Boundaries

The ecoregion includes the system of Lake Ubsa Nor [Uvs Nuur = Ubso Noor] and its tributaries (the largest one is the Tess River [Tesiyn-Gol]); water systems historically connected with this drainage in Tuva and Mongolia; Lake Ureg Nur [Uureg Nuur]; drainage areas of the Kobdo [Hovd Gol] and Dzavhan Gol [Dzabkhyn] rivers; and rivers and lakes of the central Mongolian endorheic basins located between the Hang Kai [Hangayn Nuruu] Range and Gobiiskiy Altay Range [Govi Altayn Nuruu]. In the northwest the southern edge of the Ukok Plateau and the Saylyugem Range [Siylugemiyn Nuruu] divides the ecoregion from the Chuya ecoregion [604] (Chuya and Katun’ headwaters). The Chrebet Chikhachova Range separates Lake Kyndyktykul [Khindiktig-Khol] and Lake Dzhulu-Kul (in ecoregion 604). Further east the northern border follows the Zapadnyy Tannu-Ola, Vostochnyy Tannu-Ola, and Sangilen ranges. The latter divides the Tesiyn Gol drainage from the Malyy Yenisei [Ka Hem] drainage (ecoregion 605) in the north. Between the Bulnain Nuruu Range in the north and the Tarvagatayn Nuruu [Tarbagatin Ondor Uul] Range in the south, the eastern border of the ecoregion is indented by the Ider Gol River drainage (Selenga basin in ecoregion 605). Further east the ecoregion is contiguous with the Inner Mongolia Endorheic Basins [621]. In the south, the ecoregion border with the Dzungaria ecoregion [623] follows the Gobiiskiy Altay Range [Govi Altayn Nuruu], and further in the northwestward, the Mongolian Altai Range, which divides the Kobdo [Hovd Gol] headwaters from the headwaters of the Upper Irtysh [603].

Topography

The Central Asian Internal Basin (Northwest Mongolia), a comparatively small area, is geomorphologically subdivided into the West Mongolian Great Lakes Valley and Gobi Valley Lakes (or Lake Valley). The basin consists predominantly of large lenthic water masses without outlets beyond the basin. They are fed by relatively short upland streams or rivers that drain the southern slopes of Tanny-Ula and the Hangayin (Hangay) and northen slopes of the Mongolian Altai Mountains.

Freshwater habitats

The ecoregion includes the mountain lakes of the Mongolian Altai, water bodies of the Gobi Valley, and some lakes of the Hangayn Plateau. The largest Mongolian lakes are situated in the Gobi Valley, in spite of a very low annual rainfall of only about 100 mm, and a high evaporation rate of 900-1000 mm. This region, bordered in the north by the Mongolian Altai Mountains, is endorheic (i.e. it has only internal drainages not connected to oceans or inland seas). There are three lake complexes within the Great Lakes Valley: the Altai mountain lakes, the Hangayn Plateau lakes, and the Great Lakes.

Altai mountain lakes are situated in valleys of mountain ranges of the Mongolian Altai, Tannu Ola, and Hangayn. Of the large lakes Hoton, Horgon, Dayan, Uureg, Achit, Tolbo, Uvs, and Hongor-Ulen, all but two are of a flow-through freshwater type. The saline lakes Uvs (20 m deep) and Uureg (27 m deep) are rich in fish.

The Hangayn Plateau lakes are located northwest of the Hangayn Plateau, 1700-2000 m above sea level. The largest lakes are Hangayn-Har, Ulaagchnyi-Har, Heh, Oigon, Telmen, and Sangiyn Dalai. The first four are freshwater lakes with a maximum depth of 50 m. Rotifers and copepods dominate the zooplankton, with an average annual biomass of 2.8 g m^-3. The other lakes are saline and have no outflow.

The complex of Great Lakes includes lakes Hyargas, Ayrag, Har, Har Us, and Durgun. The flow-through freshwater lakes Har and Har Us have an average depth of 2-6 m. Hyargas, Durgun, Uureg, and some other lakes are saline and have no outflow. Their maximum depth is 80 m. All these lakes have a very similar hydrobiology. Both the freshwater and saline lakes contain fish.

The lake region of the Gobi basin has over 20 lakes. They are not large and most of them dry up periodically. The largest of these temporary lakes are the saline lakes Boon Tsagaan, Orog Nuur, and Ulaan, situated between the Hangayn Plateau and Gobi Altai. All water bodies are shallow (3-5 m depth).

