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Brown trout Salmo trutta is a potent global invader and its establishments have progressively altered physiologies, life‐histories and niche‐availabilities for native fish species. River impoundments further escalate its invasion potential. The Himalayan rivers however, stay uncharted for the effects of brown trout interactions with the native fish fauna. Snow trout Schizothorax richardsonii a Himalayan cold‐water native, concerningly overlaps its range with brown trout. To understand its responses to invasion pressures, we investigated brown trout effects on the age and growth of snow trout populations in three rivers with varying levels of perturbation: (a) a dammed and (b) an undammed river with the invasive brown trout in comparison to (c) an undammed river without invasion pressures. We found sympatric snow trout in the undammed river to respond to brown trout invasion with fast life history responses, showing an early age‐at‐maturity (A50 = 1.2 years) and fast growth with a higher growth constant (K = 0.40 yr−1) and specific rate of linear growth across life. On the contrary, sympatric snow trout in the dammed river showed an explicitly slow life‐history by maturing at a higher age (A50 = 2.9 years) and a slow growth, with a lower growth constant (K = 0.26 yr−1) and specific linear growth rates. Our findings suggest that, the snow trout appear to present stronger response to brown trout invasions when the river is unaltered and free from hydropower operations and damming. Further research is strongly warranted from other high‐altitude Himalayan basins to delineate the variation in growth strategies exhibited by snow trout in sympatry with the invasives.

1. With the concept of 'riverscapes' long pending to be acknowledged in the 'landscape-centric' legislative framework of Himalayan nations, conservation of native riverine species stays practically unheeded. This necessitates urgent prioritization of stream networks to conserve the lotic taxa under invasion pressures. Himalayan riverscapes are pervaded with the invasive-exotic brown trout Salmo trutta, posing serious threats to the co-occurring native, the snow trout Schizothorax richardsonii.2. Using intensive surveys (218.7 km) and geostatistical stream network models (n = 537), we contrasted snow trout in two stream networks with and without invasives, for assessing differences in their spatial distribution.3. Our models indicate invasion-induced relegations of natives from the river mainstem into headwaters, with large sections of mainstem occupied by invasives.Furthermore, a concerningly small percentage of potential habitat left for natives to occupy in the mainstem is threatened, where a 100% overlap of native and invasive trout distributions is predicted.4. With a higher presence probability for the natives in headwaters of invaded watershed as compared to the non-invaded watershed, we highlight the headwater streams as potential refugia for the natives under invasion. Synthesis and Applications.Our approach of basin-scale dendritic prioritization provides immediate management solutions to tackle brown trout invasion threats in Himalaya. We inform decisions on delineation of headwaters as invasion refugia for native fish, with assisted recovery of their fragmented populations in the river mainstems through targeted management of invasives.

A key to the management of hillstreams in relation to Fish Species Richness (FSR) and Shannon and Weaver Diversity Index (H¢) (Shannon and Weaver, 1949. The Mathematical Theory of Communications. Urbana, University of Illinois Press, IL, USA) using a multitude of factors such as altitude, water source, bed gradient, dominant channel materials and habitat type has been proposed on the basis of 10 study sites, belonging to three north Indian rivers namely; Ghaggar, Yamuna and Ganga. The selected streams are located between the altitudes ranging from 380 to 1524 m. It has been observed that high altitude streams having steep gradient, boulder dominated substrate with rapids as the main habitat type had lower FSR (3-4) and H¢ (0.55-0.99) as compared to hillstreams located in the lower altitudes having gentle gradient, cobble dominated substrate with pools, riffles, runs, rapids and cascades as the main habitats and have high FSR (6-14) and H¢ (1.67-2.35). Types -A, B and F hillstreams were encountered when the hillstreams are classified after Rosgen (Applied River Morphology, Wildland Hydrology, Co, USA). Hydrobiologia (2005) 532: 225-232 Ó Springer 2005