Importance of unregulated rivers as last refuges of native fishes; implications of habitat alteration and exotic species introduction in Sri Lanka

Abstract

River Mahaweli in Sri Lanka was greatly altered by dams under the Mahaweli diversion Scheme. Fish assemblages in segments of five tributaries of Mahaweli; Badulu Oya, upstream of Ulhitiya Oya and Loggal Oya (unregulated streams) and, Minipe and downstream of Ulhitiya Oya (regulated streams) were compared. Sampling was carried out in three selected 100 m segments from each study stream using hand nets, drag nets and cast nets. 40 fish species in 10 families were recorded. Ulihitiya downstream recorded the lowest abundance and species richness. Unregulated streams demonstrated a higher proportion of endemics (Dry: 41 – 54% and Wet: 50 – 60%) while regulated streams had highest proportions of exotics in both seasons (Dry; 26 and 72 %; Wet: 34 and 66 % in Minipe and Ulhitiya downstream, respectively). The results suggest that native and endemic fishes are more sensitive to altered habitat conditions highlighting the importance of protecting the remaining natural habitats.