Some lakes dry out, especially in the Valley of the Lakes in the Central Asian Basin. The gradual dessication of lakes since 1978 has resulted in a virtual disappearance of lakes Orog and Taitsin Tsagaan. Lake Orog Nuur covers 140 km² and has an average depth of 3 m when full. This lake completely dried out during 1988-1989. Lake Taitsin Tsagaan also completely dried out during the same period, and stayed dry for a number of years. Lake Ulan Nuur, the easternmost lake in Lake Valley, also periodically dries out.

Terrestrial habitats

Most of the ecoregion is covered by Great Lakes Basin desert steppe and Gobi Lakes Valley desert steppe, with vegetation that includes semi-shrubs, shrubs, and some grasses. Other terrestrial ecoregions include Selenge-Orkhon forest steppe, Altai montane forest and forest steppe, Khangai Mountains alpine meadow, Eastern Gobi desert steppe, Alashan Plateau semi-desert, and Mongolian-Manchurian grassland. There are also scattered pockets of Altai alpine meadow and tundra, Sayan montane conifer forests, Sayan intermontane steppe, and Sayan alpine meadows and tundra.

Description of endemic fishes

Narrow-headed Altai osman (Oreoleuciscus angusticephalus) is endemic to lakes in the Hovd River system. This species is a large predatory osman. It is commonly misidentified or mixed up with the Altai osman (O. potanini). The genus Oreoleuciscus needs a deep revision.

Triplophysa chandagaitensis, T. compressirostris, and T. gundriseri are locally distributed. There is no data on abundance and biology. The Uvs Nuur basin may support more undescribed species of Triplophysa. A special study is needed.

Mongolian grayling (Thymallus brevirostris) is a Tertiary relict. In summer Mongolian grayling is present mainly in mountain rivers and lakes. In mountain lakes the fish is scattered throughout the water body and at all depths. In autumn, with declining water temperature, it starts to migrate from mountain rivers to mountain lakes. In the Hovd catchment the species migrates downstream to overwinter in the deep water layers of Lake Hovd and in lakes Har Us and Har. The fish may reach an age of 17 years but in other lakes and in rivers they seldom live longer than 8 to 9 years. The oldest fish are about 700 mm long and weigh up to 4 kg. Mongolian grayling is omnivorous, and occasionally predatory. It also feeds during the spawning season. It is still quite abundant and is a commercial fish, although it has been artificially stocked.

Other noteworthy fishes

Altai osmans (genus Oreoleuciscus) are widespread in landlocked water bodies of the Central Asian Internal Basin of Mongolia. This basin has an oval shape, extending from the southeast to the northwest, and is almost completely surrounded by mountain ranges. Its northeastern boundary is formed by Lake Sangiyn Dalai, which separates it from the catchment of the Selenga River. Lake Uvs is the northernmost site where Altai osman (O. potanini) is found. The Gobi Altai, also called Mongolian Altai, forms the southern and southwestern boundary of the area. It separates the area from the Irtysh River catchment. Altai osmans in Mongolia are limited to the northwestern part of the Central Asian Plateau, at altitudes from 1000 to 2238 m above sea level; outside this area they are present only in the upper catchment of the Ob River, outside Mongolia. The swampy nature of highland streams, which in some places may still connect (or in the past connected) the two catchments, could possibly have assisted the fish in crossing the divide.

Ecological phenomena

There are many lakes only populated by Altaian dwarf osman (O. humilis). The species demonstrates a wide range of adaptations, forming a number of "eco-morphs" - from a dwarf form to a large-sized cannibalistic one. The dwarf osman has a slim body that reaches a length of 138-200 mm (mean length of 140 mm), and a weight of 7 to 25 g. Dwarf osman matures when it is around 6 years old and about 75 mm long. It feeds mostly on algae and zooplankton. The so-called bigmouth osman lives in saline lakes of the Great Lakes Valley, such as lakes Hyargas, Durgun, and Uureg. It grows to 1000 mm in length, but the usual maximum size is 700-800 mm in length and 8 to 10 kg in weight. It reaches over 40 years of age. Bigmouth osman is an omnivore. Where the food supply is poor, bigmouth osman feeds on a diversity of food, and cannibalism is common.

Justification for delineation

The ecoregion includes catchments of the Hovd and Dzavhan rivers and lakes Hoton, Horgon, Horomdog, Dayan, Tolbo, Hongor-Ulen, Har Us, Har Nuur, Ayrag, and Heh Nuur. Rivers that flow into the Great Lakes Valley are remnants of an enormous water body that existed in this area during the Tertiary. They have a poor number of fish species, although the reasons for this are not clearly known. Only a few ancestors speciated in the region - only Triplophysa and Oreoleuciscus are represented by groups of fishes. Nearly 75% of species are endemic. This situation is absolutely unique.

Level of taxonomic exploration

Fair