Manta Ray Research Program
Underway since 2003, MMF’s longest running scientific research program focuses on the manta ray. As its principal scientist, Dr. Andrea Marshall, directs her team’s efforts towards key aspects of the biology and ecology of both species of Manta in Mozambique, Ecuador and other key aggregations sites across the globe. A passionate conservation biologist, Dr. Marshall also monitors anthropogenic threats to manta ray populations along the eastern coast of Africa, in southern Ecuador & the Galapagos Islands, and in areas throughout South-East Asia. She is also heavily involved in global conservation initiatives for mantas such as CMS and CITES.
As internationally threatened species, the health of many populations are now being examined and the sustainability of emerging fisheries questioned. Effective management and sustainable long-term conservation can only be properly actioned once the needs of a species and its biological constraints are properly understood. MMF’s team continue to focus their research efforts to best inform on conservation action plans and long-term management strategies in the countries of Mozambique and Ecuador using multidisciplinary approaches to define the conservation priorities of the different populations.
MANTA RAY TAXONOMY
A convoluted taxonomic history precluded the proper conservation assessment of the genus Manta. Since, MMF’s manta ray research team has focused their efforts on clarifying the taxonomic status of this genus and examining the anthropogenic threats facing its species. The genus Manta was officially split in 2009, with a resurrection of a new, second species of Manta (see Marshall et al. 2009). Dr. Marshall then lead authored a re-evaluation of their respective IUCN Red List assessments in 2011. Having already highlighted the existence of a putative third species of Manta in 2009, Dr. Marshall’s team is preparing to formally describe a third species of Manta.
Using photography to identify individual animals (Photo ID) is effective, efficient, and minimally intrusive, and is the underlying methodology used by many of MMF’s research programs. Manta rays can be individually identified by a ‘fingerprint’ of markings on their ventral surface as well as by distinctive scars on their body. These natural spot patterns do not change markedly over time and they can be used reliably to identify individuals for life. In conjunction with Dr. Chris Town of Cambridge University, Dr. Marshall researched and developed a new automated methodology for capturing and comparing the identification markings on mantas. In conjunction with ECOCEAN USA they have now launched Manta Matcher, the world’s first automated, global online manta ray database.
Over the last decade, over 900 individual rays have been identified in southern Mozambique. MMF researchers have been able to examine the population composition of Manta alfredi in this region, revealing a heavy bias towards mature female rays. While males are encountered, they represent less than 20% of the identified population and are encountered less frequently. For more information on this study please see Marshall et al. 2011.
MMF researchers were the first in the world to produce abundance estimates for a monitored population of mantas. In 2011 MMF released a study estimating that over 1,400 manta rays were using the coastline of southern Mozambique, making it one of the largest aggregation areas for rays on the planet. Researchers are now examining the home range size, habitat use and movement patterns of this population.
Our latest research publication (see Rohner et al. 2013) used observational sightings data over the last decade to examine and explain a surprising decline in reef manta sightings along the southern Mozambican coastline. Since 2003 a significant decline (88%) in our standardised sightings time series has been observed for Manta alfredi. A significant decline was not found for Manta birostris, which represents only 21% mantas identified in the region. The sharp decline in reef mantas is thought to be partially a result of an increase in the directed fishing for this resident ray species and partially due to an increase in indiscriminate gill netting along this coastline. The giant manta, a seasonal visitor to southern Mozambique has been less impacted by these anthropogenic factors.
With so few giant manta rays encountered in southern Mozambique, MMF began a jointly run Photo-ID based research project in Ecuador in 2010 to learn more about this poorly understood species. The Ecuadorian database currently stands as the largest and most comprehensive Manta birostris database in the world with over 600 identified individuals. MMF expanded this project in 2013, with a suite of PhD studies, to learn more about the biology and ecology of this elusive species.
HABITAT USE & BROAD SCALE MOVEMENT PATTERNS
Understanding the broad-scale movements of manta rays is critical to understanding both their geographical range throughout the oceans and their migratory patterns within a region. While Manta alfredi appear to be a more localized species, residing within smaller geographical areas, they are still capable of long-range movements. Manta birostris is often referred to as the offshore or oceanic manta and current MMF research has demonstrated that these giant mantas are capable of swift migrations of over a thousand kilometers, suggesting very large home range sizes. Southern Mozambique remains one of the only known locations where both species of Manta live in ‘mosaic sympatry’. To read more about the distributions of Manta species in our world’s oceans, see Kashiwagi et al. 2011.
A combination of acoustic and satellite telemetry is currently being employed to examine habitat use of Manta alfredi in southern Mozambique, rate of movement, broad-scale movements and seasonal shifts in habitat selection. MMF’s passive acoustic listening stations array spans over 250 kilometers, with twelve passive acoustic listening stations strategically deployed along the Southern Mozambican coast. This local array is linked to both the OTN array in southern Africa and Wildlife Conservation Society’s Indian Ocean ‘Meganet’, which has arrays operating in many locations in the western Indian Ocean, including Madagascar, Tanzania, Kenya and South Africa.
