Mayotte Reef Monitoring After Cyclone Chido – Fish and Coral Recovery Study

Final briefing before the dive — last reminder of the survey protocol.

After cyclone Chido, the lagoon still bears its scars. Beneath the surface, the coral gardens that once sheltered hundreds of species now show long, pale patches — the mark of a system shaken but not defeated.

This week, we joined Cyrielle, marine biologist at the Parc Marin de Mayotte, for a monitoring session focused on reef fish and algal dynamics. Her work follows a simple question: what happens when algae win? Because when the corals die, the empty space they leave behind is quickly colonised by algae. If these algae stay too long, they can block coral recovery and flatten the habitat. But sometimes, nature finds its own counterbalance — herbivorous fish move in, grazing the algae and keeping the system alive.

Observing fish activity on a dead coral reef.

Cyrielle’s team tracks these signs of resilience through visual counts and standardised transects — 15-metre lines surveyed at fixed depths, always at the same times of day to avoid bias. Each observation, each fish recorded, adds a data point to a much bigger picture: how the reef reorganises after a storm.
Alongside her, Éloïse from the IRD, and two research engineers from the University of Mayotte, study the coral response — how structures hold, collapse or regrow. Together, their work tells a complementary story: fish show the functioning of the reef, while corals reveal its memory.
For the Ambassadeurs du Lagon, this session was an initiation to the quiet side of science. From the surface, I captured two telling moments: a diver writing notes behind a field of dead coral, and a tablet used to identify fish species. Nothing dramatic, yet deeply symbolic — the patient, methodical work that underpins every conservation effort.

Fish identification on board, using notes taken underwater.

Over ten days of surveys, two sectors — east and west — are being documented. A second round in April will allow comparisons, revealing whether algal cover has stabilised, and if herbivore populations continue to rise. This is how science speaks: not in snapshots, but in curves that slowly make sense.
Beyond data, this work also carries a message about responsibility. Field teams constantly question the impact of their missions — the travel, the fuel, the carbon footprint of research itself. In Mayotte, that awareness has turned into action: sharing boats, grouping operations, and keeping the water column clear when measurements require it. Because studying life should never come at the cost of life itself.

Debrief between Éloïse and Cyrielle — assessing coral condition and fish communities.

A related survey is being led by Cyrielle at low tide, this time focusing on fishing pressure along the reef slope. The idea is simple: observe how, when, and where people fish — not to police, but to understand. By linking these patterns with ecological data offshore, scientists can identify moments when the system is most vulnerable and help adjust local practices without conflict.
In the end, algae and fish are two faces of the same story. One shows the space available; the other, who dares to occupy it. Watching both is the only way to know if the reef — and the people who depend on it — can find balance again.

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