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Articles by Denis Tweddle
Total Records ( 2 ) for Denis Tweddle
  Denis Tweddle
  The Zambezi River has a catchment area of 1.32 million km2, including parts of eight countries. Three divisions of the river are recognised: the Upper Zambezi separated from the Middle Zambezi by Victoria Falls, and the Lower Zambezi below Cahora Bassa gorge. The Okavango River is also linked to the Upper Zambezi system in wet years in an area of complex geomorphological history. Habitats include forested headwater streams, extensive floodplains, deep gorges, two large man-made lakes and an extensive delta. On floodplains, subsistence fisheries exploit the natural seasonal cycles, while the man-made lakes have commercial-scale fisheries for introduced kapenta, Limnothrissa miodon. Aquaculture is on a small scale, though with larger commercial cage culture enterprises on Lake Kariba. This paper summarises current knowledge on the fish faunas and their origins, the status of the different fisheries and their management, and the conservation status of the river's resources.
  Olaf L.F. Weyl , Anthony J. Ribbink and Denis Tweddle
  Lake Malawi's fishes are a source of food for millions and provide a livelihood for thousands by encouraging tourism, fascinating the scientific fraternity, enchanting aquarists around the world and maintaining ecosystem processes in the lake. From a fisheries and resource assessment perspective, the region is data-poor, but there is sufficient peer-reviewed and grey literature on the limnology, fisheries and ichthyofauna of the lake to provide a good overview of the state of the fishery. There are signs of over exploitation and an increasing fishing effort has resulted in decreased catch rates, depletion of larger, more valuable species in the fishery and species changes. The fishery is harvesting stocks that were formerly thought to be under exploited. Previous attempts to manage the fishery have been ineffective and long term strategies addressing overfishing will need to transform the fishery from an open-access to a limited access system. As important as direct intervention in the management of the fisheries, will be the management of catchment processes. Increased nutrient inputs; changes to the phytoplankton composition; sediment loading; nearshore water quality impacts and changing water levels threaten the ecosystem. Introduction of alien invasive organisms is an ever present threat to the ecosystem as well, due to continued development of small scale aquaculture in the region. The overriding causative factor for all these effects is the poverty of the lakeshore communities which do not have the economic privilege of being able to adapt their utilisation patterns.
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