Mesophotic Reefs and Our Deep Team
Why we went deep
A decade ago, mesophotic reefs seemed to offer one of the most promising stories in reef science. These are the reefs of the twilight zone, from about 30 down to 150 metres, where light fades but corals still grow.
The idea was simple and appealing: if deep reefs escaped the heat and storms damaging the shallows, they could shelter the same species and help reseed shallow reefs after bleaching. Scientists called it the deep reef refugia hypothesis, and we were as excited about it as anyone. It was a major reason we trained technical divers and began working on ways to survey our own deep reefs.
Research since then has told a more complicated story. Deep reefs share fewer species with the shallows than hoped, and they do not escape the damage. During the 2016 mass bleaching on the Great Barrier Reef, 40% of corals at 40 metres bleached, and bleaching has since been recorded as deep as 90 metres in the Indian Ocean. Closer to home, a study off Zamboanguita and Dumaguete that we supported in the field found that only about one in ten fish species were shared between shallow and mesophotic depths, and about one in four around Apo Island. Most scientists now see mesophotic reefs as ecosystems in their own right: rich, little explored, and in as much need of protection as the shallows.
That change in understanding shifted our priorities. Our own deep monitoring programme is on hold, and our deep team now spends its time where it does the most direct good: removing ghost nets and other lost gear from deep reefs, and supporting visiting scientists who work at these depths.
Sources for this section
- Bongaerts, P., Ridgway, T., Sampayo, E.M. & Hoegh-Guldberg, O. (2010). Assessing the 'deep reef refugia' hypothesis: focus on Caribbean reefs. Coral Reefs 29: 309-327.
- Rocha, L.A., Pinheiro, H.T., Shepherd, B. et al. (2018). Mesophotic coral ecosystems are threatened and ecologically distinct from shallow water reefs. Science 361: 281-284.
- Frade, P.R., Bongaerts, P., Englebert, N. et al. (2018). Deep reefs of the Great Barrier Reef offer limited thermal refuge during mass coral bleaching. Nature Communications 9: 3447.
- Diaz, C., Foster, N.L., Attrill, M.J. et al. (2023). Mesophotic coral bleaching associated with changes in thermocline depth. Nature Communications 14: 6528.
- Abesamis, R.A., Utzurrum, J.A.T., Raterta, L.J.J. et al. (2020). Shore-fish assemblage structure in the central Philippines from shallow coral reefs to the mesophotic zone. Marine Biology 167: 185.
What we tried
In 2018, we set out to develop a practical way to monitor the upper mesophotic reefs along the south Negros coast, between 30 and 50 metres. We started by reviewing the survey methods already in use at these depths, hoping to arrive at one that other NGOs and universities could use too. We then adapted our own shallow-reef fish and benthic surveys to the short bottom times of deep dives, so we could compare the deep data directly with what our volunteers record in the shallows.
We also tested different combinations of divers and cameras to see which gave the better picture. In June 2018, we presented the results at the Asia Pacific Coral Reef Symposium in Cebu: divers were more accurate than cameras for both fish and seafloor cover.
In 2019, the team put the method to work in Bacong, with a series of deep surveys ahead of the deployment of a large artificial reef.
The next step was to establish a standing team to monitor our deep sites year after year, but that is where the programme paused.
Play video: Deep survey in Bacong, 2019
The deep team today
Today, our technical divers go deep when there is work to do, most often to deal with ghost nets. Lost and abandoned fishing nets settle over deep reefs, where they can keep trapping and killing fish for years and smother the corals and sponges beneath them. Because the nets lie below recreational depths, few people ever see them, and fewer still can remove them.
Over the years our deep teams have removed more than a kilometre of ghost nets. In Bacong in 2019 alone, a series of dives recovered an estimated 350 kg of discarded net from mesophotic reefs, though large hauls are brought up every year. You can read more about this work on our Combating Ocean Plastics page.
Training the deep team
Our technical diving programme now focuses mainly on building skill. We train our divemaster and instructor candidates, and our own staff, in the planning, gas management and steady habits that diving well beyond recreational limits calls for. That means we have people ready to do the job properly when work comes up below 30 metres.
We train technical divers because we need them. Progressing from recreational diver to TEC 50 or Trimix level typically takes around 24 weeks, so we offer the training only to interns whose research requires it and to volunteers staying at least four months. See Technical Diving for courses and requirements.
Supporting visiting researchers
Our deep team also helps visiting scientists whose research takes them below recreational limits.
Dr Angela Stevenson, now at the University of Plymouth, studies crinoids, the feather stars and sea lilies found from the shallows to the deep sea. We have supported her fieldwork a number of times, including dives to between 60 and 90 metres. Her work here includes a study at Malatapay of how feather stars regrow lost arms, and the first recorded case of a hawkfish feeding on a feather star, photographed off Dauin.
Dr Rene Abesamis, a reef fish ecologist now at the University of the Philippines Diliman and previously at Silliman University, has worked on the reefs of southern Negros for many years. We have given his team logistical help on about half a dozen occasions, including fieldwork for a study comparing fish communities from shallow reefs down to 90 metres off Zamboanguita and Dumaguete.
We teach and dive trimix, but open-circuit technical diving has its limits. Below about 90 metres, the helium each dive needs makes the work very expensive, and open-circuit gear allows only a short bottom time. For work deeper than 70 metres that needs more than 20 minutes of bottom time, the better tools are rebreather divers, who can go a bit deeper and stay a bit longer, and ROVs (remotely operated vehicles).
