Order: Pinnipedia Illiger, 1811
Genus: Phoca Linnaeus, 1758
Subgenus: Phoca Scoroli, 1777
Common names: Russian: tyulen, nerpa; Kazakh: balak; Turkmenian: duvlen; Azerbaijani: su iti; English: seal
The Caspian seal is one of the smallest representatives of the family Phocidae. Males and females are nearly of the same size: maximum length is 160 cm, maximum weight during the period of the best sate of nourishment ranges from 90 to 100 kg. Sexual dimorphism is poorly manifested. Numerous
dark irregular, often merging spots are typical of adult animals. They are sparse or completely absent on the sides and belly.
Dental formula: i 3/2 c 1/1 p 4/4 m 1/1=34. There are 32 chromosomes in the diploid chromosome set.
Intraspecific forms. None
Related forms. The genus Phoca Linnaeus, apart from the Caspian seal, includes Baikal seal Phoca(Pusa) sibirica Gmelin, 1788; and ringed seal Phoca (Pusa)hispida Schreber, 1775. Ringed seal has circumpolar distribution within peripheral seas of the Arctic basin, in the seas of the Arctic and subArctic Atlantic, in the far northern and north-eastern parts of the Pacific Ocean, Baltic Sea, Ladoga Lake and Lake System of Saimaa. The geographic range of Baikal seal is restricted to the Baikal Lake.
The Caspian seal occurs throughout the Caspian Sea from the coastal areas of the Northern Caspian to the Iranian coast. In winter, early spring and late autumn, the greatest part of the population congregates in the Northern Caspian. In autumn seals enter the mouths of the Volga and Ural Rivers, in late spring, summer and early autumn they occur in the Middle and Southern Caspian.
Status as per International Red Data Book. Not defined
Status as per National Red Data Books. Not defined
First Record for the Caspian Sea. The species was described during Academic expeditions undertaken in the second half of the 18th century and identified as a local form of the common seal Phoca vitulina s. caspica Gmelin J.Fr. (Linnaeus Systema Naturae, Edit. XIII, 1788: 34)
Redescription of species. Phoca canina var. caspica Pallas P. (1811-1831, Zoographia Rosso-Asiatica, I: 116), Phoca caspica (1875, V. Yakovlev. Sealing in the Caspian Sea. In: “Nature”, 2: 1-35).
Ecological group. Marine mammal, semi-aquatic species
It reproduces and rests on a solid substrate (ice, islands), its feeding and migration occur in the sea.
Origin. Caspian endemic
There are two basic hypotheses that explain the origin of the species. According the first, seals colonized the Caspian Sea during the Quaternary period entering it from the north under the influence of the advancing glacier. Another hypothesis accounting for the autochthonous origin of the species is based on paleontology findings referred to Miocene and Pliocene. The Caspian seal is a descendant of seals of the genus Pusa that inhabited the Sarmat, Maeotic, Pontic and other basins that alternated each other successively and eventually became the Caspian Sea.
Distribution. The Caspian seal is an endemic of the Caspian Sea. It is distributed throughout the sea from avant-deltas of the Volga and Ural Rivers to Iranian coasts. Some animals occurred sporadically in the Volga River as far as Volgograd and 200 km upstream the Ural River.
Biotope. Similar to all semi-aquatic animals, areas of reproduction and feeding of the Caspian seal are distinctly differentiated. The Caspian seal is biologically bound to the ice where it breeds, nurses pups and molts. The location of breeding-grounds depends on ice area and may change depending on freezing conditions in the western and eastern part of the Northern Caspian. The Caspian seal also uses some areas of land (flooded islands, sand and shell islands, sea stony ridges) for rookeries. It occurs in shallow and deep-water areas of the Caspian basin. The feeding area is annually correlated by distribution pattern of food organisms (kilka).
Migrations. The Caspian seal undertakes annual migrations from north to south in late spring and backward from south to north in early autumn. Animals migrate mainly in sparse small groups. It may be suggested that there are no fixed routes of their migration.
Relation to salinity. Marine euryhaline species
Relation to temperature. Poikilothermal animal, eurythermic species
It prefers moderately cold water with surface water temperature not more than 200C. It can resist water heating up to 25-300C in summer. During winter it feels quite well at temperature below 00C.
