Some physico-chemical parameters of Bhutanese waters were studied and are summarized in Table 3 for rivers and in Appendix 1 for Lake Ho Ko Tso.
The data obtained indicate that the water of the river systems throughout the country tends to be alkaline, with a pH ranging from 6.8 to 8.7, except in one case where it was found to be 9.2. Dissolved oxygen values range from 7.8 to 11 ppm. Although it was not possible to record the water temperature at different altitudes and in different seasons, from the information collected during the assignments the water temperature of the sub-tropical areas, mostly remains at 12°C and above throughout the year. In the lower temperate zone (below 1 524 m) and the higher temperate zone (below 2 382 m) the temperature remains at 12°C and above for about nine and six months respectively.
Difficulties of communication and lack of field facilities did not enable a detailed sampling of high mountain lakes. However, the adviser paid visits to nine natural lakes in the cold area, five in Degala (3 500 m) and four in Pajoding (3 600 m); two in the temperate area, Ho Ko Tso (1 829 m) and Luchika (1 830 m), and one in the subtropical area Gulandi (366 m).
The high altitude lakes of Dagala, visited on 30 May 1976, had a water pH range of 6.1 to 6.8, a temperature of 7°C, and an electric conductivity of 8–19 u mohs and showed presence of plankton and insects. The visit to Lake Ho Ko Tso, one of the biggest in Bhutan, is described in Appendix 1. It could be tentatively classified as eutrophic and after further examination could be developed for culture of common carp and other suitable varieties. Similarly, Lake Luchika, situated in the Wangdi district and Lake Gulandi, a subtropical lake, can also be developed for carp culture.
The rivers and lakes have predominantly cold water and torrential stream fauna except in the foothills and plains. A total of 42 species of fish have been collected from the rivers and one natural lake of Bhutan and their distribution, size range, etc., are indicated in Table 4. Asla (Schizothorax progastus), a local trout, is predominant throughout the rivers of the country and is found at all altitudes. Sampled asla fry, caught in Thimbu in April, are reared at Wangchutaba.
Of the indigenous species of economic interest, catli (Acrossocheilus hexagonolepis) has been observed up to an altitude of 1 200 m, whereas mahaseer (Tor tor and Tor putitora), chepti (Semi-plotus semi-plotus), Labeo dyocheilus and other minor varieties are found in the foothills up to about 330 m.
One natural lake, Gulandi, situated in the subtropical area in the Diapham district has catli (A. hexagonolepis), mahaseer (T. tor), Mastecambelus armatus and other minor varieties.
Fry and fingerlings of species of local economic importance, such as catli (A. hexagonolepis), mahaseer (T. tor and T. putitora), L. dyocheilus and chepti (S. semiplotus), have been collected from different rivers, including the Torsa at Phuntsholing, Kalikhola in Sankosh, Sarbhang Khola at Sarbhang and Manas at Manas Sanctuary, whereas fingerlings of Catli were collected only from Mangdi Chu near Mangdi bridge and Gamri Chu near Tashigang. No breeding habits of these fishes could be studied. The best place for this study was the Manas at Manas Sanctuary but due to the excessive rains and continuous floods during this year and that the road leading to Manas from India was cut off, the adviser had to abandon the study. However, one mature catli (A. hexagonolepis) was caught in Sarbhang Khola on 7 August 1976 and a mature male and a female L. dyocheilus were caught in the Manas River at Manas Sancturary on 4 May 1976. Other varieties like buduna (Garra gotyla) and many species of Barilius also seem to be established in all the rivers of the foothills.
Brown trout (Salmo trutta fario) was first introduced in Bhutan in 1930 and just afterwards, a hatchery was established at Ha. Another hatchery was established at Wangchutaba, Thimbu in 1975. As a result of almost continuous raising of fry and fingerlings, mostly at Ha, and of continuous stocking of the rivers in the country for the past 46 years, this fish has now established itself in some rivers, like the Ha Chu, Thimbu Chu and Paro Chu. It is also present in some tributaries of the Sankosh and Manas Rivers (i.e. Mo Chu, Ho Chu, Mangdi Chu and Chamkhar Chu). Although a quantity of trout fry and fingerlings have been produced at Wangchutaba since 1975, the main source of the supply of fingerlings has been the trout hatchery at Ha. About 1 030 one-year-old fingerlings 30–40 g/12–14 cm were also collected during 1976 from Thimbu Chu during the assignment and kept at Wangchutaba fish farm to study their growth. Breeding of brown trout in natural conditions was also observed in Thimbu Chu on 18 November 1976.
Although some of the high altitude lakes have been stocked with brown trout (S. trutta fario), during visits to them the water was observed crystal clear and no trace of any fish was seen. No netting operations could be conducted as the boat was received very late. Angling was attempted once in Lake Ho Ko Tso but no fish were caught.
It was not possible to make any assessment of rivers in the subtropical and temperate zones due to swift currents and to impassable roads during the rainy season. For assessment of the trout population in cold water streams castnet sampling was carried out during May to November 1976. A 5 km stretch of Thimbu Chu, from Ramtokhta to Simtokha Monastry, was fished (35 m average width, equivalent to 17.5 ha surface area) and the following catches were obtained:
| Month | Number of fish caught | Average weight |
| May | 234 | 120 g |
| June | 614 | (A separate record for each month could not be kept, so the average weight of 630 fish caught during this period has been used). |
| July | 169 | |
| August | 43 | |
| September | 48 | |
| October | 144 | |
| November | 45 | |
| Add 2% of the above catch from angling | 26 | |
| TOTAL | 1 323 |
