Effect of large weirs on abundance and diversity of migratory Labeobarbus species in tributaries of Lake Tana, Ethiopia

Abstract

Lake Tana has a remarkable fish diversity, including 17 endemic Labeobarbus species, of which nine spawn in the inflowing rivers. Three of the migratory species are threatened, namely the endangered Labeobarbus macrophtalmus and the vulnerable L. acutirostris and L. platydorsus. In July–November 2016 during the wet season netting upstream and downstream of large weirs in two rivers and in two undammed rivers showed that weirs had a severe negative effect on migratory Labeobarbus, including the three threatened species. Of eight Labeobarbus species caught below the Gelda River weir, only L. intermedius was recorded above the weir, and in the Shini River, five Labeobarbus species were caught below the weir, but none above it. Labeobarbus diversity below the Gelda River weir (H’ = 1.4) was significantly different to that above the weir (H’ = 0), whereas in the two rivers without any weirs, Labeobarbus diversity downstream (H’ = 1.39 and 1.55) was not significantly different from that upstream (H’ = 1.52 and 1.40). Therefore, specific mitigation measures, such as fishways, need to be designed from the outset of proposed weir and instream dam construction.

Keywords: endemic species, fisheries management, fish-corridor, migratory fish