Vertical distribution. Animals move in the surface water layers. They occur both in the shallow water areas of the Northern Caspian and in deep waters of the Middle and Southern Caspian (100 m isobath) diving as deep as 80 m in search for prey.
Relation to oxygen conditions. As seal is a representative of mammals (lung respiration), oxygen regime of the water body is not important for this species.
Relation to fluctuations of the sea level. An increase in depth because of sea level rise in recent years has affected the choice of temporary shelters (ice and insular habitats) by seals. Insular rookeries in the Southern Caspian were significantly reduced. Flooded islands in the eastern shallow water area of the Northern Caspian used by a greater part of the population sometimes changed their configuration and shifted due to fluctuations in seawater level. At that, in the years of maximum decrease in seawater level flooded islands were overgrown with reeds which reduced the area of rookeries.
As the depths at former traditional areas of seal winter rookeries (Rakushechnaya, Bolshaya Zhemchuzhnaya and Kulalinskaya Banks) increased, which led to adverse environmental impact on pups’ spots, the area of seals shifted gradually to north-eastern part of the Northern Caspian where the depth does not currently exceed 5 m.
Feeding type. The Caspian seal is an ichthyophagous predator. Its diet is based on schooling fish species of
a short life cycle, mainly kilka. Crustaceans amount to 1% of the food consumed by the entire population of Caspian seal.
Feeding behavior. Because of some peculiarities of their teeth, pinnipeds can ingest a whole prey (fish). When prey is captured in nets or seines, Caspian seal can tear and swallow it in pieces. During kilka fishing season in the Southern Caspian, seals use man-made congregations of kilka (the principle of a “set table”).
Supply of food. Food supply is mostly maintained by mass species with a short life cycle: kilka (g. Clupeonella), sandsmelt (A.b. caspia), gobies (Gobiidae). The use of C. engrauliformis and C. grimmi is limited to the Middle and Southern Caspian, while other diet components may occur in the Northern Caspian as well. The proportion of ordinary fish (“chastik”) in the stomach content of the seal is rather large because of their active feeding in the Volga River avant-delta during autumn.
Food requirements of Caspian seal population
|Food organisms||Thousand tons||%|
|Clupeonella engrauliformis (Borodin, 1904)||169.7||57.0|
|Clupeonella cultriventris caspia (Svetovidov, 1941)||63.7||21.4|
|Clupeonella grimmi (Kessler, 1877)||10.1||3.4|
|Rutilus rutilus caspicus (Jakowlew, 1870)||12.6||4.2|
|Atherina boyeri caspia(Eichwald, 1838)||9.8||3.3|
|Stizostedion lucioperca(Linnaeus, 1758)||2.1||0.7|
|Other fish species||3.3||1.1|
|Caspiomyzon wagneri (Kessler, 1870)||0.2||0.1|
Quantitative characteristics of feeding. The annual amount of food for the population of 420, 000 seals is calculated as 297, 700 tons of fish, of which 90% is consumed during the feeding season in the Middle and Southern Caspian. A daily ration of one-summer-old animal varies from 1.2 to 1.8 kg (average 1.4 kg), that of immature seal - 3.2 kg (1.2 kg per intake). A mature male eats some 1.7 kg of food per feeding, 4.5 kg per day. Seals feed most actively in the morning and in the evening, but no distinct pattern in diurnal feeding activity was revealed (probably due to their dependence on densities and diurnal cycle of food organisms).
Reproduction type. Sexual
There is no strict monogamy in seals. A female chooses one of three or four competing males and mates in water not more than two-three times.
Reproduction areas. Eastern and central areas of the Northern Caspian.
Terms of reproduction. Under favorable conditions pupping begins at the end of January and ends in early February. Under unfavorable conditions in winter (above-zero temperatures, thin ice, etc.) pupping season somewhat extends - so white-coats may occur through the period of the last ten days of February - early March. The present barrenness level is estimated to be 50-70%, the reproductive interval is two-three years.
Fecundity. The Caspian seal gives birth to one pup only. Birth of twins is very rare.
Limiting factors. Insufficient food supply, environmental pollution, intensive navigation, sea level fluctuations, climatic factors (the most important - winter severity).
Life history stages.
Relation to environmental factors. The most important abiotic factor
to the biology of the species is the area of ice in the northern part of the sea where breeding occurs. Winds are dangerous because they facilitate ice
removing and its further destruction.
