Boundaries

This ecoregion includes the Central and Southern Caspian Sea as defined by the geomorphology of the sea bed, depth, and salinity. The border with the Volga Delta-Northern Caspian Drainages ecoregion [453] or North Caspian Sea (as accepted in the literature) follows the traverse from Chechen’ Island to the Tyub-Karagan Cape of the Mangyshlak [Mangghyshlaq] Peninsula.

Topography

The present 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 424,000 km². The level of the Caspian Sea fell after the last glacial period and now lies 28 m below the level of the world’s oceans. The enormous Caspian Sea basin is morphologically split into three parts: 1) the northern shallow part (less than 10 m), 2) central 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 up to 980 m and average depth equal to 325 m. Thus, the Central and Southern Caspian are naturally separated from the shallow Northern Caspian. 

The continental slope, shelf, and the bottom of hollows are prominent in the deep parts of the sea in the Central and Southern Caspian, and submarine ridges are prominent in the Southern Caspian. In terms of geomorphology the Middle Caspian is a hollow, bounded by the Mangyshlak Rift in the north and by the Apsheron Rift in the south. According to some authors, the deepwater part of the Caspian is a combination of three hollows. The deepest is the Derbent hollow with a flat bottom that is slightly inclined to the southwest.

The shelf is the narrowest (up to 11 km) and the continental slope is the steepest (up to 1°) around Derbent and Divichi. The shelf expands to the south, and the continental slope becomes declivous. Tectonic uplifts in the form of banks and islands line the shelf around the Apsheron Peninsula. The average depth of the Middle Caspian is 215 m. The shelf occupies 56% of the area of the Middle Caspian; its edge lies at approximately 100 m in depth.

The Southern Caspian is separated from the Middle Caspian by the Apsheron Rift. Forty-six percent of its area lies on the shelf at depths up to 100 m. The shelf of the western coast south of the Apsheron Peninsula has many banks, islands, and mud volcanos. The most dissected part of the eastern shelf is the one adjoining Cheleken. South of Cheleken is Ogurchinskii Island, Ulski Bank, and Gryaznyi volcano. The continental slope in the South Caspian is very steep, with the eastern slope running deeper and wider than the western slope.

The deepwater South Caspian hollow is bounded by depths of 800—900 m: in the north it appears to project between two submarine ridges. To the south the undulations gradually disappear and the bottom levels off over most of the hollow. Between the hollow and the Apsheron Rift are two ridges that extend in a south-southwest direction, and reach 200—250 m above the water level.

Freshwater habitats

The salinity of the Caspain Sea varies spatially. It increases from north to south and from west to east, which is related to freshening and evaporation. Maximum salinity occurs in shallow bays and coves of the eastern coast, reaching 14-14.15 ‰. In the open parts of the sea salinity reaches 12.8-12.9 ‰. Mean values of salinity grow slightly with depth.

The Caspian Sea is regarded as a closed basin; however, this is not completely true. In fact, it has a permanent flow into the Kara Bogaz Gol Bay, the level of which is lower than the level of the Caspian Sea by 0.5—1.0 m. The Kara Bogaz Gol is separated from the sea by a narrow sand spit, resulting in a strait with a width of up to 200 m in some places. Through this strait waters flow from the Caspian Sea into the bay (more than 20 km³ per year), which plays the role of a giant evaporator. The salinity in the Kara Bogaz Gol Bay is thus extremely high (169‰).

During the summer the Caspian Sea is thoroughly warmed, with water temperatures at the surface reaching 25-27°. In winter the sea cools slowly, retaining a positive temperature (1°) over most of its area. In the middle and southern part of the sea ice is absent.

Terrestrial habitats

From the base of the Agrakhan Peninsula a recessed coast stretches south—southeast to the Apsheron Peninsula. The Apsheron Peninsula projects eastward into the sea; its eastern extremity is bordered by banks and small islands of the Apsheron Archipelago (Zhiloi, Pirallakhi, Neftyanyye Kamni, etc.). Further south are branches of the Caucasus separated from the sea by a narrow lowland plain that merges with the extensive Kura—Araks plain in the south. The coast there is low and swampy. North of the Kura River estuary it is level. A large number of banks, islands (Baku Archipelago), and mud volcanoes line the coast on the continental slope. Further south of the Kura River estuary the coast is characterized by spits (Kura Sara) and shallow bays. The largest of these is Kyzylagach Bay.

