Thesis (M.A.)--University of Hong Kong, 1999. / Includes bibliographical references (leaves 67-68).

Thesis (Ph.D.)--University of Rhode Island, 2008. / Includes bibliographical references (leaves 230-244).

Thesis (M.A.)--University of Hong Kong, 1999. / Includes bibliographical references (leaves 67-68). Also available in print.

Thesis (M.S. in natural resource sciences)--Washington State University, May 2010. / Title from PDF title page (viewed on July 19, 2010). "Department of Natural Resource Sciences." Includes bibliographical references (p. 43-47).

Thesis (Ph.D.(Agricultural Economics))--University of Pretoria, 2006. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

Thesis (Ph. D. (Sociology))--Rhodes University, 2006.

Thesis (Ph.D.)--The Chinese University of Hong Kong, 2005. / Adviser: Jianfa Shen. Includes bibliographical references (p. 184-192)

Thesis (M.A.)--Georgia State University, 2006. / Title from title screen. Jack Boozer, committee chair; Shirlene Holmes, Marian Meyers, committee members. Electronic text (138 p.) : digital, PDF file. Description based on contents viewed June 19, 2007. Includes bibliographical references (p. 28-30).

The Okavango is a vast inland delta system in northern Botswana which receives an annual flood from the highlands of southern Angola. There are distinct communities of fish in the Okavango which can be separated from each other by the physical characteristics of the different habitat types with which they co-evolved. This thesis provides an account of the biology and ecology of selected fish species in the Okavango Delta. Their response to the annual flood regime, and the environmental factors which limit their distribution and abundance, are examined. The thesis emphasizes the importance of water fluctuations in determining the nature of the fish fauna and the reaction of the fishes in terms of community structure, movements, breeding, predator-prey interactions and feeding. Four major ecotones were studied in the Okavango Delta. In the riverine floodplain and perennial swamp ecotones a higher species diversity was recorded than in the seasonal swamp and drainage rivers ecotones where diversity was lowest and comprised mainly of smaller fish species. A greater variety of habitat types was associated with the riverine floodplain and perennial swamp relative to the seasonal swamp and drainage rivers. The variety of habitat types between ecotones is associated with the degree of flood inundation in the respective ecotones. During the course of this study, annual recruitment of fish into the drainage rivers was from refugia in the seasonal swamp whereas the greatest degree of lateral and longitudinal movement was in the riverine floodplain and perennial swamp. Movement was in response to both biological requirements, such as availability of food and spawning sites, and physical features of the environment, such as the changing water depth. The total catch per unit effort (CPUE) of fish throughout the year was more constant in the riverine floodplain and perennial swamp than in the seasonal swamp and drainage rivers where CPUE fluctutated widely. An increase in CPUE during the duration of this study was apparent and related to the magnitude of the annual flood. In contrast to most other African wetlands, the arrival of the annual flood in the Okavango Delta coincides largely with the dry winter months. This situation presented an opportunity to compare the influence of floods and water temperature on the reproductive biology of the selected fish species. The results show a definite pattern and indicate that both the flood cycle and increased water temperatures greatly influence the breeding cycles of the selected species. The tilapia Oreochromis andersonii exhibited a considerable degree of phenotypic plasticity. Fish from the seasonally inundated areas showed a smaller mean size, egg size and larger number of eggs relative to fish in the perennially flooded areas. The size at sexual maturity was also smaller. These different reproductive characteristics exhibited by O. andersonii are dependent on the degree of water retention in the different habitats. The fishes of the Okavango have adopted other reproductive strategies to survive the changing environmental conditions brought about by an annual flood cycle. These strategies include the construction of foam nests, as described for Hepsetus odoe, for guarding the young and to provide an oxygen-rich environment. Two main non-piscivorous feeding pathways were identified in the Okavango. These are a detritus pathway based on dead plant and animal material, and an epiphyte pathway, based on algae and invertebrates that are attached to plant stems. Seasonal changes in diet in relation to the annual flood were recorded. The most dramrtic change was demonstrated by the catfish Clarias gariepinus which congregates in mass aggregations in the northern regions of the Delta and hunt in packs. Pack-hunting by catfish is a regular response to the annual fluctuations in water level. It is my conclusion that the main flow of biotic and abiotic stimuli within the Okavango Delta originates from the relatively hydrologically stable riverine floodplain and perennial swamp ecotones to the widely fluctuating seasonal swamp and drainage rivers ecotones. The relatively stable ecotones allow a diverse and biotically interdependent fish community to develop, whereas the widely fluctuating seasonal swamp and drainage rivers ecotones are characterized by a less diverse and interdependent fish community. The degree of abiotic and biotic interdependence among fish in an ecotone is very important for the long term management of the Okavango Delta. Potential developers have to determine whether the effect of a given action by man is likely to result in a long term disturbance or merely in an elastic recoil to a more or less similar state. Recommendations are made on the conservation and management of Okavango fishes taking into account the ecological characteristics of the delta.

Two field plots in the Ladner area, one supporting pasture species and the other inter-tilled crops, were irrigated by sprinkler with water containing more than 13,000 parts per million of ocean salts. Eight acre inches of water were applied annually, in three equal applications, the trial being conducted for a period of two to three years. Soil samples, obtained at appropriate intervals during the course of the irrigation trials, were studied in the laboratory to determine the effects of the treatment on their physical and chemical properties. Plant yields were recorded and tissues chemically analysed to reveal alterations, if any, in the uptake of mineral elements. Analyses revealed that, as a result of irrigation, marked increases occurred in respect to salt content of the soil and osmotic pressures in the soil solution. Subsequent dormant-season rains were observed to remove a large portion of the accumulated salt, but sufficient salt residue remained in the cultivated soil after two year's irrigations to affect injuriously the growth of a non-irrigated silage crop. The exchangeable sodium percentage of the soil was increased only slightly in the pasture field, but to a dangerous level in the cultivated field. Exchangeable magnesium was increased and calcium decreased in both soils. The effects on soil calcium are considered to be important, since the soils are already low in available calcium, and a further deficiency of this element for plant growth is therefore threatened. Studies of soil structure revealed no destructive changes which could be definitely ascribed to irrigation with saline water. Calcium uptake by the plants was generally depressed by the treatment, and of the tissues studied, all, with the exception of the grasses, showed accumulation of sodium Pea and silage corn yields were reduced by irrigation, sugar beet yield was unaffected, and pasture grass yield was increased eight-fold. The quality of all crops was impaired by the treatment. The author is convinced that the continued use of this highly saline water, except in very special circumstances, will cause reduced crop yields and lead ultimately to serious and permanent impairment in the physical and chemical properties of the soils. / Land and Food Systems, Faculty of / Graduate