Exotic brown trout threatening native Himalayan fish species: Study

The exotic brown trout invasion is threatening the native Himalayan snow trout by restricting their distribution and movement in high-altitude river basins, according to a study.

The research, published in the Journal of Applied Ecology in July, shows that snow trout highly prefer the river mainstem or the downstream segment for survival.

However, the invasive brown trout is forcing the native species to move upstream towards headwaters or the source of the river.

"This is concerning, as the headwaters can at best, only provide sub-optimal habitats, not sufficient to support the entire life of native snow trout," said Aashna Sharma, from Wildlife Institute of India (WII) in Uttarakhand, who co-authored the research paper.

"This is majorly a disastrous outcome of our preferences for the invasive brown trout for sport leisure and delicacy choices we have," she added.

The brown trout is an exotic species, which is highly promoted due to its popularity for recreational angling and food delicacy, the researchers said.

The fish was introduced under the British rule mostly for sport, they added.

The researchers also emphasised the need to leave the headwaters free from dams.

"We have already tapped most of the hydropower from these fragile ecosystems," said the corresponding author of the study K. Sivakumar from WII.

"Damming the headwaters will dilute the opportunities of the natives to seek refuge, as snow trout move towards headwaters under invasion and these stream segments provide conditions not so favourable for the invasive species," Sivakumar added.

The researchers, including those from Panjab University in Chandigarh, used intensive surveys and geostatistical network models for the study.

Watersheds of Asiganga in Uttarakhand and Tirthan in Great Himalayan National Park, Himachal Pradesh were used as experimental sites to run these models.

The researchers compared snow trout in two stream networks with and without invasive species, for assessing differences in their spatial distribution.

Study author Vineet K. Dubey from WII noted that snow trout are evidently pushed from the main channel into the headwaters by the invasive brown trout.

"Our models predict disastrous situation where the natives have to compete for space with highly aggressive brown trout in whatever habitat is left for the natives to occupy," Dubey explained.

"This has made the distribution of native snow trout highly patchy and fragmented which can also be leading to potential local extinction in near future," he said.

Study co-author J. A. Johnson from WII noted that the snow trout is an iconic species of high-altitude Himalayan rivers distributed all along the Himalaya from Jammu and Kashmir to Arunachal Pradesh.

Owing to dearth of information on other high-altitude natives, the snow trout is a potential keystone species for this study to facilitate conservation of all other threatened species in the Himalayan rivers, he added.

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