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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
Coral reefs are utilised in a number of ways, particularly, for various types of fisheries, tourismecotourism, coral mining, and mariculture. Coral reefs are high density fishing grounds for a variety of finfish, octopus, sea
cucumbers, lobsters, and shellfish used both for food and curio. Since fisheries is a separate theme on its own, this use of reefs will not be discussed further here.
According to Andersson 1998, 20 of the tourists interviewed said that the main attraction for visiting Zanzibar was diving. During their visit on Unguja, 68 had dived at least once and 85 had snorkelled. Since almost all
diving and snorkelling is done around coral reefs, this makes reefs an important tourist attraction.
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Live coral is either used as a building aggregate or is burnt in open kilns to produce white lime which can be used as cement or to white wash houses. Live coral is whiter than fossilised coral rock and is therefore preferred. The
negative impacts of coral mining include loss of reef habitat for associated fish species, loss of natural breakwaters with concomitant indirect loss of adjacent sheltered marine habitats, and loss of the aesthetic value of
the reefs for tourism Darwall et al., 2000. Moreover, since coral mining simplifies the surface topography of reefs, there is also a reduction in microhabitat diversity which, of course, results in a decrease in biodiversity.
Solandt and Ball 1999 reported that several communities in Mikindani Bay are almost entirely dependent upon coral mining for their income. In Mitengo Village, the oldest coral mining community, out of a population of 100
people, 50-75 men, women and children are involved in various stages of the lime industry. The most commonly mined coral is Porites lobata. Though highly destructive, mining of live coral is, nevertheless, an all too common
use of coral reefs, particularly in the south. Coral mining in Mafia, Songo Songo and Mtwara has been documented Dulvy and Darwall, 1995; Dulvy et al., 1996; Darwall et al., 2000. In northern Tanzania, it is
primarily fossilised coral rock that is mined Horrill et al., 2000.
An indirect use of coral reefs is for the establishment of seaweed farms which are sometimes set up in lagoons e.g., Paje seaweed farm, Zanzibar between a coral reef and a sandy beach. Some are even set up over part of the
reef flat. This is because the reefs shelter the lagoon from the full force of waves, but usually have openings which allow sufficient flow of water to replenish nutrients required by the seaweeds. Seaweed farming in Zanzibar has
primarily been carried out by women and it has thus uplifted the status of women and, to some extent, has changed gender relations in the communities where it is practised Horrill et al., 2000.
4.6 Human Impacts 4.6.1
