Recurring bleaching in Mayotte

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 results have just been published in the online open access journal PeerJ. The study was conducted by David Obura (CORDIO), Lionel Bigot (U. Reunion) and Francesca Benzoni (Tara Expeditions and U. Milan)

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:

  1. >> the rate of warming experienced (0.1oC per decade) is some 2-3 times less than projected warming in coming decades,
  2. >> the interval between severe bleaching events has declined from 16 to 6 years, and
  3. >> 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:

  1. >> coral colony size should be measured,
  2. >> unaffected colonies should be included in counts,
  3. >> quadrats or belt transects should be used and
  4. >> weighting coefficients in the calculation of indices should be used with caution.