Boundaries

The ecoregion includes the drainage areas of the Upper Chu River down to the Chuiskaya Dolina Valley and Lake Issyk Kul [Ysyk-Kol]. Lake Issyk Kul [Ysyk-Kol] basin and the Upper Chu [Chuy = Shu] drainage are bordered in the north by the Kirgizskiy [Kyrghyz Zhotasy] and the Kungey Ala-Tau [Kungey Alatau Zhotasy = Kyungey-Ala-Too] ranges, and in the south by the Terskey Ala-Tau [Teskey Ala-Too = Teriskey Alatau Zhotasy] and the Dzhumgoltau [Dzhumgal-Tau] ranges, which divide the Chu drainage and the headwaters of Naryn and other rivers flowing to the Fergana Valley. The northern border of the Upper Chu drainage follows the Zailiyskiy Ala-Tau Range [Ile Ala toosu = Ile Alatauy Zhotasy] and, further northwest, the dividing ranges of Zhetizhol and Kindyk-Toos (the border with the Balkhash - Alakul ecoregion [634]).

Topography

Lake Issyk Kul lies in a depression between the Tersky Ala Tau and Kungei Ala Tau ridges at an altitude of 1609 m. The length of the lake reaches 182 m and width 58 km. The depths of the lake are quite significant, with the maximum depth reaching 702 m. Its area is 6206 km², and volume is 1738 km³. The lake is of tectonic origin. The basin of Lake Issyk Kul includes 138 small rivers and streams; of these 11 are rivers with catchment areas of more than 300 km² (the largest catchment area is 655 km²).

The Chu River is formed by the confluence of the Dzhuvan Aryk and Kochkorka rivers, rising in the alpine zone of the Tien Shan. Cutting through the spurs of the Kirghizian Ridge and Tersky Ala Tau, the Chu River emerges from the Issyk Kul Depression, from where it flows through the Boamskoye Ravine into the Chu Depression. Emerging from the Boamskoye Ravine the river receives its fullest tributary, Bolshoi Kebin, on the right and numerous tributaries on the left flowing down the northern slope of the Kirghizian Ridge.

For the past half-century the level of the Issyk Kul has dropped by more than two meters. Scientists proposed several hypotheses for the causes of this reduction. Some attribute it to the low water phase. Others attribute the fall of the water level to tectonic processes that drain water outside of the depression. Still others attribute it to growing irrigation that prevents small rivers and streams from reaching the lake.

Freshwater habitats

The water of Lake Issyk Kul is brackish. Its sources include numerous streams and rivers flowing down ridges. Because of the large water surface area the lake influences the climate of the coastal part of the basin. In particular, water evaporating from the lake surface is one of the sources of local precipitation. Moreover, in winter the lake increases and in summer reduces the temperatures of nearby areas. It is not frozen in winter.

The catchment area of the Bolshoi Kebin lies in the elevated part of the peripheral ridges, favorably oriented toward major moisture-containing air masses. Rivers flowing down the southwestern slopes of the Chu-Ili Mountains have the lowest flow rate because of the low height of these mountains.

Rivers of the ecoregion belong to the mountain zone and, by their hydrological peculiarities, are typical for the highland of Tan-Shan.

Rivers are characterized by a predominance of low water temperatures, supersaturation with oxygen, enormous mechanical force of water flow, mobility of the bottom bed, sharp fluctuations of the volume of seasonal, annual, and daily river flow, and eventual absence of plankton and low bioproduction of benthos represented mostly by fouling cenoses and invertebrates that have adapted to life beneath stones. Only in intermontane valleys (Kochkorskii, Kokmuinokskii) with their relatively small slope does the meandering Chu River form many backwaters, excavations, and karasuks (remains of lakes and river channels). These intermontane valley areas provide mountain rivers – with their slow current, well-warmed water, and relatively rich biomass of benthos – a major ecological niche for fishes.

Terrestrial habitats

The ecoregion’s exceptional ecological diversity is characteristic of landscapes of the coastal areas. Plains alternate with small hill deserts and semi-deserts, and areas of cultivated land neighbor floodplain forests. As one moves higher in the mountains, steppes are replaced by meadows, forest-montane, subalpine, and alpine habitats.

Description of endemic fishes

The long historical and geographical isolation of the lake favored the formation of endemic forms. Leuciscus schmidti and Issyk-kul’ minnow (Phoxinus issykkulensis) are the only two strict endemic species.  

Leuciscus schmidti is a species that forms a pair of endemic Issyk Kul daces with L. bergi. It has a more limited distribution in the lake since it is mostly connected with freshened regions at river mouths and shallow littoral zones. It attains a larger size than L. bergi – 500 g – and lives up to 10 years. It migrates from deepwater to shores in spring and back in autumn, and forms local aggregations. The species spawns over rocky sites about 2 m deep, and feeds mostly on mollusks. The species was once the most numerous commercial fish in the lake. Its present state is not known, but there are indications that it is close to extinction. This needs confirmation.

