Boundaries

The ecoregion includes the North Caspian Sea (as accepted in the literature), bordered in the south by a traverse from Chechen’ Island to the Tyub-Karagan Cape, and Mangyshlak Bay [Mangghystau Shyghanaghy]. The eastern part of the Precaspian [Prikaspiiskaya Nizmennost’] Lowland is also included. This area represents the former shallow bays of the Caspian Sea, which were covered by water to a greater or lesser extent until the beginning of the 20^th century; since 1930-1945 the area has dried completely. However, the coastline is again changing due to recent water level increases. The ecoregion boundary stretches from Mangyshlak Bay [Mangghystau Shyghanaghy] along the northern spurs of the Severnyy Aktau Ridge, and further north to the Western Margin ("chink") of the Ust’-Urt Plateau [Ustirt] along the coastline of the Sor Karatuley ("sor" or "shor" means a dried up, marshy bay). Mertvyy Kultuk and Karatuley sors are also included.

The ecoregion’s borders with the Volga-Ural [410], Turan Plain [450], and Aral Sea Drainages [629] are rather formal since the territory lacks water bodies (arid). The ecoregion also includes the entire Volga Delta.

Topography

The recent Caspian Sea is the largest lake in the world. It is only regarded as a sea by virtue of its size. Its water surface area is equal to 424,000 km². The level of the Caspian Sea fell after the last glacial period, and is now situated 28 m below the level of the world’s oceans. The huge basin of the Caspian Sea is morphologically split into three parts: 1) a northern shallow part (less than 10 m), separated from the middle by a line passing from the estuary of the Terek River to the Pamgyshlak Peninsula; 2) a middle part, with an average depth of 200 m and a maximum depth of 790 m; and 3) the southern and deepest section, with a maximum depth of 980 m and average depth equal to 325 m.

The system of currents of the Northern Caspian is simple, and is determined by the flow of the Volga River along the western coast and a weaker flow through the Akhtuba along the well-pronounced gutter. These two flows are separated by a shoal oriented from the Mangyshlak Peninsula to the center of the Volga Delta.

The geological history of the Caspian Lowland reveals an exceptionally dynamic landscape.

The Caspian Sea’s size varied during the Quaternary Period. During the glacial epoch the sea decreased because of the decline of flow throughout the basin. However, during the the post-glacial Khvalyn transgression its level was approximately 50 m higher than oceanic levels, or 75 m higher than current levels. At that time waters of the Caspian Sea spread far north and occupied the entire Caspian Lowland; the area of the sea was nearly two times as large as it is now.

In the very recent past the size of the Caspian Sea, particularly its northern part, changed (becoming several times smaller or larger). This naturally led to tremendous transformations of its landscapes and ecosystems, which have adapted to periodic profound changes without a considerable loss of biological and landscape diversity. This can be seen as recently as 1930 to 1945 when the water level of the Caspian Sea dropped by nearly 2 m. The huge Kaydak and Mertvyy Kultuk bays disappeared, transforming into sors (partly dried saline shoals). The water level continued to fall until 1981. During this period such transformations were regarded as catastrophic, as well as the subsequent rising of the Caspian Sea’s water level at the end of the 20th century. However, the recurrence and periodicity of these changes suggest that the biota of the Caspian Sea is well-adapted to them. In all likelihood the freshwater diversity does not suffer from these natural fluctuations; rather, only its structure changes.

The place of separation of the Buzan branch is regarded as the area where the Volga Delta begins. From there the Volga River is split into a dense network of arms and anabranches. The Volga River delta is one of the largest deltas in the region. The numerous arms, anabranches, island, and sand ridges (Baer’s hillocks) occupy an area of more than 13,000 km². Within the limits of the Volga River are a great number of lakes. These are the so-called ilmens and polois of the Volga Delta; their number totals 1000. Ilmens of the delta are relatively more resistant, and partly retain water even during dry periods, whereas polois exist only during spring floods.

