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Several measures have been suggested to mitigate the adverse impacts of a hydropower project.

4.1 Mitigations for weirs

4.1.1 Maintenance of flow level

There may be a dewatering effect downstream during the dry season due to the flow diversion and damming of the river. The effect is local and can be overcome to some extent by releasing compensation flow downstream. Compensation flow for the conservation of microflora, aquatic insects and fish in the dewatering zone should be within 10-20% of the regular flow. Regular releases of flushing flows will maintain quality of spawning gravel scouring fine sediments away. The compensation flow varies from river to river. Depending on the volume of water discharge, the recommended minimum compensation discharge ranges from 0.5-15 m3/s in Nepal (Mewa: 0.5 m3/s, Melamchi: 0.5-1.0 m3/s, Likhu: 0.6 m3/s, Budhiganga: 1.7 m3/s, Tamur: 3.5 m3/s, Dudhkoshi: 4.0 m3/s, and Upper Karnali: 12-15 m3/s). However, these are not standard figures that can be used everywhere and for all rivers globally. While compensation flow release is present in the plan, negligence in commitment was noted as in the Marsyangdi Hydropower Project.

4.1.2 Screens and fish exclusion devices

Entrapment of fish is a critical issue and some provision should be made to protect the fish against entrapment and impingement. Installation of appropriate screen devices at the intake will divert the fish from water intakes. Ideally, fish bypass facilities should be installed.

4.1.3 Fish passes

Fish passes are one of the most important remedies for assisting fish migration. They have been proved satisfactory for salmon in the northern temperate rivers, for cyprinids in the Tigris and Euphrates, and for Tor and Indian major carps in the Ganga. Hydropower projects of Nepal are generally established in areas of fish migration and fish passes are recommended to mitigate the barrier effect of the dam. A fish pass should meet the following criteria:

- it should be adapted to the requirements of the species concerned
- it should be of a pool type, rocky ramp type, or a vertical slot
- flow velocities must not exceed the swimming capacity of fish
- it should provide passage for all fish sizes - large and small,
- it should be provided with proper fencing, with total ban on fishing.

The more natural types of passes, e.g. rocky ramps or artifical rivers (bypass channels), can even enhance the beauty of the landscape.

In Nepal there are fish passes at the Koshi Barrage, also at Chandra Nahar, Andhi Khola, Gandak Barrage and Kankaimai. Some existing passes, for example at Andhi Khola, are poorly designed. This pass has inadequate through flow and attraction flow to pass large fish such as Tor and Bagarius bagarius, and the entrance is not well placed. To make it more efficient, this pass needs drastic improvement. Similarly the pass of the Kankaimai Project that is of pool and weir type needs to be improved. There are hundreds of good examples of well-designed and effective fish passes in the world. A fish pass needs to be tested for the known fish species migratory behaviour. The pass has to be monitored by fishery specialists. If the monitoring finds that fish do not use the fish pass, the pass has to be improved.

4.2 Mitigations for high dam reservoirs

4.2.1 Artificial destratification

Water quality may deteriorate with its storage. Releases of good quality water are required for maintaining well-functioning downstream aquatic habitats. Artificial destratification is one of the effective measures for maintaining a good quality of water in a reservoir which undergoes stratification. It requires a small amount of energy per unit volume to lift cooler bottom water to the surface of a stratified impoundment. This helps to mix water and to maintain uniform temperature and vertical distribution of dissolved oxygen.

4.2.2 Trapping and hauling

It involves trapping of fish below the dam and transporting them to the reservoir or further upstream to maintain fish diversity and gene pool. But it is labor intensive, prone to poaching by handlers and stressful to fish which increases their mortality. It also needs an appropriate location: facilities have to be designed at the earliest possible stage and well incorporated into the project - a later addition may be problematic or not possible at all.

4.2.3 Fish lift

In the Kali Gandaki dam a fish lift would have been technically feasible and would be more efficient than the planned fish stocking.

4.2.4 Development of fishery

Fish passes play an important role in the conservation of the native fish resource. The existing fish passes in Nepal are not satisfactory. At present preference is given to maintenance of spawning grounds and fish hatcheries as a means of enhancing the fish stocks affected by dams.

Maintenance of spawning grounds

Some resident fish such as stone roller (G. gotyla), stone loaches (N. beavani), catfish (Glyptothorax pectinopterus) and murrel (C. punctatus) utilise gravel bed areas for spawning. Considerable loss of spawning grounds of these species has taken place immediately below dams. Adequate attention must be given to the protection of the spawning and nursery gravel beds. Where needed, additional measures should be taken:

- depositing gravel to increase the spawning habitat

- manipulating angular and large boulders to create pools for spawning and as an escape cover for resident fish during low water levels

- using large boulders to alter the flow pattern downstream

- keeping gravel and boulders together to create spawning riffles to attract resident stock to rapids

- releasing flushing discharge to rewater exposed gravel beds to maintain spawning gravel quality

- enhancing the habitat by tree planting to increase shelter cover, shade and drift food.

Fish hatchery

A reservoir associated hatchery should produce seed of important native fish such as mahaseer, copper mahseer, snow trout, jalkapore and freshwater eel which are most affected by dam projects. Stocking the reservoir and tail water will replenish the losses resulting from the disappearance of the natural spawning grounds and from secession of migrations. The fishers should be provided seed from the government hatchery to grow fish in ponds to market size. This provides alternative means of subsistence and income, thus reducing the pressure of the capture fishery on native stocks.

Reservoir fishery

While regular fish stocking is one way of enhancing reservoir fish stocks, reservoir-based aquaculture is also a useful enhancement practice. Beveridge and Phillips (1988) reviewed the cage, pen and enclosure practices in reservoirs. In Nepal, a successful cage culture is being practiced in Trishuli and Kulekhani reservoirs where it provides income to the local fisher communities.

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