Understanding the Ocean Giants
By Josh Stewart, Manta Trust Associate Director
As I snorkel in the warm, blue waters off the coast of Mexico, my eyes are peeled for ocean giants. Kicking in the mild current, an underwater landscape impossibly dramatic unfolds beneath me; I have a bird’s eye—or fish’s eye—view of peaks rising out of the depths, and cliffs that plunge hundreds of meters straight down into apparent nothingness. From everything I’ve seen previously, this should be a veritable hotspot for oceanic manta rays; the topography is right, the season is right, cleaning stations abound on the rocky reefs, but where are the mantas?
Despite several decades of divers’ and snorkellers’ enthusiasm surrounding manta rays, and an increasingly popular and lucrative industry of manta ray ecotourism, we’re still very much in the dark about oceanic manta rays’ basic ecology and movements. Hotspots for oceanic manta rays—areas or sites where they can be seen frequently and with some regularity or predictability—exist all around the world, but actually getting in the water with an oceanic manta is still largely left up to chance. Reef manta rays, the smaller of the two manta species, seem to have much more confined home ranges, making them easier to study using traditional monitoring techniques such as photo-identification and acoustic telemetry.
The fine-scale movements and habitat use patterns of reef mantas are just now coming into focus thanks to a number of studies around the world. Because reef mantas often visit the same sites again and again over short periods, photo-identification allows researchers to monitor resighting rates, site use by individuals, and oceanographic conditions associated with certain behaviors or habitat use. The restricted home ranges of reef manta rays allow for the effective use of acoustic telemetry, which is the paired use of stationary, passive listening stations and actively pinging tags attached to an animal. When a tagged animal gets within roughly a kilometer of a listening station, its presence is recorded as a unique ID number associated with its tag. In situations where a manta ray’s home range can be effectively covered in a network of listening stations—for example a single atoll in the Maldives, or a single reef area in the Red Sea or Indonesia—acoustic telemetry can show us the fine scale movements of individual mantas, and general trends in the movements and habitat use of the region’s population.
But what happens if a tagged animal swims outside of the acoustic array? The passive receivers only give us presence/absence data, so once an animal is outside the range of receivers, all we know is that it is no longer inside our study area. Is it just out of range, utilizing a very similar habitat that we didn’t cover with receivers, or did it swim halfway around the world, across open ocean thousands of meters deep? While the use of acoustic telemetry is ubiquitous in studies of marine organisms, this fundamental limitation leads us to look for more effective methods of studying the movements of highly mobile marine species, which brings us back to oceanic manta rays.
Both photo-ID methods and acoustic telemetry have been employed to study the movements of oceanic manta rays in Mexico’s Revillagigedo Archipelago. As one of the world’s best oceanic manta hotspots, the Revillagigedo Islands are also one of the most popular tourist destinations for diving with these ocean giants—and regular access to these remote islands through tourism operators makes the region a critical study site for examining the movements of the species. A photo-ID database over 20 years old and almost a decade of acoustic tagging have started to reveal patterns of site use around the islands, but the long-term, large scale movements still remain a mystery. Some identified individuals have gaps of up to 18 years between resightings, while others are seen year after year during the winter season. Acoustic telemetry paints the same picture, with a regular absence during the summer months, some individuals returning in multiple consecutive years, and others disappearing after being tagged. The limitations of these methods are highlighted here: just because an individual wasn’t photographed for 18 years doesn’t mean it wasn’t present, and acoustic tags rarely last more than two years, often falling off much sooner.
So where do oceanic mantas go when they’re not visiting these seamounts and other hotspots? Do they disperse far and wide between resightings, or simply shift habitats with the seasons—a change that cannot be detected with traditional study techniques? While a decade ago this may have been a purely intellectual curiosity, today these questions have a pressing conservation undertone, the answers having ramifications both for the species and many coastal communities that benefit from manta ecotourism.
In recent years, manta gill plates—the filtering mechanisms that allow them to filter zooplankton from the water while feeding—have become a hot commodity in Traditional Chinese Medicine. The pseudo-remedy made from manta gill plates, called Peng Yu Sai, claims to improve a huge range of ailments, from acne to breathing problems, while in reality little evidence exists to suggest any true benefits. Unfortunately demand for manta gill plates has grown with the ballooning middle class in China, and this has filtered down to targeted fisheries for mantas throughout the Indo-Pacific. Catch rates for mantas have been increasing over the last decade, with targeted fisheries growing rapidly, and live mantas caught as bycatch being released less and less frequently. These growing pressures will undoubtedly have drastic and negative consequences for manta populations, given their unusually low reproductive and population growth rates. The big question is just how hard manta populations will be hit, and so far we’re having trouble nailing this down.
One of the big unknowns in conducting stock assessments and determining fisheries impacts for oceanic manta rays is the global population structure of the species. As highlighted above, we have no idea if the global oceanic manta hotspots are connected (i.e. mantas are swimming from Mexico to Mozambique and back every few years), or if the species in broken up into smaller, insular regional populations. Given a global, mixing oceanic manta population, regional fisheries will impact the entire population (manta numbers in Mexico will decline in response to fisheries in Sri Lanka), but will be drawing from a much larger pool and thus local declines will be buffered by regional input and will occur more slowly. In the alternate scenario, isolated subpopulations that are not subject to fisheries pressures will serve as refuges, while populations in fished regions will experience precipitous declines given smaller populations and a lack of migration from non-fished regions.