Freshwater habitats

The Northern Caspian is much shallower than the Central and Southern Caspian, and contains only 1% of the Caspian’s total volume. The water here is fresh, being highly influenced by the Volga River, which contributes up to 82% of the inflow. Landscapes of the Caspian Sea are formed under the influence of sea level fluctuation, dominating currents, positive and negative setups, inflow of fresh water from river flow, and the structure of the sea floor. The system of currents determines the transfer of Volga water along the western coast, with waters of the western sector being cooled and freshened. The northern currents determine the structure of the bottom and transformations of the shore. As a result, spits and their islands are formed along the shoreline, oriented toward the south. Shallow inlets and shoals that arise are frequently overgrown with reeds. Winds producing positive and negative setups periodically transform these structures and provide for the deep mixing of waters.

In the summer waters of the Caspian Sea warm thoroughly, with temperatures near the surface reaching 25-27 °C. In the winter the sea gradually cools, retaining temperatures above zero (1 °C) throughout much of its area. Only the shallow northern part is frozen, where pack ice is formed, and ice cover is established.

The very high productivity of the Northern Caspian is noteworthy, because it comprises 0.5% of the total volume of the sea and produces about 20% of its organic matter. The areas of high production in the Northern Caspian belong mostly to landscape ecotones in both bottom structure and gradient of salinity. It is also evident that the basis of biological production includes elements of mineral feeding associated with river flow.

Description of endemic fishes

The ecoregion contains one strict endemic species, Agrakhan shad (Alosa sphaerocephala), and one near-endemic genus, Hyrcanogobius. Near-endemic species to the Caspian Sea (with ecoregion 452) include Neogobius bathybius, Caspian goby (N. caspius), Caspian marine shad (Alosa braschnikowi), Saposhnikovi shad (A. saposchnikowii), granular pugolovka (Benthophilus granulosus), B. kessleri, B. leobergius, B. mahmudbejovi, Hyrcanogobius bergi, and Mesogobius nonultimus.

Agrakhan shad (Alosa sphaerocephala) is clearly distinguishable from other shads of the basin by its deep head and body, and large eyes. It is only known from the Northern Caspian, and never enters rivers. However, it may migrate to the Central Caspian in winter, although it is not recorded. Its status and distribution need further studies, but the species seems to be very rare at present (not reported in recent literature).

Other noteworthy fishes

The genus Hyrcanogobius may be synonymous with Knipowitschia (Kottelat, Freyhof, in press), but differs from the latter by its different organization of rows of cephalic sensory pores. Hyrcanogobius bergi is a small-sized goby (up to 32 mm SL) that is distributed in the Northern and southeastern Caspian Sea and along the western coast southward to Daghestan, the Ural and Volga deltas, and lakes in the lower reaches of the Atrek River. It is a near-pelagic coastal marine species; it is found in areas with different salinities, including freshwater. It lives for one year; adults probably die soon after spawning. It feeds on small crustaceans.

Neogobius bathybius is a rare species only known from the Northern Caspian Sea. It differs from other gobies by its slender body and head, and grey monotonous coloration. Its biology is poorly known.

Caspian goby (Neogobius caspius) is a typical marine species that avoids areas freshened by river discharge. It differs from other gobies by its monotonous brown coloration and the most anterior position of both its anterior and posterior nostrils. It is a rare species, but occurs all over the Caspian Sea except for the deepest areas of the Central and Southern Caspian.

Caspian marine shad (Alosa braschniowi) is a polymorphic species that consists of eight “subspecies”, forms, or varieties. The species is distributed all over the Caspian Sea (except for completely freshwater areas), but some subspecies/forms are limited by their restricted ranges, while others migrate from the Southern to the Northern Caspian for spawning, and then back to the Southern Caspian for foraging and wintering. This shad is a predatory large-sized fish (length up to 470 mm, weight up to 1100 g) with well-developed teeth. During migrations and at spawning sites it forms large aggregations. It does not eat during spawning, which lasts 60-70 days. The Caspian marine shad was an important object of commercial fisheries. Its abundance strongly depends on the main object of the diet, kil’ka (Clupeonella), which depends on zooplankton.

