Dendritic prioritization through spatial stream network modeling informs targeted management of Himalayan riverscapes under brown trout invasion

Abstract

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.

Quantifying habitat overlaps between native and non-native fishes in the Himalayas using MaxEnt models optimized for data-deficient stream networks

“…This region is warming at twice the rate of the global average, and glacial retreat is happening six times faster than in other regions of the world 62 . The geographic distribution of native snow trout is likely shrinking, and shifting, under climate change 29 . Many developing countries have extensive freshwater systems with high species diversity and endemism, which demands approaches that maximize the utility of freely available information (e.g., climatic, landcover, and other geospatial data).…”

“…Two native snow trout S. plagiostomus (Heckel, 1838)and S. richardsonii (Gray, 1832) are listed as vulnerable on the IUCN red list 9,28 due to commercial and recreational shing pressure. In addition, a considerable truncation and range shift has been observed for these snow trout species attributed to both climate change and non-native rainbow and brown trout 29 . Nonetheless, the geographic distribution of potential habitat overlap between native snow trout and non-native trout species is still unclear.…”

How resilient are waterways of the Asian Himalayas? Finding adaptive measures for future sustainability

“…Recently, a study on snow trout (Schizothorax richardsonii), a flagship-keystone species in Himalayan waterways with great commercial and recreational values, has been found to be poorly distributed within their habitat range due to the introduction of exotic brown trout (Salmo trutta) and other land use intensifications, including river valley modifications and destructive fishing practices (A. Sharma, Dubey, et al, 2021a;A. Sharma, Dubey, et al, 2021b).…”

“…The consequence posed by the loss of keystone species including Daphnia and other biota under climate warming and anthropogenic impacts, such as species invasion, land use change, and tourism is yet to be comprehensively understood. Recently, a study on snow trout ( Schizothorax richardsonii ), a flagship‐keystone species in Himalayan waterways with great commercial and recreational values, has been found to be poorly distributed within their habitat range due to the introduction of exotic brown trout ( Salmo trutta ) and other land use intensifications, including river valley modifications and destructive fishing practices (A. Sharma, Dubey, et al, 2021a; A. Sharma, Dubey, et al, 2021b). Establishing the ATB‐EF‐ES interactions response framework with the role of flagship‐ keystone species can be an important approach for understanding and managing fish habitats enhancing diversity of Himalayan waterways under climatic and anthropogenic perturbations.…”

Do metapopulations and management matter for relict headwater bull trout populations in a warming climate?

“…For freshwater stream taxa that require cold temperatures, this often translates to the truncation of former ranges at downstream extents and persistence in mountain headwater streams as increasingly disjunct relict populations (Fausch, 2008;Quaglietta et al, 2018;Young et al, 2016). Although confinement to headwaters, especially in an era of global warming, has been viewed as antithetical to long-term persistence (Buisson et al, 2008;Domisch et al, 2011;La Sorte & Jetz, 2010), recent research indicates that the slow climate velocities typical of steep mountain streams make them resistant climate refuges for a variety of taxa (Isaak et al, 2016;Muhlfeld et al, 2020;Sharma et al, 2021;Troia et al, 2019). Moreover, such locations appear to be more resistant to nonnative species invasions and less subject to many forms of habitat degradation than are lowland streams because of their climatically extreme conditions and high stream power (Isaak et al, 2015;Margesin & Schinner, 1999;Montgomery & Buffington, 1997).…”