Ecological structure of co‐evolved fish assemblages in a highland mountain stream in Sri Lanka

This study investigates the ecological structure of an ichthyofaunal assemblage within the Galmal Oya-Heel Oya stream network on the southern flank of the Knuckles Mountain Range, Sri Lanka. We examined how species composition and spatial distribution vary along an elevational gradient and assessed the influence of environmental factors - including substrate type, water velocity, habitat dimensions, turbidity, shading, canopy cover and physicochemical parameters - on assemblage structure. Species richness increased downstream, with Garra ceylonensis and Schistura notostigma dominating high-elevation reaches (~1000 m above sea level, asl), whereas Devario malabaricus and Dawkinsia filamentosa were more abundant at lower elevations (500-1000 m asl). Species within the assemblage exhibited clear differentiation in macrohabitat associations and microhabitat selectivity, reflecting distinct ecological preferences and adaptive strategies. Our results suggest that this co-evolved structure plays a critical role in resource partitioning within Sri Lankan highland stream assemblages, thereby minimizing niche overlap. This pattern is evident in both identified feeding guilds, where species with similar feeding strategies avoid direct competition through spatial-ecological segregation, as observed in the 'benthic feeders' - the loach duo Lepidocephalichthys thermalis and S. notostigma, and the pair Mastacembelus armatus and Channa kelaartii - and in the surface feeders, D. malabaricus and Rasbora dandia. In contrast, co-occurring species further reduce competitive interactions through dietary differentiation, exploitation of different relative depths (e.g., D. filamentosa and D. malabaricus), or fine-scale feeding behavioural specialization, as observed between G. ceylonensis and Plesiopuntius bimaculatus. The study also highlights the importance of biological interactions among benthic cyprinids in shaping assemblage structure. Overall, our findings provide baseline ecological information essential for understanding and conserving Sri Lanka's highland stream fish assemblages.