El-Niño 2015-2016 : Conséquences sur les récifs coralliens de Sud-Ouest de Madagascar

Abstract

The third-ever coral bleaching event was announced by NOAA in July 2015, related to the
abnormal rise of Sea Surface Temperature (SST) as well as regional warmings triggered by El Niño
in all three ocean basins (Atlantic, Pacific and Indian). Bleaching extent and severity was
monitoring in May, July, September and November 2016 in 12 stations on outer and inner reef flat,
between a depth of 5 to 10m, in the Toliara bay and Ranobe bay. A quick guide method of observing
coral bleaching in the Western Indian Ocean Region, developed by IOC, was used to assess the
bleaching and mortality rate. A temperature peak of 32.06°C was reached in February 2016 and a
SST rise above the thermal threshold (30.1°C) during 7 weeks. The bleaching rate on all stations
of Toliara bay (21.77%) and Ranobe bay (9.25%) is largely significant. The prevalence of
"Affected" corals in Toliara Bay (25.02%) and Ranobe Bay (20.88%) varies slightly between both
sites. Mortality and Bleaching Index (IMB) less than 15% was recorded on all outer external
stations and more than 15% in the inner stations, except in the Massif des Roses (10.10%) and
Akaranjelita (9.85%) stations. Bleaching taxa response showed 8 genus of corals in the stations of
Toliara bay and 12 genus for the Ranobe bay that did not shows a sign of bleaching. Pocillopora
and Physogyra are the species most affected by the bleaching and Fungia, Acropora, Turbinaria,
Seriatopora, Stylophora and Favia are the species with average sensitivity. SST and station depth
are not the sufficient parameters to explain the bleaching and IMB variation observed on the 12
stations. Other factors need to be studied to clarify the observed variables. On the other hand,
limiting anthropogenic threats on reef ecosystems and increasing the coral restoration project is
necessary to increase their recovery time and maximize their resilience capacity.
Keywords : El-Niño, coral reef, bleaching, coral prevalence, Bleaching Mortality Index,
resilience, Southwestern of Madagascar