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 Figure 2.9.4. National averages of fish abundance for 15 fish families on coral reefs in Tanzania between 1999 and 2011. Open circles represent surveyed sites and shaded areas represent the 95% confidence limits. NATIONAL CHAPTERS Macro-invertebrate monitoring has focused on sea urchins, clams, sea cucumbers and starfish. In general they are widespread but at low densities across reefs and years. The densities of invertebrates with high economic value (lobsters and large gastropods) were very low. It is not clear whether this is due to over-exploitation as many have been historically targeted for flesh and shells (Richmond 2002), or habitat destruction. Some macro-invertebrates such as octopus are quite cryptic and not easily observed. Nevertheless, the octopus fishery is a significant one in Tanzania, and occurs almost exclusively on coral reefs and intertidal rocky flats. 2.9.4 Coral Bleaching 2016 Coral bleaching started around mid-March 2016 and ceased in June, which is similar to the timing of the 1998 bleaching. Peak bleaching occurred during late March and into April. Bleaching of 80–90% was observed on some reefs, such as Sinda reef off Dar es Salaam (M. Richmond and C. Muhando pers.comm) and northern Chumbe reef, Zanzibar (U. Kloiber pers. comm.). Bleaching information was mostly collected by divers and snorkelers from MPAs, research institutes and dive centres, and reported on the online reporting form developed for the WIO (http://cordioea.net/bleaching_resilience/wio-coral-bleachingalert/). About 70 bleaching reports were made, between, March and June, and from the whole coastline from Tanga in the North to Mnazi Bay in the South. Reported bleaching levels ranged from low (5 -10%) to extreme (>90%). 136
Coral reef status report for the Western Indian Ocean (2017) Global Coral Reef Monitoring Network Figure 2.9.5. Observations of coral bleaching (n=64) and associated mortality (n=18) observations collected in Tanzania in 2016. Categories represent the severity of bleaching/mortality reported as percentage of coral cover bleached/dead estimated at the site level. The most affected corals were Acropora, common to all sites. Fungia, Pocillopora, Porites and the faviids were also greatly affected by bleaching. Preliminary reports indicate varied outcomes, ranging from significant recovery from bleaching in Mtwara and Tanga (E. Mbije pers. comm.), and little recovery in some reefs off Zanzibar town. The effects of bleaching varied spatially and between species and growth forms. For example on Chumbe reef in Zanzibar, short-branched and digitate species and massive Porites recovered quite well, while branching and table Acropora suffered significant mortality. Depth was also probably a factor. On reefs with high mortality, cover of algal turf on dead corals increased significantly. One common observation throughout the country was that the anemone Heteractis magnifica remained bleached, months after the first bleaching occurrences. Seawater temperature has been monitored for almost two decades on some reefs. A strong seasonal pattern is shown, with lowest temperatures in July–August and highs in March–April (Fig. 2.9.6). Temperatures have reached as low as 23.4 and as high as 33.9 °C. However the yearly average is 27.5 °C. In late March to early April 2016, at the peak of coral bleaching, water temperatures repeatedly reached 31°C. NATIONAL CHAPTERS 137
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GCRMN_COI_2017-Western Indian Ocean Reef Status

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