Huso huso (Linnaeus, 1758)

TaxonomyHuso huso

Phylum: Chordata
Class: Osteichthyes
Order: Acipenseriformes Berg, 1940
Family: Acipenseridae Bonaparte, 1832
Genus: Huso Brandt, 1869
Synonyms: Acipenser huso Linnaeus, 1758; Huso ichthyocolla Bonaparte, 1845: 22; Acipenser vallisnerii Molin, 1853: 366; Huso huso Berg, 1904: 666; Huso huso maeoticus Salnikov and Malyatskii, 1934: 45; Huso huso ponticus Salnikov and Malyatskii, 1934: 44; Huso huso ponticus occidentalis Salnikov and Malyatskii, 1934: 45; Huso huso ponticus orientalis Salnikov and Malyatskii, 1934: 45; Huso huso caspicus Babushkin, 1942: 131; Huso huso caspicus curensis Babushkin, 1942: 131

Common names: Russian: beluga; Kazakh: kortpa; Turkmenian: doku (akvalyk); Azerbaijani: ag-kulag-nyarya, gyuzgi-burun; Iranian: phil mahi, beluga; English: great sturgeon, beluga

General view of the organismGeneral Illustration of the Organism

Taxonomic description of species

Body torpedo-like in shape. Notochord present. Caudal fin heterocercal. Spiracles well-developed. Gill membranes connected to each other. Snout short, pointed, slightly turned-up, without bony shields. Mouth inferior, when closed - crescent-shaped, when open - slightly semilunar, does not reach sides of head. Lower lip interrupted. Barbels with leaf-like appendages. D 58-75 (in average 65.8); A 25-40 (in average 32.4). Scutes: dorsal 10-15 , lateral 40-51, and ventral 9-11. Gill rakers 19-30.
Intraspecific forms: subspecies Huso huso ponticus Salnikov and Malyatskii, 1934; Huso huso maeoticus Salnikov and Malyatskii, 1934.
Within the Caspian Sea basin the species is subdivided into three forms: Kura, Ural and Volga great (beluga) sturgeon (Karatayeva et al., 1971). They are almost alike in morphology, but there are some differences in the terms of maturation, growth, fecundity. Vernal and hiemal races of great sturgeon are also differentiated.
Related forms. Great Siberian sturgeon Huso dauricus (Georgi, 1775) is distributed in the Amur River basin from the firths to upper reaches. Occurs in the Ussuri, Sungari, Zeya, Shilka, Argun, Khanka and Orel Lakes. It has two forms: semi-migratory and freshwater (riverine).

Distribution of species within the Caspian SeaDistribution of Species within the Caspian Sea

Huso huso inhabits the entire sea, enters the rivers of the Caspian Sea basin (the Volga, Ural, Kura, Terek, Sefidrud) to spawn.
Status as per International Red Data Book: EN.
Status as per National Red Data Books: EN.
First record for the Caspian Sea. N/A.
Redescription of Species. Berg, 1911: 151

General characteristics of species

Ecologo-taxonomic group. Pelagic fish referred to nekton
Origin. Caspian autochthonous species.
World distribution. Ponto-Caspian endemic
Great sturgeon occurs in the basin of the Caspian, Azov, Black Seas. Single individuals were recorded for the eastern part of the Adriatic Sea and the rivers of Anatolia.
Habitat. Feeds in shallow zones at depths 1.5-30 m. During the winter period it occurs at depths down to 130-180 m, mostly fish keeps to waters 10-60 m-deep (Caspian Sea. Ichthyofauna and commercial resources, 1989).
Migrations. Beluga performs spawning, hibernating and feeding migrations within its range. Unlike the Volga River population, the Ural spawning population of great sturgeon consists mostly of vernal migrants (60-80%) (Pesseridi, 1971).

