With the State Government turning a blind eye to the detrimental effects of building large dams like the Tehri dam, the native fish species once found in abundance in the Bhagirathi river and its major tributary Bhilangana have now become rare, while the increasing population of the common carp in the Tehri dam reservoir is threatening the native biodiversity.
Before the construction of the Tehri dam, native fish species like the snow trout, garra, glyptothorax and various other fishes used to be found in abundance in this region. However, these native fish species have now almost disappeared from the large Tehri dam reservoir. Research has revealed that construction of dams on rivers has affected aquatic biodiversity and destroyed fish habitats. Before the construction of the dam, the Bhagirathi and Bhilangana used to flow freely till Devprayag. At present, the Bhagirathi’s 46 kilometres course from Chinyalisaud to Koti Colony and its 19 kilometres course from the dam to Koteshwar has been converted to reservoirs. From Koti Colony to Ghansali, the Bhilangana’s 22 kilometres stretch here is now a reservoir.
Alarming facts about the condition of the aquatic biodiversity in this region were revealed in a research undertaken by the HNB Garhwal Central University Swami Rama Tirtha campus zoology professor and ichthyologist Dr NK Agrawal. Before construction of the dam, native fish species like the snow trout, garra and others were accustomed to the swift flow of the river and used to get their food from the algae and weeds on the riverbed and spawn without problems in the river. However, creation of the reservoir after construction of the dam has destroyed the food sources and spawning grounds of the fishes.
Since the construction of the dam, the population of the common carp in the dam reservoir has increased considerably. The increasing population of the common carp which was introduced to this area is now threatening the native Mahseer fish. Research by Dr Agrawal has revealed that the flow of the Bhagirathi along its 14 kilometres course from Maneri to Tiloth and the 22.5 kilometre stretch from Joshiyada to Dharasu is so low at times that the river appears dry. The aquatic biodiversity and fish population in the Bhagirathi has suffered due to this. According to Dr Agrawal, “Due to construction of various hydro power projects, the rivers in Uttarakhand are now flowing through tunnels and have lost their natural flow. The decomposition of plant material and organisms in the dam reservoir is increasing the generation of greenhouse gases here. Increasing the population of Mahseer fish in the reservoir will ensure that these fishes will consume the plant, animal and carbonic elements which will mitigate the problem,” he said.