Heterogenisation of riverine ichthyofauna diversity by small hydropower dams

Dams modify river physical and chemical attributes. This disruption hampers fish migration, leading to the introduction of non-native species and losses of specialised native species. Small hydropower plants (SHPs) have been suggested for replacing large dams to minimise reservoir size and preserve natural flow regimes. We evaluated the influences of two SHPs on the ichthyofauna in a tropical river, comparing fish taxonomic and functional beta-diversity in three periods. Period-1 preceded SHP construction (Pre-2008), when the river was free-flowing. Period-2 was 1 year after completion of SHP construction (Post-2012). Period-3 was after 10 years of SHP operation (Current-2021). We calculated 10 functional ichthyofaunal attributes based on 12 quantitative morphometric measurements related to habitat use, feeding, and locomotion. The ichthyofaunal composition varied among the periods (p = .001) and seasons (p = .009), with the highest species richness in the Post period compared to the other two periods (p = .002). Abundance significantly decreased over time (p = .004). Taxonomic beta-diversity increased in the Post period remaining stable in the Current period. Functional beta-diversity did not change between the Pre and Post periods but significantly increased in the Current period. The turnover component had the greater influence on both taxonomic and functional beta-diversity, with no temporal changes observed in nestedness. The ichthyofauna appears to have undergone heterogenisation and restructuring. Changes include the emergence of more tolerant species (e.g., Hoplosternum littorale and Pimelodus maculatus), displacement of native and sensitive species (e.g., Pogonopoma parahybae and Steindachneridion parahybae), and colonisation by a non-native predator (Plagioscion squamosissimus). This indicates that small-scale projects such as SHPs harm fish populations by altering habitats and restructuring the ichthyofauna.