Status of Coral Reefs of the World 2000

Status of Coral Reefs in East Africa
El Niño Effects on Coral Diversity and Distributions
Stress from the El Niño event was large scale and intense, resulting in species-specific
patterns of bleaching and mortality across the region. The fast growing genera Acropora,
Pocillopora, Stylophora and Seriatopora showed up to 100% bleaching. Over 50% of
species in these genera are still absent from many reefs 2 years after the event. High rates of
bleaching and mortality also occurred in other genera such as Galaxea, Echinopora, and
other minor acroporids. Low rates of mortality were observed in some corals even though
they exhibited high levels of bleaching (e.g. Fungia, Coscinaraea, anemones). The majority
of other coral species exhibited variable, and moderate to low bleaching and mortality
levels, but because of their low abundance this did not contribute greatly to post-mortality
abundance estimates, e.g. the faviids, acroporids in the genera Montipora and Astreopora,
agariciids, poritids, siderastreids and most of the octocorals and zoanthids. In some
adjacent colonies of the same species showed different levels of bleaching and mortality.
This 1998 bleaching event will have profound impacts on the structure and growth rates of
these coral reefs for many years. The coral species that suffered the highest bleaching and
mortality were typically fast growing, branching species, with high rates of reproduction
(sexual or asexual) and competitive overgrowth. Many of the surviving species are slower
growing massive, sub-massive and encrusting forms, which rarely dominate reef
communities. Therefore any future El Niño events may have very different impacts on
evolving reef community structure due to overall species changes in these coral
communities. The El Niño event may be ‘selecting’ specific gene pools that are more
resistant to temperature impacts.
Recovery Following El Niño
As noted above, there has been variable recovery of coral reefs that suffered high mortality.
Coral recovery by regrowth of surviving colonies has been significant on some shallow
lagoon reefs in Kenya (where coral cover has returned to the 1997 levels of approximately
15-20%). However, recruitment of larvae to all reef zones to replenish lost corals was low
throughout 1999 according to sites surveyed. The first strong signs of major coral
recruitment were first observed in November 1999, with increasing new coral numbers in
Country Reef location Populations of COTS Impact of COTS on reefs
South Africa Two Mile Reef, Spot outbreak of COTS Coral cover under long term
Maputaland to approx. 0.62m-2 decline where affected by COTS
Mozambique Anchor Bay, 3 year infestation to 1999, Coral cover reduced to 2-5%,
Inhambane with recent feeding main survivors Pocillopora,
scars in 1999
Acropora
Inner Two-Mile Reef, Outbreak in 1995 80% mortality, with
Bazaruto
ongoing degradation
Coral Garden, Bazaruto COTS present in 1999 20% mortality, but of mixed causes
Tanzania Changuu, Zanzibar COTS at 0.8-1m-2 Coral cover reduced
from 58% in 1996 to 25% in 1997
Bawe, Zanzibar New COTS aggregations in 1999 Mainly in Acropora thickets
Kenya Shimoni COTS reports from 70s, No recent impacts
no new reports
A summary of crown-of-thorns starfish (COTS) outbreaks in East Africa in the 1990s showing that,
while these are a persistent problem, the threat is increased on reefs where coral cover has decreased
meaning that recovery could be delayed.
69