Hydropower dams, road construction and mining along the rivers, extraction of sand from the riverbed and introduction of exotic or alien species for harvesting pose serious threat to fishes, a researcher Gopal Prasad Khanal said.

Gopal Prasad Khanal, who works as a livestock production officer in the National Research Centre for Riverine and Lake Fisheries, is conducting a research on mahseers, a species of fish.

He said fish is one of the least studied and threatened fauna in the country. “It is under threat because of the rapid decline of its population in the Himalayan region from pollution, habitat loss and fishing.”

Fish is protected in Bhutan by the Forest and Nature Conservation Act of Bhutan 1995 and Forest and Nature Conservation Rules of Bhutan 2016.

Gopal Prasad Khanal said dams lead to obstruction of fish migration within feeding, spawning and refugee habitats and dam water favours environment for the establishment of other non-native species.

The physical consequences include accumulation of sediments and alteration of natural flow of water.

The livestock production officer said the cleaning of sediments through rapid flushing of water from dams will not only bring changes downstream but will also devastate downstream flora and fauna.

He said the soil and debris from road construction and mining mostly land up in rivers and streams, thereby, increasing suspended solid and sediment loads.

“The turbidity from the suspended solids will have direct impact on fish either by killing them, slowing their growth, reducing resistance to disease, or by altering the natural movement and migration,” he said, presenting the country’s fishery resources and their threats and opportunities at the national water symposium last week.

“When the quantity of extraction exceeds replenishment limits due to increase in population and developmental activities it will be a major problem to the river system,” he said.

Preliminary research findings on mahseer in Chamkharchu and Drangmechu revealed that both the Golden and Chocolate mahseers migrate long distances of more than 25kms in two days.

Gopal Prasad Khanal said that the research also showed that not all Mahseers migrate to India in winter.

The study began in April 2015 and it has surgically implanted 100 fishes, 70 golden and 30 chocolate mahseers, with a radio transmitter so far. The fishes were released in the Drangmechu.

Brown trout was the first exotic fish species introduced in the country during the 1930s. Eight species of fish have been introduced so far since the 1980s to promote fish farming.

Gopal Prasad Khanal said invasive alien species like African sharptooth catfish was introduced through tshethar a project dedicated to saving lives and Mozambique tilapia is also reported from Crocodile Rehabilitation Centre, Phuentsholing.

“In freshwater habitats, invasive alien species are considered to be the second leading cause of species extinction along with habitat destruction,” he said.

The researcher said that there is a prompt need to further strengthen the study on freshwater biodiversity for proper management in future.

The conservation status of fishes in Bhutan is yet to be evaluated at the national level and most of the information pertaining to conservation status are being adopted by International Union for Conservation of Nature’s Red List of Threatened Species.

There are 104 species of fishes recorded in the country so far, of which 93 species are indigenous and 11 species are exotic imported species.

Bhutan ranks 6th in terms of per capita internal freshwater resources with 0.102 million cubic meters including five major and five minor river system stretching about 7,200 km.

“Adequate assessment of lakes and marshlands in other parts of the country is yet to be done,” Gopal Prasad Khanal said.

Tshering Palden