Individual reef mantas are being tagged regularly with acoustic tags, which have a detection range by the receivers of approximately 500 meters. Over time, presence / absence data combined with the frequency and duration of individual visits to critical habitats, will help to elucidate the home range and movement patterns of this species in this area. Pop-up satellite tags, which archive data such as water depth, water temperature and daily positions of tagged animals over a predefined period of time (e.g. 4 months), are being used to examine broader scale movements of individual rays along the Mozambican coastline. Combining these different approaches in a far more robust study will not only provide information required for the design of a marine protected area being proposed in southern Mozambique, but will target protection measures where they are most needed, increasing the effectiveness of long-term management efforts. By sharing movement data with other regional nodes, we also aim to examine linkages between neighboring countries and begin to develop cooperative management strategies.
Globally, MMF is engaged in a groundbreaking Save Our Seas funded study, which examines the home range, broad-scale movements, habitat use and diving behavior of Manta birostris through the world’s three major oceans. On a finer scale, MMF researchers and their collaborators are using data to examine seasonal movement patterns, habitat use and anthropogenic threats to populations of giant manta rays in Ecuador, Myanmar and southern Mozambique.
Applying photographic-identification methodology, MMF’s Manta Research team have examined many aspects of the reproductive ecology of reef manta rays, Manta alfredi, over the last decade (see Marshall and Bennett 2010). In Southern Mozambique reef mantas give birth in the austral summer period after a gestation period of approximately one year. Reproductive periodicity in M. alfredi was most commonly found to be biennial, but a few individuals were seen to be pregnant in consecutive years, confirming an annual ovulatory cycle. Normally mantas give birth to a single pup, although two pups are conceived on occasion (see Marshall, A.D., Pierce, S.J. & Bennett, M.B. 2008). MMF’s long-term research in this field has significantly contributed to the limited reproductive information currently available for this species. It has highlighted the need for immediate conservation strategies, with manta rays being one of the least fecund elasmobranch species in the world.
Elucidating the trophic role of a species is vital to understanding its general ecology and the role it plays within its larger community. MMF researchers are currently examining the feeding ecology of rays along the Mozambican and Ecuadorian coastlines, including general dietary preferences and seasonal shifts in diet. The waters off both Tofo Beach in Mozambique and Isla de la Plata in Ecuador are incredibly productive and host some of the largest aggregations of mantas in the world.
MMF researchers are using a range of emerging, non-lethal, biochemical techniques from fatty acid to stable isotope analysis to examine the assimilated diet of these giant rays. Our researchers are also using remote sensing techniques to examine the oceanographic influences impacting the availability of prey in these critical habitats. Dynamic conditions in these regions are sure to influence short-term and seasonal movements of manta ray populations in these areas and understanding these relationships may help to monitor and manage these populations over space and time.
QUANTIFICATION OF NATURAL PREDATION & BODY SCARRING
Using photographic identification, MMF has also been able to conduct an in-depth study on the natural predation of mantas by sharks in southern Mozambique. In great contrast to other known aggregation sites, 76% of all identified mantas in this region bear significant injuries from shark attacks, the highest percentage to be reported in the world. These injuries come from attacks made by a wide range of species including reefs sharks, bull sharks and tiger sharks. For more information on this study please see Marshall and Bennett 2010.
Concurrent to this study, MMF researchers are using photo ID techniques to identify and monitor anthropogenic impacts on resident and seasonal populations of mantas in both Mozambique and Ecuador. By classifying and quantifying these impacts, MMF scientists hope to be able to prioritize management action plans in the respective regions and provide government with the information needed to restrict fishing methods/tackle, institute seasonal fishing bans, and regulate tourism.
Since 2003 MMF researchers have built up the world’s largest genetic database for manta rays, with samples collected from populations across the globe. To extract these samples we use highly efficient Hawaiian slings with biopsy tips that remove small plugs of tissue from the pectoral fins of individual rays. To date we have used our growing database to examine the genetic signature of the two recognized species of mantas, Manta alfredi and Manta birostris, and estimate divergence time.
MMF researchers and our colleagues are continuing efforts by using this database to genetically differentiate discrete populations across the globe, make inferences about movement patterns and estimate effective population sizes regionally. Looking towards the future, we hope to focus in on the relatedness of individuals at the population level in Mozambique and overlay data with detailed ecological information previously collected.
ECONOMIC EVALUATION OF TOURISM
Eco-tourism is proving to be an excellent way of generating a positive, sustainable balance between protecting populations of animals and creating an economically viable alternative to fishing. In 2012 MMF joined forces with other researchers across the world to produce the Manta Ray of Hope Report, which estimated global manta ray tourism at over 50 million dollars annually. These figures are astonishing when compared to the $250-500 US that a dead manta can yield in the fish markets across the world.
On the ground in Mozambique, MMF researchers are currently producing an economic evaluation of manta ray tourism in the Inhambane Province to explore and project the value of these animals to the present and future economy of this region. On both a global and more localized level, it is our hope that these studies challenge the acceptance of manta fisheries and make a more compelling case for their protection and management as an economically viable species for tourism.
Understanding the complex nature of human-induced impacts, MMF scientists are equally concerned about the negative influence of tourism on the behavior and general health of local populations. Our scientists are working towards reducing impacts on rays through the production of educational material, the development of a manta ray code of conduct, and the overall encouragement of sustainable diving practices in both the Inhambane Province in Mozambique and in Manabí Province in Ecuador. Globally, Dr. Marshall has co-developed the first manta ray awareness specialty for PADI, a course that aims to both increase awareness about the species and train divers as ‘citizen scientists’ taking and uploading ID images across the world for use in global research projects.