Why are Aotearoa’s te whai rahi oceanic manta rays in big trouble?

LYDIA GREEN (Founder and Project Director of Manta Watch Aotearoa New Zealand)

Giants of their kind

Te whai rahi, the oceanic manta ray is the largest ray in the world, reaching up to 7 metres across and weighing up to 2 tonnes. Manta rays and their cousins the devil rays (collectively known as mobulid rays), are essentially flat sharks and share some key attributes i.e., they both have rough skin instead of scales known as dermal denticles (that are actually tiny teeth) and a light, flexible skeleton made of cartilage as opposed to bone. Oceanic manta rays are considered to have one of the most conservative life histories of all shark and ray species, second only to the Greenland shark that can live for more than 500 years! This means that mantas are slow growing, long lived, late to reach sexual maturity and only give birth to one pup at a time, after an extensive gestation period of 12.5 months. Put that all together and you have yourself one vulnerable species, whose populations can’t really handle any form of exploitation.

Historically, mobulid rays have evaded commercial fishing, but in recent decades a new market has established, and they are now heavily sought after for their gill plates for use in Asian Medicine. Gill plates are the cartilaginous structures that enable manta and devil rays to filter their much smaller zooplankton prey out of the water. A combination of clever marketing and pseudo-science have resulted in a sustained demand for dried manta gills plates. As a result, 100,000 manta and devil rays are caught and/or retained as bycatch each year, which have devastated populations around the world. Moreover, it is suspected that oceanic manta rays have undergone a global population reduction of 50–79% over the past 80 years. This was further emphasised in December 2020 when they were officially upgraded from Vulnerable to Endangered on the IUCN Red List of threatened species(1).

Aotearoa’s Oceanic Manta Rays

Despite being fully protected in NZ waters since 2011 under the Wildlife Act, the biggest threat to Aotearoa’s oceanic manta rays is the lack of data. We simply don’t know enough about this lesser-known population to provide adequate protection or establish long-term conservation management. Currently, oceanic mantas have a NZ conservation status of ‘Data Deficient’, which will be eligible for reassessment in 2025. A key objective of the Manta Watch New Zealand (MWNZ) Charitable Trust is to get this domestic threat classification upgraded to ‘Endangered’, which in turn will give NZ mantas higher conservation priority and further acknowledge this species’ worldwide decline. To inform change we need data and lots of it. We can then begin to answer the many questions surrounding this population.

For starters, are oceanic manta rays native or annual visitors, how big is the population, what threats are they exposed to and how are manta using NZ waters? Are there important areas for feeding, breeding, and giving birth that require specific protections?

CRACKING THE MANTA CODE

It quite literally takes a nation and a handful of highly motivated manta enthusiasts (who also happen to be researchers), to tackle the many mysteries surrounding one of Aotearoa’s most elusive species. When MWNZ began in late 2017, we had under 30 verified manta sightings spanning over 20 years and were met with the consensus that this population was too hard to study(2)(3). Fast forward to November 2023, the start of our fourth research season and we have averaged over 200 verified sightings per year for the last 3 years.

Our database currently sits at 886 verified sightings, and we’ve photographically identified (Photo ID’d) 136 manta rays, 20 of which we have also satellite tagged(4). Initial results confirm that Aotearoa’s mantas are world record holders, they dive deeper (1,374m), travel further (1,982 km in 52 days), and go into colder water (3.9°C), than any other manta population have been shown to do! Tag data has also confirmed that at least a proportion of the population migrate into the wider tropical Pacific between late summer and early autumn, heading towards Fiji and Tonga.

Aside from our primary research partners, Conservation International Aotearoa and the University of Auckland, citizen scientists are the project’s foundation, and account for over 70% of the project’s total sightings data. Establishing and maintaining relationships with Aotearoa’s ocean users aka our manta network has enabled this small independent research project to have a significant national reach and impact.

Tikapa Moana - Te Moananui-o-Toi

To date, we’ve conducted most of our research within the wider Hauraki Gulf. Originally this was purely out of convenience, as our research team are Auckland-Aotea based. However, as our understanding of this manta population grows, so does the significance of these waters to te whai rahi. It is of no surprise that the project’s sightings data is heavily skewed in favour of the Hauraki Gulf, as this is where the majority of our survey effort occurs i.e. more people out on the water, increases the likelihood of manta sightings. That said, we were keen to start challenging the bias to better understand how and why mantas are using the Gulf. For example, are the high number of sightings simply related to survey effort or are we getting more sightings in the Gulf because there are a lot more mantas here compared to other regions?

Masters student Rikako Orzaki was the first person to start tackling these questions5. Rika produced a series of Species Distribution Models (SDMs) to determine areas of habitat suitability for manta on a national scale. She did this by running manta sightings and absence data with a whole host of environmental variables that are known to influence their distribution i.e. sea surface temperate, seabed depth, distance from the coast etc. Interestingly, her results highlighted three key areas around the North Island of high habitat suitability for oceanic manta rays. The relatively shallow, highly productive waters of the Hauraki Gulf Tikapa Moana - Te Moananui-o-Toi were shown to be the most suitable habitat for manta on the east coast of the North Island, the Craddock and Jellicoe Channels either, side of Hauturu-o-Toi Little Barrier Island the most preferential6. Through her work, two additional areas on the North Island’s west coast have also been identified as potential manta hotspots, which we’re hoping to further investigate.