From Gorgan Bay the coast turns abruptly northward. Up to 39°N it is shallow, even, and low, like the adjacent alluvial plain that was formed by ancient river deposits. Further north the coast remains shallow and low, but is interrupted by an area of large bays and coves. Cheleken Island projects into the sea. South of it lies the extensive Turkmenian Bay, which is separated from the sea by the South Cheleken Spit and the sandy Ogurchinskii Peninsula. North of the Cheleken lies the shallow Krasnovodsky Bay that is separated from the sea by the Northern Cheleken Spit and the Krasnovodskaya Spit. The coastline around the bay is strongly indented and low, except for the north where the sea closely approaches the bluff of the Krasnovodskoye Plateau.

A split separates the Caspian Sea from its largest bay, Kara Bogaz Gol (12,000 km² in area and a depth of up to 10 m). Its bottom is nearly flat due to intensive accumulation.

North of Bekdash Cape the western bench of the plateau comes close to the sea. The coast is prone, recessed throughout its length from Bekdash Cape up to Mangyshlak Bay. The coastline in that area is uneven. North of Kazakh Bay a number of capes protrude into the sea, such as Rakushechnyi Cape, Peschanyi Cape, etc. The Tyub-Karagan Peninsula lies at the northwestern extremity of Mangyshlak, and extends far into the sea.

Description of endemic fishes

The ecoregion contains many more endemic taxa than the Volga Delta-Northern Caspian Drainages ecoregion [453], including dockbill pugolovka (Anatirostrum profundorum), an endemic genus. Other endemic species include Southern Caspian sprat (Clupeonella grimmi), anchovy sprat (Clupeonella engrauliformis), Benthophiloides turcomanus, Benthophilus ctenolepidus, B. grimmi, B. leptocephalus, short-snout pugolovka (B. leptorhynchus), B. pinxhuki, B. ragimovi, spiny pugolovka (B. spinosus), B. svetovidovi, and Mesogobius nigronotatus. Benthophilus casachicus, Caspian marine shad (Alosa braschnikowi), Saposhnikovi shad (A. saposchnikowii), granular pugolovka (B. granulosus), B. kessleri, B. leobergius, B. mahmudbejovi, Hyrcanogobius bergi, and Mesogobius nonultimus are endemic to the Caspian Sea (near-endemic to ecoregions 452 and 453).

The Southern Caspian sprat (Clupeonella grimmi) is the most deepwater species among all tyul’kas (Clupeonella). It does not migrate to shallows of the Northern Caspian and never enters rivers. It inhabits depths greater than 20 m. Most stocks prefer depths from 70 to 250 m with constant salinity between 12-14‰. However, within this range it migrates to the north in summer and back to the South Caspian in autumn and winter. In contrast to other kilkas, pronounced daily migrations are typical for this species - it often migrates up to surface layers from depths over 100 m, but always avoids waters with temperatures over 14 °C.

Endemic gobiids from the genera Benthophilus, Asra, and Anatirostrum represent relict forms of the ancient marine deepwater fauna originiating with the early stages of Paratethys history.

Asra is considered a synonym of Benthophiloides by Miller (2004). It is represented by a local endemic, Asra turcomanus, which is known only from the southeastern part of the sea. Only few specimens are reliably known. Anatirostrum is a genus close to Benthophilus and represents one of the ancient forms of scaleless gobies that evolved in the Paratethis. Anatirostrum profundorum is only recorded in the southeastern Caspian region and is characterized by an elongated head and snout and naked body. It is known to occur from 290 to 600 m.

Species within the ecoregion of the genus Benthophilus are all tadpole-gobies that belong to the ancient Paratethis fauna. Almost all of them are known from deepwater localities. Benthophilus casachicus is a local endemic species from the eastern coast between Peschanyy Cape and Ogurchinskogo Island at depths ranging from 15-75 m. There are only 20 reliably known specimens of this species. Benthophilus ctenolepidus is a comparatively shallow-water species. It spawns and forages during the warm season at depths from 0.5 to 10 m, and migrates to 20-25 m in winter. It only occurs at a salinity between 12.4-13.0‰. It is a relatively widespread tadpole-goby. Benthophilus grimmi is a rare deepwater species restricted to the western Central Caspian Sea from Chechen’ Island to south of Apsheron Peninsula. Only a few specimens are known from deep waters of the south Central Caspian Sea at depths up to 200 m, although the species probably prefers more shallow (less than 100 m) waters. Benthophilus leptocephalus is a rare deepwater species that occurs more often on the western coast from Samur to Apsheron, and southeastern coast from Krasnovodsk to Gasankuli. It prefers depths between 50-300 m and salinity between 12-13‰. Benthophilus leptorhynchus is a very small (total length up to 46 mm) goby. It is a  rare deepwater species that is only known from the western coast from Sulak to Kilyazi at depths ranging from 50-128 m. Benthophilus pinxhuki is a comparatively large (up to 104 mm) rare deepwater species. It was recorded at depths up to 300 m on the eastern coast of the Southern Caspian from Ogurchinskiy Island down to Gasan-kuli and, rarely, along the eastern coast from Apsheron down to the Iranian coast. Benthophilus ragimovi is a deepwater species (30-200 m) that has a local distribution on the western coast from Chechen’ Island to Astara. It is known to approach shallows (10-20 m) for spawning. Benthophilus spinosus is a small-sized goby (total length up to 50 mm). It inhabits waters off both coasts of the Central Caspian Sea. Benthophilus svetovidovi is a rare local endemic species only known from eastern Central Caspian.