Terms of spawning migration in vernal and hiemal races of the Ural River great sturgeon
(cited: "Fish of Kazakhstan", 1986)

Race Start of run Water temperature
0C
Peak of run Water temperature
0C
End of run Water temperature
0C
Spawning temperature (spring),
0C
Vernal Late March-early April 2-30 2nd-3rd ten day period of April 6-80 Mid-May 14-150 10-150
Hiemal 2nd half of June -July 19-240 Late August- September 15-200 Late October-early November 6-700 7-110

Feeding migrations in the sea follow the movements of beluga food organisms (mainly, shads, kilka, gobies, cyprinids). The preferential feeding grounds of the Ural beluga are located in the Ural Deep Trench, Kazakh Bay, at the eastern coast of the Middle Caspian and areas adjacent to Ogurchisky Island (Fish of Kazakhstan, 1986).

Relation to abiotic environmental factors

Relation to salinity. Brackishwater euryhaline species, occurs at salinity 0.1-15.
Relation to temperature. Eurythermic species
In spring adults prefer waters of 2-160C, while young fish - 7-200C. In summer beluga is evenly distributed throughout the sea at temperature 20-310C, in autumn preferred temperature is 8-150C. The only relationship between beluga distribution and water temperature was established for adults in spring (correlation coefficient 0.58), for immature individuals - in autumn (0.69). Beluga prefers the temperature of 17-190C in the Middle Caspian, 10-290C - in the southern part of the sea, highest densities are recorded at 22-290C. In winter beluga feeds at temperature 4-50C in the middle part, at 10-120C - in the southern part of the sea.
Vertical distribution. Eurybathic species, occurs at depths from 1.5-2 m to 110-180 m (Khodorevskaya et al., unpublished data).
Seasonal migrations of great sturgeon
Relation to oxygen conditions. Oxyphilic species.
The concentration of dissolved oxygen below 60% is disastrous for great sturgeon.
Relation to fluctuations of the sea level. Sea level fluctuations produce indirect effect through food supply. When the sea level dropped in 1973-1977, beluga migrated from shallow feeding sites located in the northern part of the sea to deeper areas in the Middle and South Caspian.

Feeding

Feeding type. Heterotrophic, holozoic
Feeding behavior. Predator, active search and seizure of food organisms in the process of hunting.
Food spectrum. Juvenile fish (in the first year of their life cycle) are mixed feeders on crustaceans, mollusks, and fish. During the first month at sea (June) they feed mostly on higher nekto-benthic crustaceans, order Mysidae, and in 1976 and 1978 their diet included large amounts of gobies, which could be attributed to a decline in mysids abundance during the low-water period (Caspian Sea, 1985).

Diet composition of great sturgeon underyearlings during their first month at the feeding grounds of the North Caspian (Caspian Sea, 1985) 

Food item, % of weight 1968 1974 1975 1976
Mysidae 100.0 100.0 93.3 35.0
Gobiidae - - - 65.0
Other - - 6.7 -
Total 100.0 100.0 100.0 100.0

Beginning the second year of life, great sturgeon transfers to predation almost completely - proportion of fish in stomachs reaches 98% (Fish of Kazakhstan, 1986). During various hydrological periods, the species food spectrum changed insignificantly, fish food has always formed the basis of its diet.

Feeding habits of great sturgeon in the Caspian Sea 
(Polyaninova, Molodtsova, Zarbalieva, 2001)

Diet composition, % of weight 1970-1977 1978-1985 1986-1998
Clupeidae, including 18.5 9.8 30.6
Cl. cultriventris caspia 15.4 8.8 17.1
Gobiidae 51.5 49.9 12.3
Atherinidae 2.5 1.5 -
Cyprinidae, including 10.5 15.9 10.1
Rutilus rutilus caspicus 6.0 11.4 8.6
Percidae 0.6 - 6.0
Other Pisces 3.9 16.1 12.1
Total Pisces 87.5 93.2 71.1
Crustacea 12.2 6.6 27.7
Other 0.3 0.2 1.2
Total 100.0 100.0 100.0
Food supply. In 1980-s, the total ichthyomass in the Caspian Sea (exclusive sturgeons) was estimated as 2 million tons (Caspian Sea, 1986); it could be potentially considered as food supply for such a large predator as Huso huso.
Quantitative characteristics of feeding. Maximum weight gain and values of stomach fullness were recorded during the first month of feeding young fish at sea. Further, total indices of stomach fullness decreased, from 311.3o/ooo in fish 10 cm TL to 75.9o/ooo in fish 31-40 cm TL.
The feeding rate depends on a lot of factors: feeding area, season, water temperature, distribution pattern and density of food organisms, etc. (Polyaninova, 1997). The long-term index of stomach fullness of great sturgeon was estimated to be 80o/ooo.

