Coral reefs: Seychelles' Aldabra Atoll experiencing "severe" bleaching event

Bleached coral on Aldabra Atoll. (Jock Currie, Seychelles Islands Foundation)

Since the beginning of the year, the coral reefs at the Aldabra Atoll, one of the Seychelles' UNESCO World Heritage sites, are experiencing a severe bleaching event.

A marine researcher on Aldabra working for the Seychelles Islands Foundation (SIF), Dr Anna Koester, said the bleaching event is even more severe than the one in 2016.

Seychelles witnessed a mass coral-bleaching event from February to August 2016 after a particularly hot season coupled with an intense El Niño caused water temperatures to rise and exceed season averages by 1 to 2°C for several weeks, triggering the phenomenon of coral bleaching on the reef.

"It is still too early to say anything specific about the scale of the damage as we are only now approaching the time of the year when water temperatures peak. However, already in February, the heat stresses the corals have experienced were by far higher than during the last bleaching event in 2016," Koester told SNA.

She said, "This heat stress is measured by adding up any temperature exceeding the bleaching threshold (the usual maximum water temperature plus 1 degree Celcius over the past 12 weeks – this measure is called Degree Heating Weeks (DHW). The higher the DHW value, the higher the accumulated heat stress over the past weeks. For example, during the 2016 coral bleaching event, Aldabra's reefs experienced heat stress of a maximum of 6 DHW. Right now, they have experienced 3 DHW already."

Aldabra's reefs are an essential habitat to fish communities. (Fotonatura, Seychelles Islands Foundation) Photo License: All Rights Reserved

Koester said the heat stress "exceeds anything we have measured at Aldabra before, which clearly shows the severity of the current bleaching event. We have also observed some mortality already, however, we will only see the full scale of the mortality in a few months."

She added that they first began to see signs of bleaching at Aldabra at the beginning of January and emphasised the importance of reducing other stressors that affect coral reefs such as pollution, overfishing and physical damage through construction or anchors.

"Marine protected areas, if well enforced, and appropriate coastal management are ways to address this. Nevertheless, we do have to be realistic about what can be achieved on the local level. Coral bleaching events such as the current one are triggered by global warming, which further and further reduces the time periods between severe bleaching events, leaving essentially no time for reefs to recover in between. Addressing climate change therefore remains the primary solution to safeguard coral reefs in the long term," said Koester.

She also talked about building more resilience in corals, especially concerning heat, and said, "There are no immediate options to mitigate the effects of this bleaching event, but we can utilise it to study the processes that make some corals more resilient than others. In the long term, this information can help to better conserve coral reefs and promote their resilience to heat stress."

Recently, the Minister for Blue Economy and Fisheries, Jean-Francois Ferrari, described Aldabra as being a living laboratory when talking about the preliminary results of the Monaco Explorations expedition and how experts at the Oceanographic Institute of Monaco had stated that about 82 percent of corals found elsewhere in the region could originate from Aldabra.

During the Monaco Explorations expedition in November 2022, samples of corals from Aldabra were collected to be studied and preserved, and Ferrari said the samples are flourishing in several institutes.

He said, "It's good that apart from the Oceanographic Institute, they are also being stored in three other laboratories in Europe. If one day we lose the corals at Aldabra because of a bleaching event, these samples will be crucial for their rehabilitation."