Importance of environmental flow on feeding and distribution of juvenile Labeobarbus species in the Gumara River, Lake Tana Sub-basin, Ethiopia

Abstract

The tributary rivers of Lake Tana are important nursery grounds for larvae and juveniles of the migratory endemic Labeobarbus species, but very little is known about their functional habitat and feeding preferences. This study investigated nursery grounds, habitat shifts, and potential energy sources for juvenile Labeobarbus species. Electrofishing point abundance sampling for juvenile sampling and stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) were applied to trace potential energy sources. Habitat use shifting was observed among labeobarbus size classes from weak or no flow pool to fast-flowing riffle mesohabitats. The young-of-the-year (YOY) were mainly found in microhabitats with a weak to moderate water velocity (i.e., < 0.3 m s−1), while 1+ juveniles (>7 cm FL) adapted to velocities of > 0.3 m s−1. Juveniles suffered from limited access to run and riffle habitats during the peak dry season (March-April). They were subsequently exposed to resource competition and predation risk which led to lower catches. Juveniles of different size classes used different food sources. C1 (<3 cm) and C3 (5.1–7 cm) size classes mainly relied on algal sources, while C2 (3 – 5 cm) and C4 (>7 cm) on zooplankton. Insect larvae were a relatively important source for larger juveniles. Maintaining appropriate flow that permits juveniles to access preferred foraging and nursery habitat in the Gumara River is crucial for the sustainable management and conservation of the Labeobarbus population in Lake Tana.