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CORDIO Status Report 1999.pdf

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CORAL BLEACHING EVENT IN TANZANIA<br />

Coral bleaching in Tanzania probably occurred from<br />

February and continued through May 1998. In Zanzibar,<br />

coral bleaching started in the last week of March<br />

(pers. obs.). Coral bleaching was reported from all parts<br />

of the Tanzania coast. The extent of coral bleaching on a<br />

national level could not be accurately estimated, but<br />

individual reports seem to suggest that bleaching was<br />

not uniform. Corals in shallow water (reef flats)<br />

bleached more that those in deeper waters. Mafia coral<br />

reefs (Kitutia and Pange) seem to have suffered more<br />

than Zanzibar reefs. Similarly, corals on the western<br />

side of Pemba Island were more affected than corals on<br />

the eastern side.<br />

The response differed considerably between coral<br />

species. In general, more than 60% of the scleractinian<br />

corals showed signs of bleaching (Table 1 and 2). The<br />

Acropora bleached most. Other coral species that bleached<br />

include Echinopora, Montipora, Millepora, Pocillopora,<br />

Seriatopora, Galaxea, Astreopora, Lobophyllia and Porites.<br />

A few corals, such as Diploastrea and Pachyseris, didn’t<br />

show any signs of bleaching in Zanzibar.<br />

Two factors seem to be associated with coral bleaching<br />

in Tanzania: water temperature and rainfall (salinity).<br />

The water temperature was 30.5°C (according to a<br />

temperature logger placed at 3 m on a coral branch at<br />

Bawe, Zanzibar), about 2°C higher than in the previous<br />

year (28.5°C) (Figure 1). Even casual swimmers in<br />

Table 1: Bottom cover category summary in some of the monitored<br />

coral reef sites in Zanzibar in 1997.<br />

Benthic Bawe Changuu Chapwani Chumbe Kwale Mnemba<br />

category (W- (W- (W- (SW- (SW- (Ecoast)<br />

coast) coast) coast) coast) coast)<br />

Hard coral 53.11 50.17 44.31 51.85 29.73 13.95<br />

Softcoral 2.45 0.74 2.85 0.76 1.66 30.67<br />

Rhodactis 0.97 6.65 2.37 0.49 0 0.1<br />

Zoanthids 0.14 1.65 0.58 0.05 0.03 0.05<br />

Sponges 0.85 0.09 0.17 0.36 0.18 0<br />

Algae 2.94 10.97 3.88 8.87 28.06 2.44<br />

Others 2.04 2.27 0.74 0.12 0.39 0.03<br />

Substrate 37.5 27.46 45.1 37.5 39.95 52.76<br />

Source: IMS-LGL Coral reef monitoring programme, 1997.<br />

Table 2: The extent of hard coral bleaching on monitored plots in<br />

Zanzibar<br />

Coral category Chapwani Changuu Bawe Chumbe Kwale<br />

Hard coral before<br />

bleaching,<br />

October 1997 44.31 50.11 53.11 51.85 29.73<br />

Hard coral after<br />

bleaching,<br />

November 1998 25 33 45 42 15<br />

Normal coral 44.4 42.6 33.3 16.5 32.8<br />

Bleached 12.7 14.8 11.8 49.4 36.4<br />

Partly bleached 21.0 25.8 14.1 9.3 15.0<br />

Dead coral 19.9 5.8 33.2 11.5 7.1<br />

Partly dead coral 2.0 10.0 7.6 13.3 7.7<br />

Source: IMS-LGL Coral reef monitoring programme, 1998<br />

Figure 1. Seawater temperatur (°C) on<br />

Zanzibar coral reefs in 1997 and 1998.<br />

31<br />

30<br />

29<br />

28<br />

1998<br />

27<br />

26<br />

25<br />

1997<br />

Bleaching<br />

period in<br />

Zanzibar<br />

Jan<br />

Feb<br />

Mar<br />

Apr<br />

May<br />

Jun<br />

Jul<br />

Aug<br />

Sep<br />

Oct<br />

Nov<br />

Dec<br />

– 44 –

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