Chhimi Dema

Research conducted by four faculty members of the College of Natural Resources found traces of heavy metals and other elements in three fish species in the Punatsangchhu: Snow Trout, Brown Trout, and Copper Mahseer­.

The research findings indicate there are pollutants in the river.

“Fish are the most suitable bio-indicators for assessing quality of the aquatic environment since they are located at the higher trophic level in the food chain,” the research titled Trace elements in fish species: Schizothorax richardsonii, Salmo trutta and Neolissochilus hexagonolepis in relation to sizes and tissues of Punatsangchhu stated.

The study, conducted along Punatsangchhu from Samdingkha up to the Changchu-Punatsangchhu confluence, also highlighted that the Punatsangchhu receives a considerable amount of chemical elements from untreated municipal sewage, agricultural runoff, Punatsangchhu Hydroelectric Project I and II, sand dredging, and effluents from Punakha and Wangdue.




Samples tissues of fish muscle and gills were analysed for aluminium, arsenic, barium, cadmium, cobalt, chromium, copper, iron, potassium, manganese, molybdenum, nickle, lead, selenium, strontium, and zinc.

The fish samples were tested for the chemical elements using an analytical technique used for the detection of chemical elements at the Royal Centre for Disease Control in Thimphu.

The research found that the highest concentration of nickel and selenium were found in the muscle tissues of Snow Trout.

The concentration level of elements in the fish was based on permissible limits set by the United States Environmental Protection Agency, the United States Food and Drug Administration, the Food and Agriculture Organisation, and the European Commission.

The maximum concentration of potassium was found in the muscle tissue of Copper Mahseer and the highest concentration of copper in the gills of Copper Mahseer.




Brown Trout muscle tissue had a minimum concentration of arsenic, barium, copper, manganese, molybdenum, strontium, and zinc. Gills of the fish also recorded the least concentration of nickel, selenium, and lead.

Chemical element cadmium showed a significant positive correlation between weight and its concentration in the muscles of Snow Trout.

Cadmium is the most toxic non-essential heavy metal to freshwater organisms including fish, the research stated. “Soil, rocks, municipal waste, and industrial effluents are the major sources of Cd [cadmium].”

Chemical substances such as lead are found in water systems through runoff, industrial and sewage waste streams, and accumulated in gills and digestive tracts of fish.