| Summary: | Shannon’s diversity index H’ = pi log2 pi was calculated for 16 fish species. The index was resolved into its components, species richness and relative abundance, to determine which components played a larger role in the determination of diversity pattern. Changes in diversity were correlated with species richness (r), habitat heterogeneity, and hydrological regimes in a longitudinal stretch of 35 km along the river Giri, a major tributary of the Yamuna River system, in Western Himalayas. Abundance differs with change in habitat structures, habitat preference of fish species and water regimes. The decrease in diversity in the lower stretch of about 3-4 km upstream of the barrage was found to be associated with habitat fragmentation and as well as d/s with loss of biotic integrity of aquatic ecosystem due to water scarcity. Species richness was found to be maximum at upper and middle reaches (elevation 650-800 m, msl) of the river, whereas it was low in lower reaches. Change in water chemistry was also noticed at Jatan barrage-low head dam due to impoundment of river water. It is inferred that the regulation of water has an impact on species richness and relative abundance, and on habitat heterogeneity which has decreased due to the change in environmental condition.
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