Age of maturity. Seals become sexually mature at the age of 6-7 years.
Thermal conditions of development. Ice-dependent (pagetodous) species
Breeding occurs in winter on the ice of the Northern Caspian
Quantitative characteristics of growth. may be defined by the equation
Y – size, cm
X – age, years
A – distance between the upper (A-C) and lower asymptotes, cm
C - lower asymptote.
Sex ratio. Male : female ratio is close to 1:1. Prior to 1967, when selective hunting of females was conducted, sex ratio was distorted.
Age- size structure. Estimation of age of the Caspian seal became possible since 1960-s, when the method of pinniped age determination by dentin and cement lamination of teeth was introduced. At present, the age structure of Caspian seal population consists of different generations up to 50 years old. The mean population age is about 19 years.
Quantitative characteristics. The population of Caspian seals accounted for 420, 000 specimens by the beginning of 2001, its biomass was 15 200 tons.
Population trends. In the XX-th century, the number of the Caspian seal population decreased by a factor of 2.5, from 1 million to 400, 000 individuals. Prior to sealing regulation introduced in 1970, the main factor affecting the Caspian seal population was unregulated hunting. During the last 25 years, population abundance was affected by anthropogenic factors, e.g., pollution. Currently, the population of the Caspian seal is in a strained state. Negative processes in the population are diagnosed as “cumulative toxicosis”. In recent years, because of some infectious diseases, there have been noted cases of mass seal mortality (1997, 1998, and 2000).
The Caspian seal as a fish predator is at the top of trophic chains in the Caspian Sea ecosystem. The only competitor in its trophic niche is supposed to be beluga sturgeon, though qualitative and quantitative characteristics of the competition are not currently ascertained. Alien/ introduced species, such as crab, shrimps and mullet are consumed by Caspian seal to a different extent. Impact of Mnemiopsis leidyi that does not contact directly the higher links of the trophic chain in the sea ecosystem, might be indirect, through intermediate trophic links, particularly, kilka as a major food item of seal (up to 80%).
Marine mammals, being -ichthyophagous species, serve as peculiar regulators of the sea ecosystem.
Economic significance of species. Valuable commercial species. Not long ago it held the first place in pinniped hunting.
The main raw material is fur skins of the offspring and blubber.
Commercial characteristics of species, catches. Prior to 1968, all age groups of seal were exploited commercially, virtually in all seasons. After sealing regulation at the Caspian Sea basin, 1966-1970, only fur offspring was hunted. Seal catch throughout the XX-th century varied widely from several hundred thousand specimens in 1930-s to several thousand in 1990-s . For economic reasons sealing has not been conducted since 1998.
Fishing gears and fishing Zones. Present hunting of Caspian seals takes place only on the ice of the Northern Caspian. There are two types of hunting: using helicopter or boats.
Unregulated sealing in 1930-s depleted the population: by late 1960-s the population number reduced twice.
Human impact/Threats. One of the main adverse factors is pollution of the sea. The decrease in the population abundance (by approximately 20%) in the last 25 years is related to accumulation of pollutants in animals’ tissues and organs, which affected the reproductive system of females.
Conservation measures. The species conservation under conditions of its commercial exploitation is regulated by estimated predictions for commercial pressure. TAC (Total Allowable Catch) is determined annually, two years in advance. It is updated on the basis of additional data on the population status within this period. Since 1970, when the first quota limitation was established, TAC was reduced upon recommendations of CaspNIRKH, from 70, 000 to 15, 700 specimens. In 2001, the experimental canine distemper vaccination of seals was carried out at the Caspian for the first time. This disease caused mortality of 20, 000-30, 000 animals in 2000.
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Khuraskin, L.S. 1989. Caspian seal. P.p. 198-205. In: Caspian Sea. Ichthyofauna and commercial resources (ed. V.N. Belyaeva, A.D. Vlassenko, V.P. Ivanov. Moscow. Nauka (in Russian).
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Khuraskin, L.S. and N.A. Zakharova. 2000. Present conditions of the development of Caspian seal population bioresources. P.p. 414-418. In: Proceedings of the International Conference: Marine Mammals of Golarctic. Arkhangelsk (in Russian).
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Khuraskin L. S. (Caspian Fisheries Research Institute, Astrakhan, Russia)
Zakharova N. A. (Caspian Fisheries Research Institute, Astrakhan, Russia)