GCRMN_COI_2017-Western Indian Ocean Reef Status

Coral reef status report for the Western Indian Ocean (2017)
Global Coral Reef Monitoring Network
Fish community structure has also shown an important change. Presumably before the
1998 bleaching (and certainly before the advent of heavy fishing pressure in earlier
decades), functional group structure would have been diverse, representing a balanced
food web. It has now shifted to strong dominance by herbivores and detritivores (about
80% of fish biomass), and increasingly, by smaller-bodied individuals of these functional
groups.
Coral bleaching - 2016. The second largest bleaching event in the WIO occurred in
2016, with just over one third of reported sites affected by severe bleaching at its peak in
April-May 2016. Seychelles was the most affected by bleaching with over 50% of reported
sites showing high or extreme bleaching, closely followed by Tanzania and Mauritius.
Fortunately, the high and extreme levels of bleaching did not translate into the same level
of coral mortality, with less than 10% of sites reporting severe mortality. However, the
region did not survive completely un-scathed with over 70% of reports recording some
level of mortality.
The future? Western Indian Ocean coral reefs seem to have shifted from a pre-1998
state to a post-1998 state with 25% lower coral cover and 2.5 times algae abundance.
For almost 2 decades, coral and algal cover have been equivalent, which may be an early
indicator that at the regional scale, reefs are approaching a threshold past which they
may become dominated by algae, or by other non-hard coral invertebrates. This may be
exacerbated by the fish communities also shifting from more complex to simpler trophic
webs in which herbivory and detritivory by small-sized fish are dominant processes.
EXECUTIVE SUMMARY
Exactly what the implications of these regional trends in coral and algal cover and fish
community structure are likely to be is difficult to predict. Coral cover experienced another
step decline due to the bleaching event of 2016, but fortunately the decline in the order
of 10%, is less than the 25% decline documented in 1998. Fishing pressure on reefs
continues to increase with human population growth and migration to the coastal zones.
The latter will also drive up pollution and eutrophication on coral reefs, providing bottomup
fuel to the increasing role of algae and herbivory/detritivory as dominant processes.
Ongoing climate change is already inducing more frequent major coral bleaching and
disease events, and acidification will increasingly undermine the ability of corals to resist
other threats. In coming years, will algal cover continue to increase above the level of hard
coral cover? And if this happens, as more and more individual reefs decline, will it become
harder and harder to return coral reefs to a coral-dominated state from local to national
and regional levels?
The results of coral reef monitoring from 1992 to 2016 paint a clear picture for coral
reefs in the Western Indian Ocean. While the current state is reasonably good in terms
of coral cover, the high algal cover may be a sign of vulnerability to future changes. To
have any hope of coral reefs resisting and recovering from major threats, climate change
must be addressed globally with greenhouse gas emissions reduced sharply to meet the
Paris Agreement levels. On the other hand, local drivers of decline must also be reduced
as far as possible to limit the number of and interactions between threats. Increasing
‘effective management’ to all coral reef areas is a goal that should be attractive to key
reef stakeholders such as local and national government, fishers and tourism operators, as
this will be the only way to sustainably derive benefits from coral reefs in a warming and
increasingly populated world.
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