Genus Huso
Brandt and Ratzeberg, 1833

This genus is characterised by a large and crescentic mouth (small and transverse in Acipenser) and by the gill membranes being joined to each other and free of the isthmus (joined to the isthmus in Acipenser). The snout is short and blunt although Caspian Sea stocks have a longer snout than Black Sea ones. The barbels are flattened laterally and gill rakers are rod-like. There are only 2 species in the genus, one in the Caspian, Black and Adriatic seas and one in the Amur River of eastern Asia.

Birstein and DeSalle (1998) using cytochrome b and 12S and 16S rRNA genes found that Huso may not be distinct from Acipenser.

Huso huso
(Linnaeus, 1758)

Common names

فيل ماهي (= fil mahi, filmahi or philmahi meaning elephant fish), beluga, beloga, سگ ماهي (sag mahi, meaning dogfish), ماهي خاويار (= mahi-ye kaviar, meaning caviar fish), mahi kaviar-e bozorg (= big caviar fish).

[bolka, Kur bolkasi for natio kurensis, ag-kulag-nyarya, gyuz'gi-burun in Azerbaijanian; doku (akvalyk) in Turkmenian; beluga in Russian; great, giant or European sturgeon].

Systematics

Acipenser huso was originally described from the Danube and rivers of Russia.

Huso huso caspicus Babushkin, 1942 was described as the subspecies of the Caspian Sea basin (with natio kurensis Babushkin, 1942 from the Kura River (also spelt incorrectly cyrensis and curensis)) but Berg (1948-1949) considered Caspian-Volga populations to be typical and this subspecies description as unnecessary. No types of Huso huso caspicus are known (Eschmeyer et al., 1996).

Huso ichthyocolla Bonaparte, 1846 is a synonym (Eschmeyer et al., 1996) and a nomen nudum (Holčík, 1989). Acipenser brandtii Günther, 1870 from the "Black and Caspian Seas, with their rivers" is a hybrid of Huso huso and Acipenser nudiventris based on Acipenser schypa (in part) of Brandt and Ratzeberg (Berg, 1948-1949; Eschmeyer et al., 1996). M. Pourkazemi in PADECO (2002) considers there are two sub-populations in Iran.

Hybrids of Huso with Acipenser have been bred by the Aquaculture Department of the Iranian Fisheries Research and Training Organization (Iranian Fisheries Research and Training Organization Newsletter, 3:3, 1994; Annual Report, 1994-1995, Iranian Fisheries Research and Training Organization, Tehran, p. 6, 1996; Annual Report, 1995-1996, Iranian Fisheries Research and Training Organization, Tehran, p. 41, 1997) and natural hybrids with A. gueldenstaedtii, A. nudiventris and A. stellatus are reported from the Caspian Sea (Berg, 1948-1949).

Key characters

This species is identified by its very large, crescent-shaped mouth (small and transverse in other sturgeons) and the gill membranes being joined as a fold across the isthmus.

Morphology

The greatest body depth is slightly anterior to the middle of the body and large fish appear humpbacked. The lower lip is interrupted at its centre. Barbels are flat posteriorly, reach almost to the mouth and have foliate appendages. Experiments on ablading barbels (clipping one, two and four barbels) in 1+ age fish showed no growth differences with an unclipped control (Abasali Zadeh, 2003). The dorsal scutes are covered with skin in sexually mature fish, lateral scutes are smooth and ventro-lateral scutes hidden beneath the skin.

Dorsal fin rays 48-81 and anal fin rays 22-41. Dorsal scutes 9-17, lateral scutes 28-60 and ventral scutes 7-14. Scutes in adults may be reabsorbed. The skin is covered in small denticles. Gill rakers 16-36.

The chromosome number is 2n=118 ± 3 or 115 ± 1 (Annual Report, 1994-1995, Iranian Fisheries Research and Training Organization, Tehran, p. 43, 1996; Iranian Fisheries Research and Training Organization Newsletter, 8:5, 1995), 2n=116 ± 1 (Nowruz Fashkhami, 1996), 2n=118 ± 2 or 2n=116 ± 4 (Klinkhardt et al., 1995), 2n=117 (Nowruz Fashkhami and Khosroshahi, 1999). Sex chromosomes are absent or weakly differentiated in the genome and DNA markers cannot be used to sex fish; minor surgery has to be used (Keyvan Shokoo et al., 2004; Keyvanshokooh et al., 2007).

Sexual dimorphism

None found in morphometric and meristic characters although females are said to be longer and heavier than males of the same age.

