Drs. M. de Graaf : Lake Tana's piscivorous Barbus

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2 Sep 2003 14:00
Unit: Wageningen University
Location: Aula (gebouw 362), Gen. Foulkesweg 1, Wageningen
Co Promotor: dr. F.A. Sibbing

 The 15 Barbus species of Lake Tana, a large shallow lake located at an altitude of 1830 m in the north-western highlands of Ethiopia, form the only remaining intact species flock of large (max. 100cm) cyprinid fishes. Lake Tana is the source of the Blue Nile and high waterfalls (40 m) at Tissisat (‘smoking water’), 30 km downstream from the outflow, effectively isolate the lake’s ichtyofauna from the lower Nile basin. Lake Tana and its Barbus species flock from a natural laboratory and provide an unique opportunity to study the selective forces driving speciation and diversity in freshwater fish communities. The rapid ecological diversification and speciation of Lake Tana’s Barbus is at least as spectacular as the adaptive radiation of the cichlids in East-Africa’s Great Lakes (Victoria, Malawi and Tanganyika). One of the most intriguing aspects of these Barbus is the large number (8) of piscivores. Cyprinid fishes are not well designed for piscivory. Furthermore, one of the key innovations in the bodyplan of the evolutionary successful cyprinid fishes (>2000 species), the voluminous palatal organ lining the pharynx, turned out to have come at a cost: their reduced competitive abilities to function as piscivores. However, Lake Tana lacks potential piscivorous competitors, rendering the piscivorous Barbus by far the “best” and apparently highly successful. They have adapted to all available macro-habitats, using different techniques, a unique scenario for barbs. The lack of fisheries legislation and regulations after the introduction of a small-scale commercial gillnet fishery in Lake Tana towards the end of the 1980s, resulted in the rapid and dramatic decline (75%) of Barbus during the 1990s. The gillnet fisheries mainly targets the spawning aggregations of the barbs and is now threatening the existence of this remarkable species flock.
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