Browsing by Author "Ussi, Ali Makame"

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    Population distribution and impact of Crown-of thorns starfish, Acanthaster planci (L), on some coral reefs of Zanzibar
    (University of Dar es Salaam, 2009) Ussi, Ali Makame
    Distribution patterns age-size structure and population density of Crown-of –thoms starfish The starfish, Acanthaster planet (L), were determined on Murogo on the western side and Mnemba on eastern side of Zanzibar Island. Feeding rates of young, sub-adult and adult starfishes, preferred prey (coral) species and sizes, and densities of the starfish within and between two reefs were compared. showed clumped and regular distribution at Murogo and Mnemba reefs, respectively. Sub adults (20-30 cm) starfishes dominated the populations in both reefs. With 6 1% for Murogo and 54% for Mnemba. The habitat mean densities (individuals/hectare) at Murogo were 3125 ± 12 13, 248 ± 140 and 2280 ± 1163 for lagoon, 116 ± 114, 44 ± 35 and 67 ± 31 for slope, 220 ± 162, 30 ± 20 and 60 ± 55 for crest, and 13 ± 5, 40 ± 22 and 87 ± 96 for flat during first, second and third sampling, respectively. For Mnemba, mean densities were 28 ± 30, 22 ± 31 and 27 ± 23) for first, second and third sampling, respectively. Densities at Murogo showed significant differences among and within habitats . The differences were in reef flat and lagoon for among habitats and in lagoon and reef slope for within habitat s. Between sites, results showed that Mnemba densities were significantly lower from that of Murogo habitats. Mean feeding rates (cm2/day) were 114, 232 and 244 for young, sub adults and adults, respectiveI). The means were not statistically different due to high variances Pontes and Ethiopora of middle (30-60 cm) and large (> 60 cm) size were preferred at Murogo while small (<30 cm) and large (>60 cm0 size were preferred at Mnemba roof. There was positive correlation between the size of the starfish and size of the coral colonies preyed. This study indicated that the density of the starfish at Murogo is above threshold level and poses threat to coral reefs porites is the most vulnerable coral genera during starfish feeding Moreover, the study point out that type of growth form of coral colony determine preferences.
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    Recruitment and growth of scleractinian corals in relation to Bio-physical processes in reefs of Unguja island, Zanzibar, Tanzania
    (University of Dar es Salaam, 2014) Ussi, Ali Makame
    This study was conducted to examine the dynamics of key ecological processes that effect the recovery of reef communities, which include the recruitment and growth of juvenile corals, and their relationships with reef herbivory, corallivory, and sedimentation stress. The study was undertaken on Chumbe, Changuu and Mnemba reefs in Zanzibar, from October 2010 to June 2012. The juvenile corals and invertebrate herbivores assessments were done in Situ in permanent quadrats whereas herbivorous and corallivorous fish were assessed using under water visual sensor method. The results revealed significant differences in juvenile coral densities and survival rates among reef sites. There was significant site habitat variability in juvenile coral density at Changuu and Mnemba. Corals of the Genus Acropora at Chumbe, Porites at Changuu and Mnemba. Growth rates (cm2 6-month-1) of juvenile corals species ranged from 8.1 (for Porites lutea) to 155.7 (for echinopora lamellosa). High growth and survival rates were observed on reeef slope relative to the reef crest and reef flat. Sea urchin density and diversity was highest at Changuu whereas the lowest sea urchin diversity and density were recorded at Mnemba and Chumbe, respectively. Herbvorous fish densities at Mnemba. At Chumbe, juvenile, corals were positively influenced by herbivorous within sites. Sedimentation rates (mg cm-2 day -1) were highest at Chumbe reef (4.67) and lowest at Mnemba reef (!.22), with marked seasonal and habitat variations within sites examined. The results indicated considerable natural recovery potential differences among reef sites. However, appropriate management measures to the degraded reefs can facilitate reef recovery process.