PhD Theses

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Browsing PhD Theses by Subject "Fishes"

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    Epizootiology of microbial diseases of wild and cultured fish in the Mtera dam catchment area, Tanzania
    (University of Dar es Salaam, 2015) Shayo, Salome Daniel
    Bacterial diseases are responsible for heavy mortalities of fish in wild environment and cultured systems worldwide, thus causing significant economic losses threatening livelihood of people. In the present study, causes of outbreak of ulcerative infections and mortalities in Nile tilapia (Oreochromis niloticus) at Mtera dam and in fish farms around the Mtera catchment were investigated. Collections of fish samples for examination and measurement of environmental parameters were done for a period of eighteen months to cover seasonal variation in epizootic occurrence. Conventional and molecular techniques were employed in the confirmation of the causative agents. Furthermore, virulence of the recovered bacteria was studied in vitro using similar fish species to confirm the causative agent of ulcerative infections in Nile tilapia at the Mtera dam. 50 bacterial isolates belonging to 13 genera were obtained. Virulence experiment confirmed that the causative agents of ulcerative infections in the dam were Aeromonas, and Pseudomonas species. Temperature, DO, Turbidity, and conductivity showed significant effects on the observed infections pattern in the dam. Occurrence of fish mortality and clinical symptoms were profound during the dry period. The presence of infectious bacteria in the fish samples indicate a potential health risks for consumers as bacteria such as Aeromonas hydrophila are associated with human infections. Fishers/consumers are advised to avoid ulcerative fish from the dam particularly during the dry season. The high number of the isolated opportunistic bacteria from the dam is an alert for proper fish health management to evade great losses in case of stress oriented disease outbreaks.
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    Organic matter sources and trophic interactions among fish species in Pangani estuary – Tanzania
    (University of Dar es Salaam, 2009) Mwijage, Alistidia Paul
    Estuaries are primary habitat that serves as feeding and nursery grounds for most juvenile marine fish. However, estuaries in Tanzania have been affected by anthropogenic activities in the upstream of the river catchments including the use of these areas as fishing grounds by the artisanal fishers. These human activities contribute to determine the abundance and diversity of OM sources that support the nutrition of fish; and also to define the complexity of the estuarine food webs. The four complementary methods: stomach contents, stable isotopes (Ֆ13C and Ֆ15N) fatty acid (FA) biomarkers and trophic flow modeling approach were used to describe the OM Nutritional sources and trophic interactions among the dominant marine fish in the Pangani estuary. Among the Fish species investigated (Hilsa Kelee, Valamugil buchanani, Arius africanus, Carangoides chrysophrys and Epinephelus malabaricus), PERMANOVA, Pseudo – F = 125.27: P = 0.001), stable isotopes (PERMANOVA, pseudo –F 300.29: p = 0.001), and FA composition (PERMANOVA, pseudo-F = 74.75; p = 0.001). Moreover, the stable isotopes results revealed slight differences in the most important OM sources relied by individual species from depleted in Ֆ13C values to relatively enriched in Ֆ13C OM Sources among the estuarine zones. Yet, the combined methods – stable isotopes, FA biomarkers and Eco path modeling emphasized that terrestrially – derived OM Supplement the main benthic micro-algae basal food sources is an indication resources to sustain the overall estuarine food web. The reliance on benthic diatom – dominated estuarine and marine basal food sources in an indication of low estuarine food connectivity to the fresh water related food web. This situation is most likely threatening the resilience and stability of the estuarine food web structure. This was also highlighted by Ecopath trophic flow model which indicated that the Pangani estuarine food web structure is less resilient to any perpetuation when compared with the other tropical estuarine systems