Mayotte’s 4th major bleaching event in 4 decades … and the reefs are showing a mirror to the social turmoil and changes that the island is going through. The expedition was planned for just after the peak of thermal stress in the region, which occurred during March/April 2016.
This expedition on IRD’s SIRÈME programme led by Dr. Pascale Chabanet had as its intent to renew the long term monitoring record for Mayotte, and build up further data for mapping and biodiversity (genetic) sampling.
I was particularly interested to join the trip as this would make it my third trip here, after a first one in 2005 through the GIS Lag-May programme and a second in 2010 with the Tara Expeditions – the latter also occurring at the peak of the last major bleaching event. Thus, we would get to see how Mayotte’s reefs recovered in the 6 years since major bleaching (and 40% mortality) in 2010, and the degree of impact from this latest event. Together with Dr. Lionel Bigot of the University of Reunion, we implemented the same methods as applied in 2010 – conducting long 1m-wide belt transects to record the condition of all coral colonies from the tiniest recruits to the largest adults, in a pre-selected range of genera.
On the one hand, results seem good – recovery of corals since 2010 has been fantastic, displaying a burgeoning recruitment and regrowth that we think is due to high connectivity in this northern part of the Mozambique Channel, the center of biodiversity for the Indian Ocean. Further, though this year is hotter than 2010 the degree of bleaching and mortality seems to be less, preliminary estimates suggest up to 30% final impact compared to 40% in 2010.
However, the picture is complicated by other threats affecting the reefs. We could easily recognize coral mortality due to bleaching in the last weeks, but in several sites there was evidence of strong mortality some months earlier – perhaps due to a first pulse of high temperatures in January?; but also due to Crown of Thorns seastar predation and disease syndromes (i.e. not the clear band-diseases); and siltation/degraded water quality from the main island. The fish team found almost no reef predators in their surveys, having been completely removed by fishing.
So the reefs of Mayotte are looking heavily stressed – all by human-derived factors. Their rebound from bleaching in 2010, and the high abundance of recruits even now are indicative of the recovery potential and resilience they still contain – but for how much longer?
We hope that the results of the surveys, and the spatial analyses, provide additional guidance for management and identifying priority sites for the new Marine Protected Area of Mayotte to make a real impact in conserving these beautiful reefs, so they can also support the burgeoning population and development needs on this wonderful island!.