Reproduction

Reproduction type. Gamogenesis
Reproduction areas. Huso huso is a lithophilous fish: eggs are deposited on hard substrate (stones, gravel, pebble, coarse sand, etc.).
Spawning grounds in the Ural River are located 400-1200 km upstream Atyrau City, about 70 spawning grounds (922 ha) have remained.
The Volga great sturgeon spawns in the area below the Volgograd dam (from Volgograd City to Kamenny Yar Settlement), at depths 10-30 m, flow velocity 1-2.5 m/sec.
Terms of reproduction. Hiemal beluga spawns in the Ural River at water temperature 7-110C, vernal spawners - at 10-150C. Spawning interval is 3-4 years for males, 4-6 years for females (Fish of Kazakhstan, 1986). The Volga great sturgeon spawns in spring (late April-early May) at water temperature 6-160C.
Fecundity. The mean absolute fecundity of great sturgeon in the Volga River is currently equal to 680,000-800,000 eggs

Qualitative characteristics of the great sturgeon population in the Ural River

Years Length,
cm
Weight,
kg
Fulton Condition Factor Proportion of females in catches
%
Absolute fecundity,
thousand eggs
1996 224.2 78.7 0.67 28.9 621.1
1997 220.0 81.9 0.65 34.9 625.0
1998 212.6 73.1 0.69 17.0 790.8
1999 219.8 88.3 0.71 23.6 520.7
2000 206.6 89.7 0.69 39.0 765.7

Limiting factors
Abiotic:

Biotic:

Anthropogenic:

Life history and development

Life history stages. Embryonic development lasts 200-240 hours at 8-110C, 130-140 hours at 15-160C.
Prelarvae: newly hatched beluga is larger (11.0-12.0 mm TL) than the other sturgeon species at this stage; the larval stage lasts until transition to active feeding (up to 25 mm TL).
Larvae migrating downstream usually appear in the Ural River delta during the first ten days of May (25-50 mm TL).
Fry is traced in the Ural River delta during the first 10 days of June.
Underyearlings attain 10-15 cm TL in the Ural River by July.
Relation to environmental factors. Embryonic, larval and fry stages of the life cycle are most vulnerable to pollution, oxygen deficiency, and predation by fish. Great sturgeon has no enemies in the sea.
Age of maturity. Huso huso reaches sexual maturity rather late. Some males perform their first spawning migration to the Ural River at the age of 9-11 years, females - at the age of 12-13 years. Most males reach maturity at the age of 12-15 years old, females - at 15-20 years (Fish of Kazakhstan, 1986). Mature males in the Volga River are aged 10-15 years, females - 16-22 years.
Thermal conditions of development. Development of eggs and larvae proceeds at temperature 9-150C, underyearlings and juveniles develop at 15-260C. 
Quantitative characteristics of growth. Huso huso in the South Caspian grows less rapidly than the other populations. The size and weight of great sturgeon within age groups vary considerably.

. Age and size characteristics of great sturgeon in the North Caspian and Ural River, 1961-1972 (Fish of Kazakhstan, 1986).

