Stenodus leucichthys, (Guld.)

Taxonomic description of subspecies

A large fish up to 120 cm TL (total length), weight � up to 20 kg.
Body elongate, fusiform. Body sides and belly silvery white. Adipose fin present. Mouth large, terminal. Maxillary does not extend to the vertical of eye posterior. Teeth develop on jaws, vomer, palatines and tongue. Generally 21-25 gill rakers on the first gill arch. Scales large, 99-120 in lateral line. Pyloric caeca 191-193. Sexual dimorphism develops weakly. At spawning, nuptial tubercles appear on the head and body sides of males.
Intraspecific forms. 2 forms are supposed to exist � spring and autumn, which are differentiated by the time they enter the Volga river for spawning (Berg, 1948).
Related forms. The only related form is subspecies Stenodus leucichthys nelma (Pallas, 1773) that differs from inconnu by certain osteological features (Shaposhnikova, 1967). Nelma is widely distributed in the basin of Arctic Ocean from the White to Bering Sea and the inflowing rivers (Northern Dvina, Pechora, Ob, Irtysh, Lena), in the systems of rivers Mackenzie and Yukon. Nelma occurs in freshened zones of the seas, in rivers and lakes with different environmental conditions. This species makes anadromous, semianadromous and land-locked (lacustrine) forms.

Distribution of subspecies within the Caspian Sea

Inconnu is seldom found in the sea; that is mainly at the western and eastern shelves of the Middle Caspian at depths less 60-65 m; in the Volga river during the spawning run � from the river mouth up to the Volgograd city. Individual specimens enter the Ural river.
Status as per International Red Data Book. Introduced into �IUCN Red Data Book�
Status as per National Red Data Books. Russia � N/A; Azerbaijan � EN; Kazakhstan � EN; Turkmenistan � DD; Iran � N/A
First record for the Caspian. Pallas (1771:31) mentioned inconnu under the name of Weisslachs.
Redescription of subspecies. See: synonyms

General characteristics of subspecies

Ecologo-taxonomic group. Nekton
Origin. Arctic immigrant.
Inconnu penetrated into the Caspian from the Arctic Ocean basin in the late glacial period.
World distribution. Caspian endemic subspecies.
Habitat. Pelagic area up to the depth 60-65 m, favorable oxygen (90-100%) and lowered temperatures (8-20^oC) conditions. Dwelling areas mostly coincide with those of Caspian kilkas/ sprats, which are the main food item of inconnu. Unlike adult fish, juveniles occur in the sea coastal zones and bays (Mangyshlaksky, Kazakhsky). In the fall, when water temperature decreases to 8-10^oC, inconnu appears in the North Caspian.
Migrations. Anadromous subspecies. Upon attaining maturity inconnu migrates northward. The majority of fish migrates along the eastern coast of the Middle Caspian, the other part � along the western coast. In August the fish is registered at Mangyshlak peninsula. Spawning migration into the Volga river begins in September and lasts through the fall, winter and the first part of spring. After spawning (in November) the fish migrates downward to the sea. Feeding migration in the Middle Caspian occur mainly within the zones of circular streams.

Relation to abiotic environmental factors

Relation to salinity. Brackishwater euryhaline subspecies.
In the course of life history, salinity of environment varies from 0⁰/₀₀ (in the river) to 12-13⁰/₀₀ (at sea). Increase of salinity tolerance in fingerlings is evident at the age of 30-50 days, when they are able to adapt to the Caspian water.
Relation to temperature. Eurythermic cool-water subspecies.
Occurs at sea water temperature below 20⁰C. The narrowest temperature range is observed in the courses of spawning period (0.1-6.0⁰C) and embryonic development (0.1-1.2⁰C). When reared in ponds, inconnu fingerlings tolerate temperature rising up to 25⁰C.
Vertical distribution. Stenobathic subspecies, which was not registered at sea depths below 65 m; optimal depths are 25-45 m.
Relation to oxygen conditions. Oxyphilic subspecies
Relation to fluctuations of the sea level. Indifferent

Feeding

Feeding type. Heterotrophic
Feeding behavior. Active predatory species
Food spectrum. Inconnu feeds mostly on fish (up to 90% weight and more), less � on invertebrates. Transition to predatory feeding occurs at early stages. Stomachs of 30-days old fingerlings contain larvae and small juveniles of the other fish species. In the sea adult inconnu feeds on kilka and silverside (97-99%), less � on Caspian roach/vobla juveniles and gobiids.