There is no new data on the abundance and distribution of Issyk-kul’ minnow (Phoxinus issykkulensis). However, it is very interesting in terms of the history of dispersal of Phoxinus in different basins of Central Asia. A phylogeographic study is needed.

Other noteworthy fishes

The whole former fish community is an example of a native lacustrine ecosystem that lacked a predatory member. Only larger-sized individuals of osmans and L. schmidti fed occasionally on smaller fishes. Introductions of the predators destroyed the ecosystem completely.

There are a number of endemic subspecies in the ecoregion, including Gymnodiptychus dybowskii var. primitive, G. dybowskii lansdelli, Issyk-kul naked loach (Triplophysa "strauchii" ulacholica), and Issyk-kul marinka (Schizothorax pseudoaksaiensis issykkuli). Near-endemics include Ili marinka (S. issykkuli), Leuciscus bergi, Gobio lepidolaemus nikolskyi, Diptychus maculatus oschanini, Gymnodiptychus dybowskii bergianus, and Triplophysa stoliczkai elegans.

Leuciscus bergi is a near-endemic Issyk Kul dace. It has a wider distribution than L. schmidti since it is a pelagic, mostly planktivorous species. It inhabits the whole littoral zone, as well as depths down to 120-150 m. It spawns in shallow waters at depths between 1-8 m. It is a small fish (up to 175 mm) that was once the most numerous fish in the lake. It migrates from deepwater to shores in spring and back in autumn, and forms local aggregations. Its present state is not known, but there are indications that it is rarer now than before.

Phoxinus issykkulensis relictus is a relict form from the Upper Chu River. It was described from a single locality. However, there are reports that it was not found in its type-locality later. There is no other data.

Gymnodiptychus dybowskii var. primitive is only known by the original description from the Karakol and Karkara rivers of the Issyk Kul basin. There is no other data. Commonly identified as Dyptychus dybowskii.

Gymnodiptychus dybowskii lansdelli is the main form of lacustrine osmans. It has two ecological morphs: those that spawn in the lake, so-called winter osman; and those that enter rivers for spawning, so-called summer osman. In the lake it feeds on mollusks over muddy or loamy bottoms 15-30 m deep. It overwinters in holes 40-90 m deep. It migrates for spawning, and winters and forages in groups. It attains an age of 20 years. It was once the main object of fisheries, but it is now rare. The state of the population needs further study, as well as special protection.

Issyk-kul naked loach (Triplophysa "strauchii" ulacholica) is a small fish, once numerous in the shallows among water vegetation. There is no recent data.

Ecological phenomena

The Issyk Kul ecoregion can be split into two parts: a lake part and a river part. They have different ecological conditions, even though the region on the whole demonstrates obvious historical unity among the fauna. For example, the tributaries are inhabited by naked osman (Gymnodiptychus dybowskii typ.), whereas the lake is inhabited by Gymnodyptychus dybowskii lansdelli.

Vertical zonality is markedly pronounced within the rivers as well as, respectively, the vertical shift of fish assemblages. Triplophysa stoliczkai elegans is predominant in the river source of the Chu River drainage (Kara-Kudzhur, Suek, Eastern Karakol, a tributary of the Kochkorka River). Schizothorax issykkuli chuensis occurs, although seldomly, and Gymnodyptychus dybowskii bergianus occurs where the river spreads out into the valley. In the Kochkorka Valley (Dzhuvan-Aryk, Chu, Kochkorka) Schizothorax issykkuli chuensis and loaches are quite common; Phoxinus issykkulensis relictus and gray loach (Triplophysa dorsalis) inhabit small streams. In tributaries downstream from the Boomskoye Ravine (Ala-Archa, Alamedin, Kemin rivers) Triplophysa stoliczkai elegans and Gymnodyptychus dybowskii bergianus are numerous.

Justification for delineation

Lake Issyk Kul is located 1607.9 m above sea level and is one of the largest high-elevation lakes on earth (area 6236 km², length 178 km, width 60 km, depth up to 668 m). Until 1930, the native fauna was typical of the local Central Asian fish complex that originated from the Central Asian Montane Fauna (a term by Berg), characterized by the presence of the loaches of Triplophysa sensu lato and cyprinids of the subfamily Schizothoracinae, with a small addition of leuciscins of Siberian origin. Piscivorous and herbivorous ecological forms were absent.

The lake proper was inhabited by the following endemics and near-endemics: Leuciscus schmidti, Issyk-kul’ minnow (Phoxinus issykkulensis), Leuciscus bergi, Ili marinka (Schizothorax issykkuli), Gymnodiptychus dybowskii lansdelli, and Issyk-kul naked loach (Triplophysa "strauchii" dorsaloides). This ecoregion also covers the lake tributaries and upper sections of the Chu River, which have the same complex of mountainous forms (Schizothorax, Diptychus, Gymnodiptychus, Triplophysa), but lacks typical lacustrine forms (both Leuciscus species).

Level of taxonomic exploration

Poor. A lot of subspecies and eco-morphs have been described, and most of them synonymized, but the forms are probably valid. The fauna of Phoxinus, Triplophysa and Schizothoracinae urgently need revision.