Age group, years Males ( n = 1048) Females (n =835)
Length, cm Weight, kg n Length, cm Weight, kg n
variations average variations average variations average variations average
9-10 142-160 152.6 19-32 24.5 21 - - - - -
11-12 146-169 159.5 24-36 29.3 44 158-174 167.3 26.6-36.5 31.8 14
13-14 155-181 168.8 25-41 33.9 81 161-179 113.8 23.8-39.1 35.9 38
15-16 164-194 177.0 28-68 37.8 118 169-198 182.8 28.1-68.5 42.3 54
17-18 170-201 186.3 31-71 45.3 145 174-216 194.8 31.3-71.2 49.6 79
19-20 178-208 194.5 30-73 50.1 181 181-238 208.5 30.8-87.7 58.0 104
21-22 183-217 203.1 36-78 56.5 138 196-248 220.3 38.5-84.0 64.9 126
23-24 185-230 214.6 41-88 64.2 121 211-246 227.4 50.0-124 78.5 130
25-26 209-245 225.3 48-90 70.3 94 216-252 238.3 68-135 89.0 102
27-28 231-260 241.4 51-103 78.4 43 228-284 253.2 88-160 108.3 84
29-30 240-266 250.8 55-101 79.1 31 239-295 264.3 94-185 122.0 32
31-32 251-280 262.3 61-114 87.2 16 246-294 271.5 101-193 136.5 26
33-34 254-285 271.2 70-129 99.3 9 259-311 380.6 114-205 143.8 21
35-36 251-290 276.1 76-134 108.5 5 268-318 284.1 118-201 147.3 11
37-38 - - - - - 276-324 291.3 126-218 153.8 7
39-40 - 284.0 - 131.0 1 281-328 298.0 146-264 174.5 5

Structural and functional population characteristics

Sex ratio. In recent years, the proportion of females in the Ural River population has increased from 17.0% in 1998 to 39.0% in 2000.
Number of females in the Volga River does not exceed 13% of the spawning population. Sex ratio at the sea feeding grounds is close to 1:1 (the proportion of females reaches 45-50%).
Age-size structure. The life span of Huso huso is very extensive, some individuals attain the age of 100 years and even more. In late years, the mean TL of females in the Ural River was 245.0 cm, males - 210.6 cm TL; the minimum and maximum age - 12 and 39 years, respectively. Immature individuals predominate at sea feeding grounds.. The maximum age recorded currently at sea: males - 35-40 years, females - 16-55 years. The mean age of females in the Volga River population is 22.7 years, weight - 114.6 kg, 239.7 cm TL, corresponding values for males are: 17.7 years, 62.8 kg (Wt), 204.7 cm (TL). 
Quantitative characteristics. In 2000, the abundance and biomass of beluga spawning population in the Ural River were: 2,660 specimens and 226,7 tons, respectively; in the Volga River (1991-1995) - 8,000 specimens, biomass - 500 tons.
Population trends. The population number of Huso huso is dependent on a complex of factors, such as: conditions of reproduction, abundance and quality of spawners, food supply . In 1990-s, a decline in the spawning population of the Ural River was recorded, by the year of 2000 it consisted of 2,660 individuals only. The Volga River population also shows enormous reduction as compared with the previous periods; 1971-1975 - 20,700 specimens, 2,000 tons; 1991-1995 - 8,000 specimens, 500 tons. In 1980-s, the abundance of great sturgeon in the sea was over 18 million individuals, presently - 8 million specimens only.

Interspecific relations

Huso huso is a predator in relation to the other fish species, specifically, roach, kilka/ sprats, gobies. Cannibalism may occur at early developmental stages. Beluga juveniles are benthic feeders in the first year of their life cycle, hence they are food competitors to young sturgeons, gobies, cyprinids. The most strained trophic competition exist between adult great sturgeon and another predator - Caspian seal; the total index of food affinity is 61.3% (Zakharova et al., 2000). On the other side, young seals are prey to large beluga, especially in winter and early spring (Badamshin, 1949).

Importance of species to bioresources production of the Caspian Sea

Economic significance of species. The importance of great sturgeon for the economy of the Caspian littoral states is defined not only by the volume of harvest, but for the value of products made from this fish as well.
Commercial characteristics of species, catches. Great sturgeon makes third in catches of the Caspian sturgeons (succedent to the Russian and stellate sturgeon). A steady decline in beluga catches in the Ural-Caspian basin was recorded.