Saposhnikovi shad (Alosa saposchnikowii) is a small-sized species that belongs to the group of predatory marine Caspian shads. It winters in the Central and Southern Caspian at depths over 30 m, and starts the run to the Northern Caspian in early spring. It never enters rivers. Most stocks of this shad spawn in the eastern part of the Northern Caspian: east and west from the Ural River mouth, at Buzachi Peninsula, in Saratysh Bay. Spawning takes place at 14-17 °C until the middle of May. In autumn, both yearlings and adults migrate back to the Central Caspian. This species feeds mostly on fishes, and also crustaceans.

Granular pugolovka (Benthophilus granulosus) is a small-size (less than 66 mm SL) goby. As all its congeners, it is a bottom dweller. It lives about a year, reaching maturity during the spawning season following its hatching. Some individuals can attain maturity at an age of 6-7 months. All individuals die after spawning, with females dying earlier than males. All tadpole-gobies are characterized by a lack of scales, and a body covered with smaller and/or larger rough bony elements that disappear in mature males by the spawning season. Granular pugolovka differs by having numerous, dense and small bony granules covering the entire body, but absent large, thorny platelets. It inhabits the sea along all coasts; is common along the northern and eastern coasts and at river mouths, especially around the Ural and Volga rivers, as well as the Volga Delta up to Astrakhan. During the warm season it inhabits shallow coastal waters and shallow channels of the delta. In winter, it migrates to deeper areas (up to 60-70 m). It feeds mostly on crustaceans.

Benthophilus kessleri is a species that may be rare in the Northern Caspian (only a single record is known). See ecoregion 452.

Benthophilus leobergius, compared to B. granulosus, has few, sparse small bony granules and large, thorny platelets on its head and body. It is a larger-size goby (up to 135 mm total length), which is commonly sympatric with B. granulosus in the Caspian Sea along most of its coast. It is most common in the northern regions and the Volga Delta.

Benthophilus mahmudbejovi, a small-spine tadpole-goby, is distributed along most costs of the Caspian Sea at depths between 3-15 m in the North Caspian, and up to 50 m in the Middle and South Caspian, at salinities up to 13 ⁰/_00.  It is especially numerous in the Volga Delta. It is a small-sized goby (up to 66 mm SL) distinguishable from its congeners by a markedly depressed head, small, dense bony granules covering the whole body, and relatively small, thorny platelets.

Mesogobius nonultimus is a poorly known species, with only a few specimens recorded in the Northern and southeastern Caspian Sea.

Ecological phenomena

Spawning areas. The Volga River in its lower reaches is an important migration route of anadromous and semi-anadromous fishes. The delta and the adjoining avandelta are areas of concentration of semi-anadromous fishes during spawning.

Feeding migration areas. Floodplain areas adjoining the deltas of the Volga and Ural are feeding migration areas for both semi-anadromous fishes and for floodplain river fishes. Feeding migration areas are thoroughly warmed shallow areas with depths up to 10 m.

Wintering area. Volga populations of sturgeons concentrate in the autumn winter period along the western shore of the region. Some of the semi-anadromous species winter in the delta of the Volga River where there are several dozens of wintering dips.

Justification for delineation

This ecoregion is distinguished by its developed river delta, which has an infinite number of branches, channels, and shallows. Biological productivity is also remarkably high. This ecoregion contains the main spawning and foraging sites for around 60 species of semi-anadromous and anadromous fishes. The first group inhabits the delta and the Northern Caspian and performs short spawning migrations upstream. It then undergoes pre-wintering and feeding migrations back to the sea or downstream in the delta. The second group of true anadromous fishes includes species from Petromyzontidae, Acipenseridae, Clupeidae, Salmonidae, and Cyprinidae.