So, what are the key drivers influencing manta movements? Satellite telemetry is a great research tool to quickly visualise how mantas are using their habit, both horizontally and vertically. Furthermore, understanding a species’ movement ecology helps us predict their responses to future environmental changes and human impacts. Our current Masters student Tamsin Cooper has been analysing the data from 6 satellite tags to progress our knowledge of fine scale manta movements within the Hauraki Gulf Marine Park. Her results reinforce both our sightings and modelling data, showing extensive use, likely due to foraging behaviours throughout the inner and outer Hauraki Gulf. Tamsin’s analysis also showed that most mantas spent their time in the top 5 metres, in water less than 200m deep, periodically diving down to the seafloor. Additionally, one tagged manta hit all our known manta hot spots, traveling up to the Hen and Chicks, on to the Poor Knights, before heading south to the Coromandel and the Bay of Plenty, nicely confirming regional connectivity.

How can you support our Manta Mahi?

The more data we can collect, the quicker and easier it will be to inform change though long-term conservation management(7). Which will enable greater protections for this globally endangered species, not only in Aotearoa but throughout its entire population range.  None of the research findings above would have been possible without citizen science data. Initially, the project lacked verified sightings, the basic where and when. Now we know where mantas are likely to be (most of the time), so our next focus is to collect more data at an individual level. As, mentioned we have 886 verified sightings compared to only 136 photographically identified individuals, with only 3 of those manta rays having been resighted i.e. Photo ID’d a second time. We can identify manta rays by the unique spot patterns on their belly. Much like a human fingerprint, these markings remain unchanged throughout the manta ray’s life and by cataloguing these images we can passively track an animal’s life history overtime. Key insights into maturity, growth rates, and overall population size, can only be answered with high quality data collection at an individual level. Obtaining high quality images and videos are the most useful, with video being our preferred medium. We can learn so much about a manta ray in 10 seconds of footage (but the longer the better). Getting a good belly shoot is ideal, but if that isn’t possible, make sure to get a good image of the manta ray’s head. Through extensive drone work we have confirmed that the markings and colouration of the manta rays dorsal or top side are also unique, and can also be used to identify individuals, so just do the best you can.

The main take aways are please keep doing what you are doing but think quality over quantity. Get your cameras and/or Go Pros at the ready, stuck to the end of a pole does the trick! And if you are lucky enough to encounter a manta ray, you’ll be prepared. Check out our Manta Ray Data Collection Guide(7) this nicely outlines all the information we need and the many ways you can submit your data to the project. Jump onto our website www.mantawatchnz.org to submit sightings directly and learn how else to get involved and support the project. When you’re out on the water enjoying mantas and collecting lots of lovely data, we want to actively encourage that you do so safely and respectfully for all concerned. For interaction tips please see our Best Practice Guide For Manta Encounters(7). Finally, please spread the word and tell people about the project, so that our collective understanding continues to grow, and we have more eyes on the water looking for te whai rahi.

Follow us @mantawatchnewzealand

Acknowledgements

Special thanks to Ngāti Manuhiri, Ngāti Rehua Ngātiwai ki Aotea, Ngāti Kuri and Ngati Te Rangi / Tuhua Kaitiaki for their support of this project. Massive thanks to our research partners, Dr Mark Erdmann and Dr Edy Setyawan of Conservation International Aotearoa for their invaluable contributions and enabling the project to grow from strength to strength. Big thanks to the University of Auckland and the wonderful Professor Rochelle Constantine and Dr Alice Della Penna for supporting our mahi and mentoring our Masters students, past and present. Speaking of which, big shout out to Rikako Orzaki and Tamsin Cooper for your commitment and hard work towards the project. Many thanks to our project supporters and funders, the Manta Trust, James Kline, Live Ocean Foundation, Pub Charity Ltd and the Lions Foundation. Finally huge, thanks to everyone that has ever let MWNZ jump on your boat to survey manta and to all of our wonderful citizen scientists, we’ll look forward to hearing from you soon!

References

1. Marshall,  A. et al. (2020) Mobula birostris. The IUCN Red List of Threatened Species 2020: e.T198921A68632946 https://dx.doi.org/10.2305/IUCN.UK.20203.RLTS.T198921A68632946.en

2. Duffy, C. & Abbott, D. (2003) Sightings of mobulid rays from northern New Zealand, with confirmation of the occurrence of Mantabirostris in New Zealand waters. New Zealand Journal of Marine and Freshwater Science 37: 715-721. https://doi.org/10.1080/00288330.2003.9517201

3. Manta Watch New Zealand Sightings Map (2023) https://mantawatchnz.org/sightings-map/

4. The Truth About Tagging, Dr Mark Erdmann (2020) https://www.youtube.com/watch?v=avI0JXO98xE

5. Orzaki, O. Modelling the distribution of Oceanic manta and Spinetail devil rays in Aotearoa (2022) https://rikaozaki.shinyapps.io/myapp/

6. Green, L. et al. (2021) Overview of Mobula birostris sightings in Aotearoa New Zealadn and initial observations from a suspected critical foraging area. https://mantawatchnz.org/wp-content/uploads/2022/02/Overview-of-Mobula-birostris-sightings-in-Aotearoa-NZ-2021.pdf

7. Manta Watch Aotearoa New Zealand. https://mantawatchnz.org

Note: Satellite Tagging research was conducted under DOC Wildlife Permit 96119-FAU and University of Auckland Animal Ethics AEC23490.