CORDIO Status Report 1999.pdf

Coral mortality in the Chagos Archipelago
DR CHARLES SHEPPARD
Department of Biological Sciences, University of Warwick, UK
SUMMARY
Most of the corals and soft corals on the seaward reefs of
all six Chagos atolls have recently died. The reefs of
Chagos were previously known to be among the richest
of the Indian Ocean, as well as the least affected by man.
However, 1999 comparisons between the present
situation and that found by earlier surveys, particularly
during 1996, showed that only about 12% of the substrate
on the seaward reefs is live coral today. Up to 40%
was identified as dead coral, and 40% as unidentifiable
dead substrate, much of which is almost certainly
severely eroded, dead coral. In 1996, an average of about
75% of the substrate was living corals and soft corals.
Some reefs have fared better than others, but on many
of these once spectacular reefs, such as the seaward reef
of Nelson Island, no living coral or soft coral (or any
living coelenterate at all) was seen during a 20 minute
snorkel in clear water. Lagoons contained greater
amounts of living coral (about 28% living coral, with
67% of the substrate covered by dead colonies or bare).
Most corals are becoming heavily eroded, leaving
extensive rubble. Rubble on reefs is highly erosive and
tends to prevent recolonisation. Reef fish were not
counted systematically, but on many reefs there were
clearly less than 25% of the former abundance and
diversity. The same applies to other invertebrate groups,
such as starfish, urchins and molluscs, which appear to
have become very uncommon in Chagos waters, at least
above 20 m.
This situation represents unprecedented destruction
and change of coral reef communities. Possible consequences
are outlined below. It is clear, that the situation
requires a substantial re-think of present approaches
and research priorities in coral reef science and conservation.
INTRODUCTION
Following the rise in surface seawater temperature
(SST) in 1998, there have been numerous reports of
coral bleaching and mortality in tropical waters. It is a
world-wide phenomenon, but of varying extent. In the
Indian Ocean, the temperature rise occurred between
April and June 1998, and reports of affected reefs have
now been posted from many locations in East Africa,
the Maldives, Sri Lanka and throughout South-East
Asia.
This report documents the phenomenon in the reefs
of the Chagos Archipelago, where effects have been
severe. First, a brief background to the Chagos reefs is
given.
REEFS OF THE CHAGOS ARCHIPELAGO
All atolls and submerged banks of Chagos appear to be
actively growing reefs. Most reef flats dry out at low
tide, especially those on the seaward side of the islands,
and compared to many other reefs in the Indian Ocean
they are depauperate, partly due to the occasional
coincidence of low spring tides with solar noon. Areas
lying close to shore which dry, have the poorest biota.
Seaward of this is a boulder zone with storm tossed reef
fragments colonised by the surge-resistant cup sponge
Phylospongia and the alga Turbinaria. The boulder zone
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