also concerned by the threats associated with tourism, and this needs to be managed sustainably to ensure that any interaction between human and animal is not going to disturb the behavior, especially during feeding and cleaning, which are essential processes to their health. Another big concern is pollution: if they don't have healthy reefs then we don't have healthy mantas. We have noticed that if we think about plastic in the terms of fishing line and fishing nets, we find that a lot of our mantas are impacted in the sense that they do get entangled with fishing lines and nets that are passing through the waters.
Reef manta ray dorsal markings © Guy Stevens | Manta Trust
“If we don’t have healthy reefs, then we don’t have healthy mantas”
ARE WE TALKING ABOUT ‘GHOST’ FISHING NETS, LEFT TO DRIFT IN THE OPEN OCEAN?
We've just actually recently done a study on the sub-lethal injuries to manta rays in the Maldives, and we found that 89 percent of the reef manta rays classified with injuries from anthropogenic impacts originated as a result of fishing line and hook entanglement, and in the case of oceanic manta rays, 91 percent were classified with injuries from anthropogenic impacts. This can have devastating implications on their health: it can cause drag if the line is still entangled; it could get hooked on the reef or anything floating in the water column, and could stop or limit the manta from moving. Fishing lines also wrap around the cephalic fins – those fins on the front of the face – and can amputate the fins mantas need to be able to feed. Also, if a fishing line goes around the pectoral fins, it could affect their mobility – it could cut or slice some of the wings at any given time. Ghost nets will probably stop them from moving, and they need to be perpetually moving to breathe, so that is a huge concern as they will inevitably die if they don't get out – they can't swim backwards.
HAVE YOU CONDUCTED ANY RESEARCH ON THE IMPACTS OF MICROPLASTICS ON THEIR HEALTH?
Not in the Maldives, but the Manta Trust are studying it in the Mexican Caribbean – they're still in the process of assessing the data. There's also work being done in Indonesia, where they have found that mantas are capable of ingesting plastic, obviously – but they're also lucky in that they can expel the plastic through excrement and vomiting. But that's not to say that some of the plastic – or any sharp material – might be remaining in their bodies, affecting their nutritional uptake, causing destruction within the body, or transferring toxic chemicals. That's also a big concern.
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Hanifaru Bay reef manta season: May through October, depending on monsoons.
Manta ray photography courtesy of Manta Trust.