Major coral bleaching has been reported in Mayotte four times – in 1983, 1998, 2010 and 2016. During the 2010 event a survey carried out by the Tara Expeditions documented the bleaching in detail, estimating the full impact of the bleaching event.
The study found that 52% of coral area was impacted – 19% pale, 11% bleached, 5% partially dead and 18% recently dead. Acropora, the dominant genus, was the second most susceptible to bleaching (22%, pale and bleached) and mortality (32%, partially dead and dead), only exceeded by Pocillopora (32% and 47%, respectively). The majority of genera showed intermediate responses, and a small number almost no response.
A linear increase in bleaching susceptibility was found from small colonies (< 2.5 cm, 83% unaffected) to large ones (> 80 cm, 33% unaffected), across all genera surveyed. Maximum mortality in 2010 was estimated at 32% of coral area or biomass, compared to half that (16%), by colony abundance.
Mayotte reefs have displayed a high level of resilience to bleaching events in 1983, 1998 and the 2010 event reported here, and experienced a further bleaching event in 2016. However, prospects for continued resilience are uncertain as multiple threats are increasing:
- >> the rate of warming experienced (0.1oC per decade) is some 2-3 times less than projected warming in coming decades,
- >> the interval between severe bleaching events has declined from 16 to 6 years, and
- >> evidence of chronic mortality from local human impacts is increasing.
In relation to monitoring coral bleaching, the study produced four recommendations to reduce bias and strengthen credibility of results:
- >> coral colony size should be measured,
- >> unaffected colonies should be included in counts,
- >> quadrats or belt transects should be used and
- >> weighting coefficients in the calculation of indices should be used with caution.