March 2023

When the perfect recipe of prevailing winds, currents, tides and moon phase occurs, Hanifaru Bay becomes a melting pot for large megafauna to feast on tiny nutrient-rich zooplankton. This unsuspecting small reef inlet the size of a football pitch in the Maldives hosts the world’s largest aggregation of manta rays! Faced with the looming deadline of the zooplankton dissipating into the deeper waters at any moment, manta rays race around scooping up as many speedy animals with their unfurled cephalic fins and mouths wide open while this ephemeral hotspot lasts. As a snorkeler being surrounded by 150 large individuals (some the size of a flat VW Beetle car) is nothing less than surreal.

Drone photo of Hanifaru Bay. ©Farih Rasheed

But how and why do the prevailing currents and changes in water properties promote the concentration of zooplankton and ultimately manta rays in Hanifaru Bay? This year we set out to investigate the perfect recipe for mass feeding manta rays in Hanifaru Bay. Lead by Associate Professor Phil Hosegood, an oceanography study was launched in Baa Atoll which combines biological sciences with physical oceanography. The study was designed to monitor the physical oceanographic conditions that are driving manta ray behaviour and zooplankton availability in and around Hanifaru Bay. We used a combination of zooplankton sampling, aerial surveys, manta ray photo-ID and oceanography equipment to address these large overarching questions.

Acoustic Doppler Current Profiler deployment. ©Sophie Owsianka

Oceanography sounds glamorous. As a surfer and sailor, I’ve always dreamed of combining my love for the physical ocean with marine biology. But hold my hand while I take you for a walk down the reality highway of oceanography school.

Think hundreds of kilograms of equipment, large batteries, weird shaped tools, sweaty work-rooms, 5am starts and boat break-downs. It’s far from glamorous, quite the opposite actually. It’s physically a lot of work and preparation, you’re playing with expensive toys that you hope you turned on correctly and some the concepts are tediously information heavy. But did I love it? Oh yes I did! More than I ever expected.

Equipment ready on the boat. ©Hannah Moloney

To be able to combine what animals do with the reasons why they do it is the medicine that I didn’t even know my brain needed. It also helped having an incredible teacher and mentor like Phil Hosegood and a fabulous manta-mad team from the Maldivian Manta Ray Project.

We had a 10-day window with Phil onsite at Four Seasons Landaa Giraavaru to deploy 2 x signature 500 moorings at 50metres, 2 x signature 1000 Acoustic Doppler Current Profilers (ADCPs) tripods at 15metres, and to map the bathymetry of Hanifaru Bay and surrounds. Did we get it all done? No we did not, but we got to a point where Phil could fly home on the plane and sleep easy.

Acoustic Doppler Current Profiler deployment.

Many of these days were filled with moving equipment around on golf buggies in the squelching sun and working on the tools in a sweaty common room in the staff living quarters of Four Seasons. Each piece of equipment, tools, extra batteries and spare parts was all meticulously prepared and planned by Phil and sent over from his workshop at Plymouth University in the UK to our team at Four Seasons. Working in an island nation with a delicate ecosystem and a social system separated by bodies of water; makes conducting a massive study with big pieces of funny-looking equipment challenging. Four Seasons staff from the Purchasing and Engineering departments assisted greatly with logistics. But it was the support from the Four Seasons General Manager Mr Kraenzlin who encouraged our new approach to studying manta rays that made all of this possible.

The many pieces of equipment needed for oceanography research.

During the field work, the ADCPs on tripods were deployed in and outside of Hanifaru Bay in 15metres of water. ADCPs survey the water column using sound, similar to the way that Fish-Finders on a boat detect schools of fish. The data from this equipment will help us to resolve currents (water movement, temperature and velocity) and zooplankton density inside the bay. Recording at a high resolution will allow us to look at the zooplankton biomass in the water column and what may be preying on it. On October 6th we observed a hungry whale shark and 30 manta rays chain feeding (in a line) over the top of the ADCP in Hanifaru Bay. But we will not know what the recording will look like until the data has been processed! If this study shows that we can successfully identify manta rays, whale sharks and zooplankton to a fine-scale resolution, it could possibly change the way we conduct megafauna research.

Whale shark and mantas swim over the Acoustic Doppler Current Profiler. ©Hannah Moloney

At the end of the three-month battery life span of these ADCPs, it was time to do a swap over, but in this instance it was without the supervision of Phil. The MMRP team lead the retrieval of two ADCPs. As we were working with heavy pieces of valuable equipment, it meant impeccable teamwork and timing was a must to make the retrieval as smooth as possible. Once retrieved and back on dry sand at Four Seasons, the tripods and ADCPs were cleaned and taken back to the sweaty common room where the team and a virtual Phil conducted the battery change, data download and the new system set-up on both the ADCPs. These were two very long and tiresome days of work with all team members taking turns to be onsite so we could break for meals and rest.

When the equipment was ready, we triple-checked everything and crossed our hearts. We could say with 95% confidence that they were ready to redeploy. The redeploy mission required scuba-diving, freediving and surface safety for the ADCP outside of Hanifaru Bay due to the “strange” water movement and bad visibility that occurs in this area (take note, this may be a key to unlocking the reasons behind the zooplankton concentrations in the bay). The physical mission was a success but now we all must wait until February when we do the second swap-over to find out if the MMRP team set all the equipment up correctly (please cross your fingers for us!).

With the dark cloud of climate change and the intensity of systems such as El Nino/La Nino looming above us, it is more important than ever to understand what drives these natural systems. A changing climate threatens to decrease zooplankton biomass by up to 50% in some tropical regions, affecting the availability of food for manta rays and other planktivores. This study will help us to understand what drives the world largest feeding aggregation of reef manta rays and how we can aid conservation efforts into the future protecting the habitats that they call home and the food they call dinner.

The Salt, Sweat, And Tears Of Oceanography Research

March 2023

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