Dynamics of stock characteristics of the Ural great sturgeon 

Years Annual volume of the Ural River runoff, km3 Total abundance thousand individuals Catches Number of spawners let to thespawning grounds,
thou. ind.
Number of fingerlingsmigrated downstream,
million ind.
thou. ind. thou. tons
1991 10.6 7.5 3.6 0.29 3.9 0.333
1992 6.0 6.2 3.1 0.28 2.8 0.216
1993 15.0 13.2 6.9 0.39 6.3 0.603
1994 15.4 3.9 1.7 0.13 1.4 0.430
1995* 6.0 N/A N/A 0.31 N/A 0.333
1996 3.8 3.2 1.4 0.11 1.2 0.189
1997 5.7 4.3 1.1 0.09 1.9 0.864
1998 12.0 3.1 1.2 0.09 1.9 0.073

1999 5.0 2.1 0.7 0.06 0.8 -
2000 11.0 2.66 0.67 0.06 1.3 0.920

* studies were not conducted in 1995.
Annual catches taken from the Volga-Caspian region decreased from 20,500 individuals in 1970 to 4,100 individuals during 1991-1995.
Fishing gears and fishing zones. Riverine beach seines are used in the lower reaches of the Ural River at permanent fishing sites; fixed nets - in the Ural estuary.
In the Volga River great sturgeon was harvested with riverine beach seines at fishing sites located mainly in the estuarine part of the delta. Sturgeon fishing has been halted in the Volga-Caspian basin since 1999, in the Ural-Caspian region - since 2001.

Impact of fisheries on the population status

Unsustainable fishing was the main reason for depletion of the Ural and Volga River populations of great sturgeon. Harvest should take 40% of the population at most. At present (2001), the commercial harvest of beluga is banned. Great damage to the stocks is incurred by poaching at rivers and the Caspian Sea. 
Human impact/Threats. Damming of the Volga River resulted in restricted access to the spawning sites so that the natural reproduction/ recruitment decreased to a critical limit. Overfishing, rampant poaching, pollution of rivers and sea caused a drastic decline in population number.
Conservation measures

  1. Management of water releases in the Volga River to increase the natural reproduction of great sturgeon, reclamation of sturgeon spawning grounds in the Volga and Ural Rivers.
  2.  An increase in the annual hatchery production of great sturgeon juveniles up to 15-20 million.
  3. Increased control over poaching.

References

Babushkin, N.Ya. 1964. Biology and fishing for Caspian great sturgeon. VNIRO Proceedings. Vol.52, 1: 183-258. Food Industry. Moscow (in Russian).
Badamshin, B.I. 1949. Some new data on the biology of Caspian seal on the ice. J. Rybnoye Khozyaistvo (Fisheries), 3: 39-41 (in Russian).
Fish of Kazakhstan. 1986. Vol.1. Nauka Press. Alma-Ata pp. 57-71. (in Russian).
Khodorevskaya, R.P. and A.A. Polyaninova, 2000. Estimation of feeding conditions for great sturgeon (Huso huso L.) in the north-western part of the Northern Caspian. In: Marine hydrobiological studies. VNIRO Press. Moscow. Pp. 205-208. (in Russian).
Legeza, M.I. 1970. Quantitative distribution of sturgeons (family Acipenseridae) in the Caspian Sea. TSNIORKH Proceedings Vol.2. Food Industry. Pp. 57-63.
Legeza, M.I. 1973.Distribution of sturgeons in the Caspian Sea. J.Voprosy Ikhtiologii (Problems of Ichthyology). Vol.13, 6(83): 1008-1015.
Polyaninova, A.A. 2000. On occurrence of Caspian kilka in great sturgeon diet. J. Voprosy Rybolovstva ( Problems of Fishery). Vol.1, 2-3(2): 91-92. National Fish Resources Press. Moscow (in Russian).
Polyaninova, A.A. 1996. Importance of the trophic factor for the development of great sturgeon stock in the Volga River. In: State and prospects of scientific-practical developments in the field of mariculture of Russia. VNIRO Press. Moscow. Pp. 254-256. (in Russian).
Zakharova, N.A., L.S. Khuraskin and A.A. Polyaninova, 2000. Trophic relations of sturgeons and Caspian seal. In: International Conference: Sturgeons on the Threshold of the 21st Century. Book of Abstracts. CaspNIRKh Press. Astrakhan. P. 54. (in Russian).

Compiled by:

Yu.A. Kim (KazNIIRKhAtyrau branch, Kazakhstan)
R.P. Khodorevska (CaspNIRKh, Astrakhan, Russia)
I.M. Aminova (KazNIIRKh Atyrau branch, Kazakhstan)

Acknowledgements:

Authors are thankful to A.A. Polyaninova for providing data on great sturgeon feeding.