Food composition of inconnu in the North Caspian, weight percentage
(Kiselevich, 1926, cited: Podlesny A.V., 1947)

Food supply. Until recently, food supply of inconnu (small pelagic fish � kilka, silverside) did not restrict feasible increase of species abundance. Invasion of Mnemiopsis into the Caspian and decline of kilka stocks led to sharp degradation of inconnu food supplies.
Quantitative characteristics of feeding. Data for the Caspian Sea on feeding rate of inconnu are rather scarce. In spring 1979, inconnu captured in the western part of the Middle Caspian, had rather high values of index of stomach fullness, which varied in the range 100-223 ⁰/₀₀₀. Feeding ratio of adult inconnu is assumed to be 10 (Karpevich, 1975).

Reproduction

Reproduction type. Sexual/ Syngenesis. Semi-adhesive eggs are deposited on the bottom.
Reproduction areas. Before regulation of the Volga river stream, the main spawning grounds for inconnu were located in the Kama river system, i.e., in the river Ufa, 3,000 km away from the mouth of the Volga. Spawning occurred in shallow zones, at bottom covered with sand or gravel. Currently, spawning takes place at the small area of 7-10 ha and at the artificial spawning site (0.5 ha) in the vicinity of the Volgograd hydroelectric power station.
Terms of reproduction. Spawning season begins in the end of October � beginning November as temperature falls to 6.0⁰ C. Each individual spawns twice during life cycle, with an interval of 2-3 years.
Fecundity. Average fecundity - 250 thousand eggs (160*10³-400*10³)
Limiting factors:

• Low number of spawners
• Insufficient spawning areas
• Unstable hydrological conditions/operating mode at the lower reach of the Volgograd HEPS. Abrupt fluctuations of the water level and flow velocity result in decline of spawners� physiological status; some portions of eggs get buried in sand and mud.
• Damage caused by fish and crustaceans: sterlet as well as Pontigammarus obesus and Dikerogammarus feed on inconnu eggs. Amphipods were estimated to eliminate daily up to 1.5 million eggs at the area of 0.72 ha (Letichevski, Dubinin, 1978)

Life history and development

Life history stages. Development of eggs (2.2-2.4 mm in diameter) lasts for 180-200 days. Larvae hatch from the end of March up to the early May. Larvae possess egg-shaped yolk sac with large oil drop. On the 7-th-10-th day, larvae start feeding on plankton. At the age of 30-40 days, transformation to fingerlings stage is initiated. Fingerlings migrate downward to the sea without delay (i.e., without stops in the river). Maturation period lasts from 3 to 6 years.
Relation to environmental factors. The most vulnerable life history period is embryonic development, when the eggs are exposed to unfavorable abiotic (sharp drops of the current velocity, pollution of the water and substrates) and biotic factors (inconnu eggs are consumed by invertebrates and fish). Survival rate of eggs is several times more on artificial spawning ground of crushed stone compared with that on the open sand bottom. During migration down the river larvae and juveniles are considerably ate up by predatory fish (perch, zander, catfish). The same reason attributes for the low coefficient of commercial return in hatchery-released fingerlings (Letichevsky, 1983). The main factor restricting abundance at sea is food supply.
Age of maturity. Maturation at sea does not occur simultaneously. Males get mature at the age of +5 � 6+ years; females attain maturity at the age of +6 � 7+ years.
Thermal conditions of development.
Embryonic � 0.1-1.2⁰C
Larval � 1.2-10.6⁰C
Fingerlings � 10.6-25.0⁰ C
Juveniles � 8-25.0⁰C
Mature fish (marine period)� 8-20.0⁰C
Spawners (river) � 25-0.1⁰C
Quantitative characteristics of growth. General description of inconnu linear growth was provided by Kiselevich (1936). At hatching, the mean larval length is 11 mm, at the age of 5 years inconnu reaches 89 cm TL, which constitutes 80-fold increase.

Linear growth of inconnu
(Kiselevich, 1926, cited: Podlesny A.V., 1947)

When reared in ponds of the Lower Volga hatcheries, weight of fingerlings increases from 13.3 to 950 mg for the period of 40 days, average daily growth increment is 11.2% (Letichevsky, 1983). Weight of juveniles captured at sea ranges from 27g to 223 g at the age of 1year, 750-1,750 g � 2 years, 1,600-2,293 g � 3 years (K.N. Prokhorova, unpublished data).

