Spatio-temporal pattern of coral bleaching in Salary Nord, south-west of Madagascar
Creators: name: Rakotomanga, Tahiana
name: Randrianarivo, Mahery
name: Botosoamananto, Lebely Radonirina
name: Penin, Lucie
name: Adjeroud, Mehdi
name: Todinanahary, Gildas
Publisher: name: Zenodo
Year: 2022
Subjects: Coral reefs
Scleractinian corals
Coral bleaching
Spatio-temporal variability
Marine Protected Area
Madagascar

description: Madagascar's coral reefs have been declining for 50 years. Increased fishing pressure,pollution, and hypersedimentation, accentuated by climatic disturbances, have led to the lossof 40-80% of coral cover, particularly along the southwestern coast where the population ishighly dependent on marine resources. The Malagasy government committed itself to mitigatethis decline in 2003 by increasing the surface area of its Marine Protected Areas (MPAs) dueto their assumed benefits in enhancing coral reef community resilience. However, some failuresnoted regarding MPA effectiveness call this assumption into question. Coral bleaching, one ofthe major threats to coral reef health, is not thwarted by any protection status. As in manyregions of the world, bleaching is increasingly severe and frequent in southwesternMadagascar. A better understanding of the extent and intensity of this phenomenon is essentialfor the conservation and management of these coral reefs.The prediction of an El-Niño phenomenon for the beginning of the year 2020 led us to conductthis study, aiming to assess the extent and impact of the bleaching phenomenon on coralassemblages of Salary Nord, in southwestern Madagascar. The study is based on an adaptivebleaching monitoring method derived from the Rapid Guide to Coral Bleaching MonitoringMethods in the Southwest Indian Ocean. We installed six stations in March 2020, includingthree stations in fished areas and three in unfished, no-take zones. Post-bleaching monitoringwas then conducted on the same stations in March 2021. At each station, 20 quadrats of 1 m²were used to sample corals. We identified coral colonies at the generic level and classified themaccording to four categories of health: “healthy”, “partially bleached”, “totally bleached”, and“dead”.A total of 35 coral genera were recorded in 2020 and 32 genera in 2021. The taxonomiccomposition in the stations during both years shows a similarity in the coral community. Coraldensity showed a significant temporal variation (χ² = 36.051; p < 0.001), whereas spatialvariation among the six stations was similar (χ² = 8.696; p = 0.121). Significantly more coralcolonies were recorded in 2020 (17.1 ± 6.46, mean ± SD) compared to 2021 (10.6 ± 5.14). Theoccurrence of bleaching intensity varied in terms of colony density (χ²= 271.022; p < 0.001).Healthy colonies (18.2 ± 6.51) are significantly more numerous compared to completelybleached colonies (3.78 ± 2.85, Estimated Marginal Means post-hoc; p < 0.001). Similarly,more healthy colonies (18.2 ± 6.51, EMM; p < 0.001) were counted than dead colonies (2.07± 2.02). Bleaching intensity varied significantly between the two years (χ²= 72.560; p < 0.001),with significantly more bleached colonies in 2020 (3.32 ± 2.72) compared to 2021 (0.47 ± 0.88,EMM; p < 0.001). Fishing pressure was a significant factor (χ²= 12.263; p = 0.006), with ahigher abundance of dead colonies in unfished areas (2.42 ± 2.47) compared to fished ones(1.72 ± 1.35, EMM; p = 0.009). More bleached colonies were also found in the unfished areas(3.87 ± 3.32) compared to fished ones (3.7 ± 2.32), though the difference was not significant(EMM, p= 0,06). The density of healthy colonies was not significantly different between fished(19.40 ± 5.79) and unfished (16.90 ± 6.99) areas (EMM, p= 0.155). Among the main generarecorded, the most susceptible taxa were Pocillopora (30.43%), Porites (11.61%), andAcropora (9.59%). The most partially-bleached genera were Montipora (16.78%), Porites(14.69%), and Acropora (11.71%). Those with the most deaths were Pocillopora (18.84%),Stylophora (17.72%), and Acropora (8.14%).In conclusion, bleaching intensity was more severe in 2020 than in 2021. This study highlightedthat Pocillopora, Stylophora, and Acropora are the most "vulnerable" genera, while Poritesand Montipora are the most "resistant". Contrary to our hypothesis, our results showed that theMPA at Salary Nord fails to protect coral reefs from climate-induced threats such as bleaching.Nevertheless, these outcomes should be supported by additional data from a long-termmonitoring of coral assemblages.