December 2020

Over the last two years we have made some fantastic progress as a project, we have increased our photo-ID data collection capabilities by fostering new relationships across Fiji, and we are seeing the fruits of our labour, we recorded 852 manta sightings in 2019 and in 2020 we have recorded 1,665! As we continue to expand our photo-ID work we begin to gain a better understanding of the population demographics across the country, how many mantas are there in Fiji? What is the ratio of males, females and juveniles? What percentage of the population display melanism? All questions which can be answered through photographic identification, something which we are constantly striving to roll out in ore areas of the country.

The manta’s movements however are a little harder to understand from photo-ID alone as you need people in the water at exactly the right time to capture the ID and mantas do not wait for us to migrate! Despite this, over a long period of time it is possible to get some movement data through photo identification work, and we have! We have managed to record 5 individuals making migrations off over 250km one way since 2012. Understanding these animals’ movement and habitat use is crucial when attempting to design robust management strategies that will have a meaningful impact on the species survival, we want to protect and manage the critical habitats, the sites that the mantas visit frequently for foraging, cleaning or courting.

However, to really get a handle on habitat use and the movement ecology of these rays we need to utilise the latest technology, enter the satellite tag, by attaching a GPS transmitter to the back of a manta we can get detailed movement data without having to be in the water. The problem is these tags are expensive and of course like tagging any creature, data is not guaranteed, a myriad of problems can arise…

Through a successful grant application and our collaboration with Kokomo Private Island Fiji we managed to secure two pots of money for satellite tagging this year in two locations, the Yasawa Islands and around Kadavu Island.

Once the tags arrived in country we set about calibrating, testing and getting all necessary equipment prepared for the deployment. The tags are deployed on the back of the manta rays by inserting a small metal anchor under the skin, which then is attached to the tag by a tether. The tags we decided to use were SPOT tags, this specific type of tag needs to breach the surface to transmit a location, so this begs the big question, to use a long tether? Or short? Both have pros and cons, a long tether will be more likely to breach the surface when the manta comes shallow, however will have considerably more drag, is more prone to entanglement and even predation, meaning the tag might not stay on the animal that long. A shorter tether however is the opposite, less data but less chance of entanglement, predation, and less drag. Conferring with other researchers and the literature we decided on a variety of lengths between 30-40cm.

All tags were deployed on snorkel while freediving as most of the sites we see the mantas are shallow. The first tag to be deployed was on the ever famous ‘Ell’, she has been our most sighted manta since her first sighting last year. She was seen 103 times during 2020 in the Yasawa Channel, the most of any manta in a single year since our records began. She clearly has a high site fidelity to the channel and might even be a resident of the island chain so we were very excited to see what she would show us. The nervous wait for the first data points began…

With Ell we didn’t have to wait long, within two hours we had our first data, and over the next weeks she gave us many locations, heavily concentrated around the channel which we expected. As the weeks progressed, we have managed to gather a large amount of movement data that has never been captured before. Below you can see a snapshot of Ell and ManTue’s (another tagged manta) initial data.

As of November, we are still receiving lots of fantastic data and are already starting to identify further areas of interest for future surveys to assess if these habitats are in fact aggregation sites. We hope to gather lots more data over the coming months and years so that we can best assist local stakeholders and policy makers in the conservation of our favourite flat fish!