Colour

The back is ash-grey, blue-grey to greenish or dark brown, sometimes black, fading to a white or cream belly. The contrast between the dark back and lighter rest of the body is marked. Young often have a metallic sheen which fades with age. The snout is yellowish.

Size

Attained weights of 1228 kg yielding 246 kg of caviar or 7.7 million eggs (Berg, 1948-1949), even 1600 kg (Farid-Pak, no date), and there are newspaper and other reports of fish 1200 kg and 6 m (Ottawa Citizen 14 May 1986) or even 3200 kg and 9 m but such large fish are not seen today and the largest sizes are probably exaggerations. Modern catches are mostly much smaller than these exceptionally large fish. A recent record with the specimen preserved in the Astrakhan Museum in Russia is given in Sternin and Doré (1993) for a fish from the Volga River in 1989 weighing about 980 kg, 4.3 m long and yielding about 110 kg of caviar (Iran News, 14 July 1998, gives 988 kg, 120 kg of caviar and an age of 60 years, presumably the same fish). A photograph of a 1908 capture at Astrakhan in Stein and Bain (1981) shows a fish weighing about 400 lbs (181.4 kg) containing 200 lbs (90.7 kg) of caviar worth more than $69,000 in 1981. Tsepkin and Sokolov (1971) give some examples of large fish from former Soviet waters. Birstein et al. (1997) consider this species to be the largest freshwater fish.

The mean weight of Caspian Sea fish decreased from 110 kg in the early 1970s to 57 kg in 1991 (De Meulenaer and Raymakers, 1996).

Up to 2.83 m and 450 kg generally in Iran (Azari Takami et al., 1980) but see below for news reports. Belugas up to 960 kg tried to enter the Atrak River in 1836 (Vladykov, 1964). The longest fil mahi caught in Iranian waters is apparently one taken on 23 February 1989 by Turkmen fishermen at Shilat-e Nahee 4 in Mazandaran (see Abzeeyan, Tehran, July 1991, page 3). It had a fork length of 4.5 m, a total weight of 725 kg and a caviar weight of 98.2 kg. This individual was worth U.S.$140,000 (Abzeeyan, Tehran, November 1992, page 57). The heaviest fish from Iran is one reported by Hossein Aimani at 3000 lbs (1360.8 kg) from near Babol in 1973 (www.amarillonet.com/stories/120599/bus_LQ7659.shtml, downloaded 7 March 2000). Mobayen (1968) gives the largest Iranian specimen as 4.2 m and 850 kg. Anonymous (1991a) and Sternin and Doré (1993) cite a fish of 1742 lb (= about 791 kg), 7.5 feet long (= about 2.3 m) and yielding 220 lb (= about 100 kg) of caviar from Iran in 1989, the largest caught for 20 years; this may be the same fish as the previous one as confusion in weights and lengths are common in reports of large fishes. Other large specimens were taken at Mahmudabad, Mazandaran on 28 October 1992, measuring 3.2 m, weighing 430 kg and with 61.2 kg of caviar (Abzeeyan, Tehran, November 1992, page 13), at Bandar-e Torkeman (= Bandar-e Shah) weighing 320 and 410 kg giving 110 kg of caviar for the two fish (Abzeeyan, Tehran 4(1):IIX, 1993), at Bandar-e Torkeman, Mazandaran on 27 March 1993, measuring 4.0 m, weighing 550 kg and with 81 kg of caviar (Abzeeyan, Tehran, 4(2):47, 1993), and in Mazandaran one measuring 3.0 m fork length and 3.4 m total length, weighing 960 kg and yielding 62.5 kg of caviar (Iranian Fisheries Research and Training Organization Newsletter, 5:8, 1994). Newspaper reports in 1996 listed a fish of 500 kg with 54 kg of caviar worth $107,000 and a fish caught in October 1997 at Babol Sar weighed 300 kg, measured 3 m in length and had 45.1 kg of caviar. In 1998, one fish 3.4 m long yielded 43 kg of caviar (Reuters), a fish caught off Bandar-e Torkeman on 2 February measured 3.75 m, weighed 405 kg and yielded 50 kg of caviar (IRNA (Islamic Republic News Agency), 3 February 1998), one caught off Bandar Anzali on 25 October weighed 360 kg, was 3 m long and yielded 24 kg of caviar and "meat" worth 3.6 million rials (IRNA, 26 October 1998), one caught off Nour, Mazandaran on 15 November measured 3.5 m, weighed 450 kg, yielded 53 kg of caviar and was 30 years old (IRNA, 16 November 1998), and one caught off Kianshahr, Gilan weighed 290 kg, was 3.5 m long and yielded 50.6 kg of caviar worth 100 million rials (IRNA, 24 November 1998). In 1999 newspaper reports included one caught off Bandar Anzali weighing 155 kg, carrying 31 kg of caviar worth $12,400 (IRNA, 31 October 1999), one caught off Talesh weighing 120 kg with 23.5 kg of caviar worth 150-200 million rials (IRNA, 5 December 1999), and one caught off Bandar-e Torkman weighing over 405 kg with over 52 kg of caviar worth 500 million rials (IRNA, 14 December 1999). One fish caught near Bandar Anzali in weighed 370 kg and yielded 51 kg of caviar (IRNA, 28 October 2002).

