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

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.
Using intensive surveys (218.7km) 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. 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. 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