GCRMN_COI_2017-Western Indian Ocean Reef Status

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Coral reef status report for the Western Indian Ocean (2017) Global Coral Reef Monitoring Network Three groups of countries are apparent, considering trends in the post-bleaching period from 1999 to the present: • Increasing coral cover is shown strongly for the Seychelles and Kenya, both undergoing significant recovery of coral communities from 1998 to 2015. The line for Rodrigues is ambivalent: a slow increase is apparent in sites reported up to about 2010, but the omission of low-cover sites in the last year of monitoring imposes a larger increase that is misleading. Hard coral cover has increased over the last 25 years in South Africa, perhaps due to slight warming from colder conditions considered marginal for hard corals, and thus improving conditions for coral growth (Celliers and Schleyer 2002, Porter and Schleyer 2017). • No overall change in coral cover is shown for Comoros, Reunion and Tanzania, though there is clearly high variation around the mean in each year in all three countries, and both increases and decreases across subsets of time during the monitoring period. For Comoros and Tanzania, inconsistency in sites being monitoring is apparent (e.g. in 2009 in the former, and 2002-2007 in the latter). Reunion has the most consistent dataset with 14 sites monitored annually throughout the period, with only one missing site-byyear combination. • Declining coral cover is shown in Madagascar, Mauritius and Mozambique, though the dataset in Mozambique is too sparse to draw firm conclusions. Recent monitoring and reef assessments in coral reefs in the north of Mozambique do not have sufficient years of data, so have been excluded. In Mauritius, two lines are shown to distinguish two different sets of data, with each of these showing a decline. Madagascar has a data set that is very geographically dispersed, with initially a strong decline in the early 2000s, slight increase in coral cover from 2008-2010 then a sudden drop likely due to bleaching in 2010 and recovery since – but the linear trend is an overall decline. REGIONAL CHAPTERS Across all countries, the 1999 – 2015 period displays overall stability of coral cover with some reefs and countries improving, and others declining. This period of stability is sandwiched between the pre-1998 period of healthier reef condition (but not shown well in the included datasets) and the post-2016 period, when a significant decline in some countries occurred due to widespread bleaching in 2016 (see section 1.3). Unfortunately, with reduced effort in coral reef monitoring in the last 3-5 years, only a few countries were able to repeat monitoring of reefs in 2016 to document coral cover in a way that could be included in these long-term time series. The high coral cover in countries that started coral reef monitoring after 1998 may reflect biased selection of sites that did not suffer high mortality, as new monitoring programmes tend to preferentially select ‘good’ reefs to track future trends, rather than a random or stratified selection of sites that would include some low-condition reefs. High-cover reefs may stay stable or suffer a decline in coral cover, resulting in a net negative trend. By contrast, reefs that suffered high mortality in 1998 might either remain in a poor state, or show recovery, which may vary from low to high. Thus, the early years of data (1999- 2005) in this monitoring series may overstate the level of coral cover, and the latter years may under-represent recovery from a low state. 14
Coral reef status report for the Western Indian Ocean (2017) Global Coral Reef Monitoring Network 1.2.3 Fleshy algae Algae data were not reported from all countries, but an overall comparison with coral cover is possible (fig. 1.2.4). Here, fleshy algae combines turf and filamentous algae, macro-algae and calcareous algae or Halimeda. However, different monitoring programmes recorded algal cover quite differently, so caution is necessary in interpreting combined results. The general decline in coral cover spanning the 1998 bleaching event, from the early 1990s to about 2001 is mirrored by a clear increase in fleshy algae from 1994 to 2000/2001 (fig. 1.2.4). After 2001, algal cover remains high across the region, at a similar level to coral cover. Figure 1.2.4. Regional averages of Hard coral and fleshy algae cover using countries where algal cover was collected (mean and 95% confidence interval). Excludes monitoring stations with < 4 years of data. Plate. Algal competition with corals following coral mortality from various causes. Upper row - heavily grazed eroding surfaces with thin turf and coralline algae. Lower row - growth of fleshy-algae competing with corals. Photo credit: lower right - Pierre Perries, other images - David Obura. REGIONAL CHAPTERS 15
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GCRMN_COI_2017-Western Indian Ocean Reef Status

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