Mesogobius nigronotatus is only known from one specimen off Mangyshlak (eastern Central Caspian). It may be a synonym of Mesogobius nonultimus.

Other noteworthy fishes

Benthophilus kessleri is the most abundant species among tadpole-gobies along the coast of the eastern Caspian between Urdyuk Cape (44⁰ 20’N) and Kuuli Cape (40⁰ 10’N). It has only been reported from two localities in the Northern Caspian (46⁰ 07’N, 50⁰ 10’E; 44⁰ 40’N, 50⁰ 10’E). It prefers 11.5-13.2‰ salinity, and depths between 25-75 m. It feeds mostly on Dreissena.

Anchovy sprat (Clupeonella engrauliformis) was once the most abundant fish of the Caspian Sea, and was the primary food source for predatory shads, sturgeons, and Caspian seal. It inhabits the Central and Southern Caspian Sea and enters deep waters of the Northern Caspian in summer. It avoids coastal regions and depths less than 10 m. In winter, it forms aggregations at depths from 50 to 750 m, with maximal concentrations in layers with temperature between 8-13 °C. In spring and summer the species actively migrates and forages throughout the whole Central Caspian. Concentrations of this tyul’ka were once large and wide, covering the whole central part of the sea delimited by the circle current with temperatures between 7-20 °C, at depths of 15 to 50 m. In September-October, the tyul’ka migrated back to the Southern Caspian. However, the anchovy sprat has experienced a catastrophic decline, similar to the Southern Caspian sprat (C. grimmi) (see Endemic Fishes section).

The ecoregion also contains two endemic subspecies, Alosa caspia knipowitschi and Alosa caspia persica.

Ecological phenomena

The Southern and Central Caspian are the main forage areas for many groups of fishes that either migrate to the Northern Caspian for spawning (some clupeids, tyul’kas) or move northward to enter rivers of the northern coast (acipenserids, anadromous shads). This is particularly important for supporting the entire complex ecosystem of the Caspian basin.

The productivity level of the Caspian Sea in the recent past has declined considerably. For example, as compared to the period from 1936-1940, the total phytoplankton biomass declined nearly 3-fold by 2002. The changes in the productivity level in the Middle and Southern Caspian have been accompanied by structural shifts in the composition of phytocenoses. Thus, over the past 70 years, biomass of the phytoplankton species of Exuviella (a dinoflagellate), its uptake by consumers being the highest, declined in the Middle Caspian 85-fold. The decline in population levels of producers also affected consumers of the first order (zooplankton and mollusks). Thus, over the past 70 years the food supply of semi-anadromous fishes declined by nearly 40%, comprising 1,732,000 tons as compared to 2,679,000 tons in the 1930s. Catches of semi-anadromous fishes declined from 299,000 to 8,000 tons (37-fold), comprising only about 3% of catches compared to the 1930s. The general decline of food productivity of the Caspian Sea also affected sturgeons.

Justification for delineation

The Central and Southern Caspian are naturally separated from the shallow Northern Caspian. Central Caspian is a middle part with an average depth of 200 m and a maximum depth of 790 m. The Southern Caspian is the deepest depression with a maximum depth up to 980 m and average depth of 325 m.

The fish fauna contains mostly brackish water forms, both migratory and residential (inhabiting only the sea). The number of native species is slightly less than in the Northern Caspian. However, the ecoregion contains a relatively high number of endemic taxa, including species from Clupeonella,  Benthophilus, Mesogobius, and Alosa. The ecoregion also includes endemic gobiids from the genera Benthophilus, Asra, and Anatirostrum that represent relict forms of the ancient marine deep water fauna connected in its origin with early stages of Paratethis history.

Level of taxonomic exploration

Fair