These scenarios also have implications for management and conservation. A globally mixing population is much harder to manage at a government level, requiring international cooperation that is near-impossible to enforce and fraught with all of the issues of the classic tragedy of the commons. Meanwhile, regional subpopulations may be manageable at the national level or with small cooperative agreements between several countries, brightening the outlook for long-term conservation and management efforts. Similarly, fishing regional populations that are more susceptible to human-induced decline threatens a lucrative and booming ecotourism industry that is valued at almost 30 times the trade in gill plates, providing an incentive for local communities to protect the species or adhere to government legislation.
And so here I am, snorkelling off the coast of Mexico, pole-spear in hand, hoping to answer some of these pressing questions. At the tip of my spear is an archival satellite tag, the latest model of an improving technology that is allowing us to track the large-scale, long-term movements of large pelagic species such as oceanic manta rays. As I make my way to the down-current side of the seamount, I find my self face-to-face with a 4-meter female, swimming up out of the blue to approach a cleaning station. As she passes underneath, I position myself above her, cock the spear, and insert the satellite tag. She flinches slightly, but continues on to the cleaning station. I’m able to grab a photo ID while she hovers above the reef, allowing cleaner fish to pick parasites off of her skin, gills, and even the inside of her mouth. She circles around a few times to take a look at me, and finally glides gracefully down current and into the blue.
For the next six months, the satellite tag will collect data on temperature, depth and light levels. Based on the recorded day length and time-at-noon, we can estimate a daily latitude and longitude, narrowing down the uncertainty by comparing sea-surface temperatures recorded by the tag to those recorded by satellites. Depth data provides insight into the habitats these animals are utilizing on a regular basis, and swimming mechanisms to maximize efficiency while traveling great distances. At the end of the deployment, the tag pops off of the animal by running an electric current through a corrodible pin, and floats to the surface where it transmits the last six months’ data via satellites.
Satellite tagging studies are becoming increasingly important for determining the geographic ranges, habitat use and connectivity of oceanic manta populations. In conjunction with genetic studies—which help us determine connectivity between populations at much longer time scales—and stable isotope analysis—which gives us a general picture of the trophic ecology and diet of an animal—satellite tag data allows us to paint a more complete picture of where oceanic manta rays are spending their time, how far they travel, whether nearby and geographically disparate populations are connected, and what critical habitats can be targeted for special protection. In contrast to acoustic telemetry, satellite telemetry gives us the full track of a highly mobile animal without requiring passive listening stations throughout the animal’s extensive home range. This information will provide the baseline for determining the impact of targeted fisheries for oceanic manta rays, as well as inform management strategies and conservation plans for the species.
As research sites and tourism destinations often overlap for oceanic manta rays, I’m frequently asked about the wellbeing of the mantas that I’m tagging or sampling. These are important questions that need to be asked, especially considering how critical tourism revenue is to the willingness of stakeholders to protect the species. However, I’ve also seen legitimate concern for these animals turn into fanaticism and aggression towards researchers, based on unfounded fears that tagging or biopsying will scare a manta off for good, never to interact with another tourist again. To alleviate these fears, before we finish here I think it’s important to address some of the major concerns associated with moderately invasive research techniques such as tagging and tissue sampling.
To attach a tag to a manta, I use a small titanium anchor, which is inserted into the dorsal musculature (essentially the shoulder muscle) using a pole spear. Every manta reacts differently, but for the most part they give a small flinch in response to the tagging. Some swim away slowly, some carry on doing whatever they were doing before being tagged, and some actually interact with us after we tag them. As for tissue sampling, many mantas do not react at all, while others also flinch, but they rarely swim away in response to a biopsy. To put this in perspective, we have numerous photo IDs of mantas that have shark bites up to one meter across on the trailing edge of their pectoral fins, which we’ve seen heal in as little as one year! These animals are massive, growing up to 7 meters across, and equipped to survive in often-hostile environments. Therefore it’s safe to assume that a biopsy less than 1 CC in volume—or a flat tag anchor two inches long—will not have any meaningful long-term impacts.
There’s also plenty of evidence to suggest that concerns about tagged individuals shying away from divers are unfounded. We keep extensive photo ID databases that include all of our tagged manta rays, and resighting rates of tagged individuals by divers are no different than those of individuals that have not been tagged or biopsied. Furthermore, both researchers and recreational divers have interacted with mantas that retain their tags long after the initial tagging, and I recently biopsied an oceanic manta, which then interacted with a group of us for almost an hour while we filmed him.
Satellite telemetry is an increasingly vital tool for studying the basic ecology of manta rays, and the knowledge gained through tagging will serve as a foundation for management efforts that have the potential to prevent devastating population declines. As scientists, we recognise the importance of keeping the public informed and involved in this important research. As an organisation, we are developing new opportunities for supporters to get involved in our studies, and invite you to check out our Manta Expeditions, which offer divers and snorkellers the chance to observe this critical science at work, as well as hands-on training in other research techniques such as photo ID collection.
The bottom line is that I am passionate about manta rays! I am driven by a desire to help further the conservation of these increasingly threatened animals and therefore the last thing I would ever want to do is to harm the very animals I am trying to protect. I would never undertake an invasive activity upon these animals if I did not think the minor injuries inflicted upon a few individuals was not worth the large gains which can be made towards furthering our understanding and protection of these graceful giants as a result.