Do short-term ecological alterations affect fish diversity in the long-run? A study from a sub-tropical river in the Eastern Himalayas

The present study was carried out to assess the effect of short-term ecological alteration on fish diversity of River Siang, Arunachal Pradesh in the Eastern Himalayas. Fish diversity of six stations along the river (Puging, Yingkiong, Boleng, Komsing, Pasighat and Oiramghat) was assessed across pre-monsoon, monsoon and post-monsoon seasons along with all major physico-chemical attributes of water. We recorded 78 fish species belonging to 48 genera under 17 families. On a spatial scale, the maximum number of species was recorded from Oiramghat (65 species) followed by Pasighat (56 species) along the lower stretch of the river. The family, Cyprinidae formed more than one-fourth (26.92%) of the total number of species followed by Danionidae (21.79%), Bagridae and Sisoridae (7.69% each) and Channidae (6.41%). During 2017-18 mean values of Shannon-Wiener index (Hʹ), Margalef’s richness index (dʹ) and Pielou’s evenness index (Jʹ) ranged from 1.877 to 2.420, 2.446 to 3.369 and 0.727 to 0.769, respectively, while for the period 2018–2019 values ranged from 2.654 to 3.00, 4.351 to 5.638 and 0.865 to 0.886; and from 2.731 to 3.083, 5.032 to 6.607 and 0.858 to 0.887, respectively during 2019-20. A total of 11 water quality variables were analyzed during the period. During 2017-18, water quality of the river was characterized by high turbidity and low transparency values, which improved in subsequent years. Significant variations were recorded in mean values of transparency, turbidity, and total chlorophyll between 2017-18 with 2018-19 and 2019-20 (p < 0.05). Statistical analysis affirmed that water quality attributes like depth, pH, dissolved oxygen and turbidity have strong association with fish community compositions in the river. We have observed that following the short period of environmental degradation, as the river regained its pristine status, fish diversity also improved concurrently, which suggests that short-term ecological degradation do not affect fish diversity in the long-run.