Structural and functional population characteristics

Sexual structure. At the start of spawning run females/males ratio in the Volga river delta was approximately equal to 1:1 in 1970, 1973, 1976 and 1980, in the other years males prevailed (Letichevsky, 1983). At the spawning grounds below Volgograd the share of females fluctuated from 31 to 70% in 1970-1979.
Age-size structure. In 1991-2000, linear characteristics of migratory spawners in the Volga river delta ranged within 68.0-107.0 cm, weight � 4.7-13.2 kg, age � 4-12 years.
Quantitative characteristics. The number of reproductive part of the population sampled in the Volga river increased from 1.5-2.0 thousand specimens in 1970-1971, to 22.5 thousand in 1976; in succeeding years it reached 80.9 thousand specimens (Letichevsky, 1983).
Population trends. Decrease in population number is observed.

Interspecific relations

At sea inconnu feeds on pelagic fish (kilka, silverside, shads) as well as juveniles of semianadromous fish and gobiids.

Importance of species to bioresources production of the Caspian Sea

Economic significance of species. A very important commercial fish due to extremely delicate rich flesh (fat contents: 18-26%) best for smoked products (balyk).
Commercial characteristics of species, catches. Annual commercial catches in 1931-1940 ranged from 0.8 to 1.46 thousand tons (100.0-182.5 thousand specimens). In 1950-s, under impact of degraded conditions of reproduction (restricted access of spawners to the spawning grounds due to construction of hydroelectric power stations, contamination with oil and liquid industrial waste of main spawning areas in the river Ufa), catches of inconnu declined abruptly to 0.4 tons in 1959. So, the ban on inconnu harvest was established in 1959. In 1970-s, the abundance of spawning population increased from 1.5-2.0 thousand specimens in 1970-1971, to 11-22 thousand specimens in 1976-1977, as a result of efforts undertaken for artificial reproduction. In 1980-s, commercial fishing of this species was recommenced. In 1990-s, due to reduction in the number of fingerlings released from hatcheries and increase of illegal fishing, the catches dropped abruptly: 20.0 tons in 1997, 10.0 tons in 1998, 3.0 tons in 1999, 4.0 tons in 2000. Currently, fishing is carried out only for reproduction purposes (providing hatcheries with eggs and breeders) at the experimental fishing site �Glubokaya�, lower reaches of the Volga river.
Fishing gears and fishing zones. Currently no commercial catches.

Impact of fisheries on the population status

Currently no fishing.
Human impact/Threats.

• Regulation of the Volga river runoff
• Contamination of the river and the Caspian Sea
• Poaching/ illegal fishing.

Conservation measures. Inconnu is one of the most important fish species for artificial reproduction/restocking at the Caspian. During the period of 1992-1940, a number of 615*10³-13*10³ larvae were released annually in spring into the river Belaya. In 1950-s, rearing of fingerlings in ponds of the Lower Volga hatcheries was developed and initiated in large scale (Belyaeva, Milstein, 1960). 47 million fingerlings appr. were reared and released into the Volga River for the period 1959-1978. Maintaining and rising inconnu abundance is feasible under hatchery conditions only, which implies prolonged keeping of spawners in tanks until maturation occurs. Since biotechnology of inconnu reproduction is already developed, level of efforts will be determined by the number of the breeders and environmental situation at sea (lack of contamination and sufficient food supplies). To enhance efficiency of natural reproduction artificial spawning ground with the area of 7-10 hectares was proposed to be constructed in the lower race of the Volgograd hydroelectric power station.

References

Belyaeva V.N., Milstein V.V., 1959. Rearing of inconnu fingerlings in the Volga river delta. -Moscow: �Rybnoye Khozyaistvo� (Fish Industry Journal) publishers � 18 Pp.
Berg L.S., 1948. Fishes of fresh waters of USSR and adjacent countries. V.1. USSR Academy of Sciences publishers, Moscow-Leningrad. P.3-466.
Letichevski M.A., 1983. Reproduction of inconnu. Moscow. �Lyogkaya I pischevaya promyshlennost� (Light and Food Industry). P.30-112.
Podlesny A.V., 1947. Inconnu Stenodus leucichthys Güld. Bio-ecological outline essay //Proc. Siberian Department VNIORKh (All-Union Research Institute of Lake and Riverine Fisheries), V.7., 1:3-195.
Shaposhnikova G.H., 1967. Comparative characteristics of nelma/white salmon Stenodus leucichthys nelma (Pallas) and inconnu Stenodus leucichthys leucichthys (Güldenstädt)// Voprosy Ikhtiologii (Problems of Ichthyology)-V.7,2(43):225-239.

Compiled by:

E. S. Belyaeva (CaspNIRKh, Astrakhan, Russia)