Hajangoua islets' biodiversity under threat

The Hajangoua islets, located off the coast of Dembéni, are home to a remarkable yet fragile terrestrial and marine biodiversity. Since July 2023, Les Naturalistes de Mayotte, the site managers, have been implementing measures to combat black rats, which threaten the reproduction of the yellow-billed tern, with support from GEPOMAY for monitoring seabirds. The preservation of these ecosystems raises questions about their future in light of coastal developments.

Monitoring efforts on Kolo Issa

On the largest of the three Hajangoua islets, Kolo Issa, Alicia Sedani and Nakchi Dimassi from Les Naturalistes de Mayotte check wooden boxes placed on the ground with cameras inside, looking for signs of black rats. "The bait, a piece of coconut placed there a month ago, has disappeared. But this does not necessarily mean a rat ate it; we need to look at the camera footage to know for sure," explains Alicia Sedani, who is responsible for ecological restoration of the islets, including the fight against invasive species.

Funding and management of the project

Les Naturalistes de Mayotte has been managing the three islets since July 2023 under a contract with the Conservatoire du littoral, the landowner. Nearly 120,000 euros are allocated to the project through funding from the Direction de l’Environnement de l’Aménagement, du Logement et de la Mer de Mayotte (DEALM) under the Green Fund.

Preventing rat re-invasion

Improved rodent control measures are being implemented to prevent re-invasions. "Rats can enter the islets aboard boats, drifting on floating debris, but they are also very good swimmers and can swim short distances," observes Alicia Sedani, inserting the SD card from a camera into her computer. In total, five wooden boxes, called "rat motels," are scattered across the 3.3 hectares of Kolo Issa. Five others are on Pengoua (3 hectares) and eight on Pouhou (0.6 hectares), an important nesting site for the yellow-billed tern.

Historical rodent control efforts

Rodent eradication operations on the islets began in 2004. In 2005, a campaign eliminated more than 1,500 rats. Despite these results, rats were observed again in 2008. A new operation was conducted in 2012 on the three islets, with rat eradication reported on Pouhou. However, on September 17, rats were spotted again on the islet, to the dismay of the yellow-billed terns.

Ongoing monitoring and biosafety

"Until now, there was no biosafety device with control of potential re-invasions," adds Alicia Sedani, who visits the islets every month to complete her data and check the motels, cameras, and traps, including small cardboard filled with peanut butter. This morning, no signs of rats were found across the islet. A promising sign.

Nest survey on Pouhou

After Kolo Issa, the team heads to Pouhou to perform the same exercise under the scorching heat. Alicia Sedani boards a small kayak from the Mayotte Explo boat, which accompanies scientists during each monitoring of the islets to keep the equipment dry. Nakchi Dimassi jumps into the water to reach the islet and cool off. On site, they meet two members of the Groupe d’étude et de protection des oiseaux de Mayotte (GEPOMAY) and Jérémy Amiot, the head of the Conservatoire du littoral, who are conducting a nest survey of the yellow-billed tern.

Yellow-billed tern nesting habits

On a rugged terrain made of red and porous volcanic rock, they venture to the edge of the cliffs, searching for nests. Some are marked by numbered metal plates placed during previous visits. Occasionally, the shrill cry of a yellow-billed tern, startled by the presence of researchers while brooding in a rocky cavity, startles them.

Impact of rat predation

"At each nest, we note whether there is an adult bird, a fledgling, eggs, whether it is empty or contains broken eggs," explains Elsa Barrandon, coordinator of the Observatoire des oiseaux marins de Mayotte at GEPOMAY. "We check the nests once a month, especially since the return of rats in September. Rats eat the eggs and chicks, which has a significant impact on the reproductive success, which occurs all year. Approximately 95% of breeding attempts fail before the young even fly."

Current status of the yellow-billed tern

"In Mayotte, the yellow-billed tern is doing relatively well. On Pouhou, we estimate the number of nests to be about 150, making it the largest breeding colony in Mayotte. However, the bird is also present in other areas of the territory," continues Elsa Barrandon. "The real question is how long this rodent eradication project will last. We know rats return, it's a continuous battle. But for that, we need funding."

Biodiversity of the islets

Multiple threats weigh on the richness of the islets. In addition to the yellow-billed tern, the Hajangoua islets host a remarkable terrestrial and marine biodiversity. The terrestrial flora includes 83 species of vascular plants distributed across 42 families, including the golden fern (Acrostichum aureum), listed as vulnerable, the white aristolochia (Aristolochia albida), and the Comoros ebony (Diospyros bernieriana). Underwater, 546 marine species have been recorded, including 134 species of hard corals, 177 species of benthic fauna, and 119 species of fish. These waters also host green and loggerhead turtles, as well as the Indo-Pacific humpback dolphin.

Environmental threats to the islets

This richness is threatened by invasive species, but also by pollution, lagoon siltation, fishing and poaching, as well as climate change and marine heat episodes, which weaken coral reefs.

Legal challenges to a desalination plant

These risks are compounded by the threat posed by the future desalination plant of Ironi Bé, whose environmental permit was suspended by the administrative court of Mayotte on July 31. Sued by several environmental associations, including Mayotte Nature Environnement, France Nature Environnement, and GEPOMAY, the court found three illegalities: the lack of information provided to the Marine Nature Park on the chemical substances discharged into the lagoon, insufficient compensation for the destruction of wetlands, and the absence of a protection device against suspended materials during maritime work, as well as specific measures to protect a protected mangrove species. The syndicate Les Eaux de Mayotte and the prefect have six months to regularize these points, pending a final decision.

Uncertain future for the islets

From the Hajangoua islets, the desalination plant's pier is visible on the coast. But what will be its real impact on these natural environments? Hydrodynamics, dispersion of discharges, effects on marine ecosystems... These are questions that are still difficult to answer with certainty.

Proposed expansion of the nature reserve

Les Naturalistes de Mayotte are considering extending the national nature reserve of the M’Bouzi islet to the three Hajangoua islets and their coral reefs. A feasibility study, financially supported by the French Office for Biodiversity (OFB), has been launched to assess this possibility. In the long term, such a classification would allow for the strengthening of the protection of these ecosystems, as well as the long-term implementation of conservation and restoration actions, particularly against invasive species such as black rats. For now, no official request for classification has yet been submitted.

Balancing water needs and conservation

But can such protection be considered when, for the state, the Ironi Bé desalination plant is one of the main responses to ending water shortages on the territory? Between the need for potable water supply and the preservation of marine ecosystems, can the two projects coexist?

Ongoing conservation efforts

In the meantime, Les Naturalistes de Mayotte, GEPOMAY, the Conservatoire du littoral, and the Marine Nature Park continue their efforts to preserve the islets.

Final nest survey

Nestled in a cliff overlooking the lagoon, a young yellow-billed tern watches warily as the GEPOMAY researchers arrive. "I know, I know, I'm sorry, I'm leaving, you're too cute," says Elsa Barrandon aloud before sitting down to record her observations. "That was the last nest, we've done a tour of the islet today. We counted and numbered 128 nests in total. About 30% of the nests checked are occupied by an individual."

No signs of rats found

After an hour and a half, Alicia Sedani and Nakchi Dimassi also finish their checks of the "rat motels" and other control devices. "No signs of the rodent," celebrates Alicia Sedani.

Future monitoring plans

A new round of monitoring is scheduled for November. In the sky, the yellow-billed terns resume their flights, while the boat moves away.