As visible in the project title of Hydro4U, sustainability is a crucial part of the developments in small-scale hydropower in Central Asia, particularly ecological sustainability. Ecological impacts of HP are for instance hindered migration, altered flow regimes (reduced flow, fast flow changes) or changed river morphology (sediment retention, increased embeddedness). The investigations performed by the European partners BOKU (A), EVINBO (B), Hydrosolutions (CH) and SJE (D) in cooperation with local partner TIIAME (UZB) in the demonstration sites Shakimardan at Koksu river in UZB, and Atbashy close to Naryn City at Atbashy river in Kyrgyzstan) focus on the mitigation of ecological impacts.

Hydrology in Shakimardan and Atbashy
The Koksu River, part of the Shakimardan basin, is influenced by a natural dam created by rockslide deposits, leading to the formation of lakes Kurbankul and Yashikul. The Koksu’s discharge, measured from 1948 to 2020, shows a minimum monthly flow of 1.25 m³/s in spring and a maximum of 16 m³/s during summer. The river’s annual sediment load is low due to upstream dam filtration, and it remains unfrozen throughout winter. The Atbashy River, a tributary of the Naryn in Kyrgyzstan, spans an area of 1,496 km² with elevations ranging from 2,455 to 4,843 meters above sea level. From 1970 to 1995, the Atbashy’s mean discharge was 16.6 m³/s, characterized by a nivo-glacial and strongly seasonal regime, with higher flows in warm months (mean discharge of 24.8 m³/s) and lower flows, accompanied by significant ice cover, in winter.

Fish populations in Central Asia and target species
The Mountains of Central Asia biodiversity hotspot consist of two major mountain ranges: the Pamir; and the Tien Shan with a total area of about 860,000 km² covered, including the 2 demonstration sites Shakimardan and Atbashy. Much of the biodiversity and natural ecosystems are in remote mountain areas and have still to be better studied. Therefore, any human impact directed on the change of aquatic habitat in these ecosystems has to be thoroughly assessed before implementation.

During 2021-2023 the ecological conditions and diversity of the ichthyofauna of mountain and foothill sections of rivers that are promising for the development of environmentally sustainable small-scale hydropower have been studied. Special attention was paid to the conservation of the diversity of fish key species and other aquatic organisms. To assess this integral component of the sustainable functioning of river ecosystems a special focus has been laid on the related habitat conditions.

One of main steps to achieve sustainable SHP is the identification of target aquatic key-species to be protected, having high importance for the protection of wildlife biodiversity. Collected field data on fish diversity, taxonomy and ecology of more than 50 river catchments in Kyrgyzstan and Uzbekistan have allowed to identify for the first-time main key fish species in upstream sections of Central Asian Mountain rivers (see also illustrations below):

1. Snowtrout – Schizothorax eurystomus (Kessler 1872);
2. Sewertsow rare-scaly osman – Diptychus sewerzowi (Kessler 1872);
3. Triplophysa ferganaensis (Sheraliev & Peng 2021);
4. Stone loach – Triplophysa sp.;
5. Glyptosternon oschanini (Herzenstein 1889).

Figure: Central Asian fish species as target of HP impact mitigation within Hydro4U (© Jennifer Clausen, www.jacdraws.com)

First findings on fish ecology of snow trout
Snow trout (Schizothorax sp.) belong to the most important fish species in Central Asia. They are adapted to fast-flowing, high-sloped mountain rivers. These fish, which reach up to 60 cm in length and 4 kg in weight, have a diet comprising algae, detritus, macroinvertebrates, and smaller fish. Their life cycle includes reaching sexual maturity at 3-4 years and a notable spawning period from April to September.

A habitat preference study of Schizothorax eurystomus in the Shakhimardan basin performed by Hydro4U researchers using point electrofishing revealed that juvenile snow trout prefer shallow waters around 20-40 cm of depth, while sub-adults and adults favor deeper areas of >30 cm and >50 cm, respectively. All size classes showed a high plasticity for diverse flow velocities and inhabit various substrates, with juveniles showing a preference for mid-sized substrates. This research is significant as it expanded the known distribution range of S. eurystomus and provided baseline data for environmental flow assessments.

Telemetry studies
Fish use various habitats to complete their life cycle such as spawning habitat, wintering sites and foraging areas. These habitats can be located at different parts of a river system and used by fish at different, seasonal-specific moments in time. Knowledge on the movement and habitat use of snow trout species Schizothorax eurystomus in the rivers of the Shakhimardan enclave is currently lacking. Hence, the effect of the planned hydropower plant in the River Koksu on the habitat use of snow trouts is unknown.
We used radiotelemetry to tag and track (position) 29 snow trouts in October 2022 to reveal their habitat use over the course of one year so we could learn when they reside and Koksu and why.

As the dataset is almost complete, we learned that the snow trouts seasonally visit the Koksu River, in particular during autumn months, which reasons for are to be clarified. However, these data suggest that the construction of the hydropower plant needs to take into account the life cycle of snow trout. In the next months the tracking data will be analyzed deeper to learn when and under which environmental conditions snow trout visits specific habitats. This information is not only crucial for the Shakhimardan enclave but can help future planning of river regulating structures in fish-ecologically, similar places in Central Asia.

Migration facilities and EFlow
The information for target species Schizothorax eurystomus has been used in Shakimardan for simulations with the habitat model CASiMiR to find a seasonally adapted E-Flow providing fish habitats in adequate quality and availability when the HPP is diverting part of the natural flow. The existing weir will integrate a state-of-the-art fish way and a bypass installation that enable up- and downstream migration for snow trout. Another artificial migration barrier within the river will be made passable as well. In Atbashy the modernized irrigation weir, equipped with a shaft turbine, will also integrate upstream- and downstream migration facilities. The attraction flow leading fish into the bypass channels and preventing them from entering the turbine inlet is investigated using a newly developed module of CASiMiR to possibly optimize the inflow conditions. The findings from these studies together with the results from the monitoring of the HPPs that both go into operation during the project period will deliver information for the adaptive management as part of the EIA.

Authors: Matthias Schneider (SJE), Tobias Siegfried (HSOL), Daniel Hayes (BOKU), Pieterjan Verhelst (EVINBO), Bakhtiyor Karimov (TIIAME-NRU), Erkin Karimov (TIIAME-NRU), Otabek Omonov (TIIAME-NRU).

Further contributors: Iana Kopecki (SJE), Tobias Haegele (SJE), Beatrice Marti (HSOL), Bernhard Zeiringer (BOKU), Johan Coeck (EVINBO), Ine Pauwels (EVINBO)