Spatio-temporal pattern of coral bleaching in Salary Nord, south-west of Madagascar

Madagascar's coral reefs have been declining for 50 years. Increased fishing pressure,
pollution, and hypersedimentation, accentuated by climatic disturbances, have led to the loss
of 40-80% of coral cover, particularly along the southwestern coast where the population is
highly dependent on marine resources. The Malagasy government committed itself to mitigate
this decline in 2003 by increasing the surface area of its Marine Protected Areas (MPAs) due
to their assumed benefits in enhancing coral reef community resilience. However, some failures
noted regarding MPA effectiveness call this assumption into question. Coral bleaching, one of
the major threats to coral reef health, is not thwarted by any protection status. As in many
regions of the world, bleaching is increasingly severe and frequent in southwestern
Madagascar. A better understanding of the extent and intensity of this phenomenon is essential
for 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 conduct
this study, aiming to assess the extent and impact of the bleaching phenomenon on coral
assemblages of Salary Nord, in southwestern Madagascar. The study is based on an adaptive
bleaching monitoring method derived from the Rapid Guide to Coral Bleaching Monitoring
Methods in the Southwest Indian Ocean. We installed six stations in March 2020, including
three stations in fished areas and three in unfished, no-take zones. Post-bleaching monitoring
was then conducted on the same stations in March 2021. At each station, 20 quadrats of 1 m²
were used to sample corals. We identified coral colonies at the generic level and classified them
according 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 taxonomic
composition in the stations during both years shows a similarity in the coral community. Coral
density showed a significant temporal variation (χ² = 36.051; p < 0.001), whereas spatial
variation among the six stations was similar (χ² = 8.696; p = 0.121). Significantly more coral
colonies were recorded in 2020 (17.1 ± 6.46, mean ± SD) compared to 2021 (10.6 ± 5.14). The
occurrence 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 completely
bleached 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 a
higher 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 genera
recorded, the most susceptible taxa were Pocillopora (30.43%), Porites (11.61%), and
Acropora (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 highlighted
that Pocillopora, Stylophora, and Acropora are the most "vulnerable" genera, while Porites
and Montipora are the most "resistant". Contrary to our hypothesis, our results showed that the
MPA 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-term
monitoring of coral assemblages.