Distribution

Found in the Adriatic, Black and Caspian seas and their drainages. Derzhavin (1934) reported it from the Babol, Sorkh and Gorgan rivers but it was rare in the Safid River, although reported up to Kisom and quite abundant in the sea off its mouth. Nedoshivin and Il'in (1927) record this species from 10 river mouths while A. stellatus and A. gueldenstaedtii are reported from 18; the 10 river mouths are Yusufabad, Musachai, Hasan Kiadeh, Dastak, Safid, Kasumabad, Chalkarud, Sardabrud, Chalus and Kheirud. Kozhin (1957), Rostami (1961) and Armantrout (1980) stated that it enters the Astara, Safid, Babol and Gorgan rivers and the Anzali and Gorgan mordabs. It comprised only 0.5% in numbers and 2.5% in weight of the Safid River catch in 1914-1915 (Nedoshivin and Il'in, 1927). Large numbers were caught in the sea off Gasan-kuli in Turkmenistan near the Iranian border (Berg, 1948-1949). Also reported from Hasan Kiadeh by V. D. Vladykov based on field work notes made in 1962. More recently reported from the Gorgan and Safid rivers, the southeast Caspian Sea, southwest Caspian Sea and south-central Caspian Sea by Kiabi et al. (1999), from the Safid River by Abbasi et al. (1999) and from the Safid, Gorgan and Tedjen rivers. This species was not caught in a survey along the Iranian coast in 2001 (Ivanov and Katunin, 2001). In 2004 there were plans to introduce this species to isolated, natural waters bodies in Fars Province (H. R. Esmaeili, in litt., 2004).

Zoogeography

Presumably a relict of past isolation of the Black-Caspian seas from the world ocean.

Habitat

This sturgeon is found in large concentrations in the eastern coastal region of the south Caspian Sea in all seasons. It is rare in trawl catches, possibly because it has a more pelagic life than other sturgeons. Fil mahi descend to greater depths than other sturgeons, 100-140 m in the Caspian and to 180 m in the Black Sea. There is no seasonal variation in depth distribution in the south Caspian Sea in contrast to the middle Caspian (Legeza, 1972; 1973). Only the young are found in shallow, warm areas. On the spawning migration, this sturgeon usually follows the deepest part of the river.

Most of this sturgeon's life is spent in the sea and it ascends rivers only to spawn. The new-born sturgeon returns to the sea. Farabi et al. (2007) examined salinity tolerance and physiology of juvenile fish in Iran. Only the youngest fish showed mortality on direct transfer from fresh to estuarine and Caspian sea water. Adults are typically found on silty or muddy bottoms in the sea but may be found on shelly and coarse sand at a temperature range of 5.6-29.3°C and depths of 5-140 m. In the southeastern Caspian it remains below 30 m in winter, entering shallower water at depths of 10-20 m in spring as the temperature ameliorates, dispersing throughout the southeastern Caspian in summer and migrating into Iranian waters in autumn (Legeza, 1972; Filippov, 1976). Depth distribution depends in large part on the available food supplies.

Oxygen requirements are high, averaging about 14 mg/l, but they can survive at 2-3 mg/l. Salinities up to 22‰ are tolerated. Feeding occurs over a temperature range of 0.5-30°C and the spawning migration at a range of 6-21°C. The highest densities in the southern Caspian Sea occur at 22-29ºC, feeding in winter at 10-12ºC (Caspian Sea Biodiversity Database, www.caspianenvironment.org).

