Coral recruitment Natural impactsthreats

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4.5.1 Distribution of coral reefs

Hamilton 1975 collected, described and illustrated 88 species of scleractinian corals in East Africa. Hamilton and Brakel 1984 reported that the Region has 140 known species belonging to 55 genera 88 species belonging to 34 genera found in the Dar es Salaam area alone. In general, the exposed windward reef slopes were dominated by acroporids and pocilloporids in the turbulent upper zone and by poritids and faviids in the lower zone. Acropora and Galaxea clavus dominated sheltered reef slopes, while Pocillopora, Porites, Psammocora, and Pavona dominated reef flats. More recently, distribution studies in southern Tanzania have identified 51 scleractinian coral genera Horrill and Ngoile, 1992; Darwall et al., 1996; Guard et al., 1997, Darwall et al., 2000. Again, Acropora was found to dominate the shallow, sheltered inner reefs. In the Songo Songo Archipelago, Galaxea dominated many patch reefs, while two particular patch reefs, Poiasi and Pwajuu, were completely dominated by the fungiid Halomitra pileus . This species was also found to dominate an entire 200-m section of an outer reef adjacent to Msamgamku, Mtwara. Muhando 1995 found the dominant coral genera on reefs off Fumba Peninsula near Menai Bay in southwestern Zanzibar to be Acropora, Montipora, Porites, Millepora, Lobophyllia, Echinopora, and Favia. Along Tanga Region, there is 407 km of coastline, of which, 199 km are broken by bays and estuaries. Of the unbroken 208 km, 97 km 47 is bordered by 41 distinct sections of coastal fringing reef. In addition, there are 55 patch reefs, of which, 30 are outer patch reefs adjacent to the continental shelf and 25 are inner patch reefs located in near shore waters where water depth is less than 25 m. In total, there are 96 reefs in Tanga, having a total of 376 km of reef edge Horrill et al., 2000. Just north of Dar es Salaam city there is a series of reefs known as the Dar es Salaam Marine Reserves System which includes Bongoyo, Pangavini, and Mbudya Islands, which are surrounded by fringing reefs, and the patch reefs, Fungu Yasin and Fungu Mkadya Kamukuru, 1997. There is a number of other smaller patch reefs in the vicinity. Just south of Dar es Salaam city there are sections of fringing reef, particularly on the headlands ras in Swahili. There are also three chartered patch reefs as well as a number of other unchartered and unnamed reefs. Further north of Dar es Salaam, there are a number of patch reefs off the coast of Mbweni. Near Bagamoyo, there are stretches of fringing reef as well as several offshore patch reefs. Unguja Island, one of the two main islands of Zanzibar, is surrounded by scattered coral development. Along the northern, eastern and southern sides of the Island, there is a fringing reef, which is interrupted by Chwaka Bay. In addition, there are reef formations near Mnemba, a small island to the northeast, Leven Bank to the north and Bedford Bank to the south. On the western side of Unguja, there are a number of islands and patch reefs between Ras Nungwi and Menai Bay Horrill et al., 2000. Pemba, the second of the two main islands of Zanzibar, is estimated to have 1100 km of reef, representing 45 of the coral reefs of Tanzania. Coral growth has been observed as deep as 64 m. Misali Island, a small island on the western side of Pemba, has reefs of very high diversity of coral and fish species Horrill et al., 2000. In southern Tanzania, there are numerous patch reefs and fringing reefs around islands in the Mafia and Songo Songo Archipelagos. There are also numerous reefs along the Mtwara coastline.

4.5.2 Coral recruitment

108 Nzali et al. 1998 found that coral recruitment is better in areas where there is higher coral cover than in degraded areas e.g., areas where there has been dynamite fishing. Moreover, coral recruitment is highest in April, which coincides with higher temperature, higher rainfall, and lower sedimentation rates.

4.5.3 Natural impactsthreats

Natural impacts on coral reefs include storms, outbreaks of the Crown-of-Thorns starfish, and coral bleaching. Damage due to strong wave action is common on exposed fringing reefs and on the seaward side of patch reefs and islands all along the coast of Tanzania. The Crown-of-Thorns starfish, Acanthaster planci, feeds on coral polyps and has, from time to time, devastated reefs in various parts of the world, such as the Great Barrier Reef of Australia. Occasional aggregations of this starfish have been sighted at Tanga, Mafia and Zanzibar Ngusaru and Muhando, 1994, though it has not, so far, posed a serious threat. Nevertheless, vigilance is required. The coral bleaching event March to May 1998 coincided with higher than normal seawater temperatures and increased rainfall lower salinity Muhando, 1999a. Coral bleaching was reported on all parts of the Tanzanian coast with variable severity. Bleaching was worse in shallow waters reef flats than in deeper waters. In Zanzibar, overall more than 60 of the scleractinian corals showed signs of bleaching, with Acropora being most affected; while a few corals such as Diploastrea and Pachyseris were seemingly unaffected Muhando, 1999a. Some species of Porites were affected, while others were not Wilkinson, 1998. After the bleaching event, the dead corals were colonised by filamentous algae. By November 1998, these were replaced by macroalgae and coralline algae. By January 1999, some areas showed the recruitment of small corals, while others were colonised by corallimorpharia and soft corals. On the economic side, some dive operators reported a decline in tourist potential due to the bleaching event Muhando, 1999a. At Mbudya Island, the large percentage of dead hard coral 40-60 was thought to be mainly due to coral bleaching Wagner et al. , 2000a. According to studies conducted at Tutia Reef by Ohman et al. 1999, following the bleaching event, 88 of the corals died. A year after the event, a large proportion of the dead corals was still standing. As surviving and dead corals were from different clones, results suggested that genetic variation might influence bleaching tolerance. After the coral bleaching, there was a change in fish community composition and a 39 increase in fish abundance Ohman et al., 1999. In particular, there was an increase in the abundance of herbivorous fish due to the growth of algae on the dead coral. Species diversity remained almost constant. There was a significant correlation between reef structural complexity and both fish abundance and the number of fish taxa. This indicates that the reef may uphold an abundant fish population as long as the architectural structure remains intact. However, if dead coral is eventually degraded and turned into rubble, with consequent loss of structural complexity, fish abundance is likely to decrease greatly.

4.5.4 Types of Resource Use