Age and growth

Males become sexually mature at age 9-16 years and females at 12-22 years, varying with the spawning river. This is a very late maturation age among fishes world-wide. Spawning intervals are 4-7 years for males and 5-7 years for females (Vecsei et al., 2002; see below for other ranges but certainly intervals are long for a fish species). Spring-spawning females (see below) first spawn at 201-209 cm, 50-60 kg and 17 years. Winter-spawning females first spawn at 181-190 cm, 30-39 kg and 16 years. Most spring females are 230-300 cm long, weigh 80-160 kg and are 23-28 years of age. Most winter females are 201-300 cm, 50-160 kg and 17-26 years (Raspopov and Dubinin, 1990). Spawning populations have a complex age structure, the Volga River in 1936 had 50 age groups for example but only 28 in 1964. There has been a trend for spawners to be younger. Average catches in former Soviet waters of the Caspian Sea now weigh only 77 pounds (34.9 kg) each, a decline caused by overfishing (Los Angeles Times, Part A, page 1, 28 August 1993). A life span of 150 years was reputed for this species but the greatest known age for a Caspian fish is 75 years (Berg, 1948-1949). Most Caspian fish are now less than 20 years old and made up of individuals from re-stocking programmes (De Meulenaer and Raymakers, 1996). Raspopov (1993a; 1993b) gives the life cycle of Volga River fish as 56 years, although this is not the maximum age. Kura River sturgeon grow more slowly and mature later than sturgeon from the Volga River. Growth in this species is rapid with 1-year-old fish in the Caspian being 51 cm long and weighing 571 g. Growth is slower in the Caspian than the Black Sea because of the decrease in numbers of Alosa spp., the prime food item. Growth is also slower in the south Caspian than the north (Caspian Sea Biodiversity Database, www.caspianenvironment.org).

Levin (1997) summarises the spawning population of the Volga River over the last 10 years as follows although he notes this population is almost extinct. Rarely spawners enter between August and October and breed after a winter hibernation. Other fish enter from December to May with a peak from February to March. Peak spawning is in May with a downstream migration to the Caspian Sea from June to September. Females, comprising 20-24% of the spawning population, average 236-261 cm and 106-160 kg and are 17-21 years old with fish larger than 400 cm being very rare. Males are 199-204 cm and 48-55 kg and are 11-18 years old. Spawning occurs at 9-11°C.

Farid-Pak (no date) gives approximate weights for Iranian beluga of 75-100 kg and 2.0-2.5 m, and a yield of 17-20 kg of caviar per female. 2608 beluga from Astara in Azerbaijan averaged 168 cm for males and 192 cm for females.

Von Bertalanffy growth parameters in Iranian females are L_∞ = 320 cm and K = 0.065 for juveniles, 450 cm and 0.029 for the middle stanza and 533 cm and 0.023 for older fish and for males 270 cm and 0.086 or 302 cm and 0.072, depending on the methodology used. Total mortality (Z) was 0.21-0.67 for females and 0.22-0.75 for males, natural mortality (M) was 0.03 for females and 0.05 for males, fishing mortality (F) was 0.45 for females and 0.33 for males, and optimum fishing mortality was (F) 0.07 for females and 0.16 for males (Iranian Fisheries Research and Training Organization Newsletter, 16:4-5, 1997). Taghavi Motlagh (2001) gives more complete data (on which the previous summary was based) on growth, mortality and yield-per-recruit on this species from 1995 to 1999 in the Iranian Caspian Sea. He concluded that fishing mortality should be stopped. Maximum age in his sample was 46 years.

Food

In contrast to other sturgeons, this species is a pelagic predator as adults. Even sea birds and seals may be eaten. However, the introduced polychaete worm Nereis is now a mainstay of the diet of this species in the north Caspian Sea. Other foods are molluscs, formerly a main food, and small fish such as Rutilus rutilus and gobies (Gobiidae). Fish are the main diet item when large, invertebrates when young. This species needs to find thick concentrations of small or large fishes in order to feed actively; in the north Caspian these are kilka and fish on migration at fishways and in the midde Caspian spawning atherinids and commercial herrings (Polyanina et al., 1999). The fish found by Azari Takami et al. (1980) in Iran were gobies, Cyprinus carpio, Liza, and Rutilus. Gobies are a favourite food item but bivalves and crustaceans are taken if fish are absent. Filippov (1976) notes that large specimens eat sturgeons such as sevryuga, kopur (Cyprinus carpio), mullets (Mugilidae), birds such as coots, and baby seals and because of its pelagic life takes the clupeids Alosa braschnikowii and Clupeonella cultriventris and also the shrimp Leander adspersus. Crabs are also eaten. The principal food as percent by weight in the southeastern Caspian was Neogobius fluviatilis (up to 78.1%), gobies accounted for up to 81.2% and fish 81.6-100%. Crustaceans accounted for up to 7.8% and molluscs only up to 0.2%. The cyprinid, Chalcalburnus (= Alburnus) chalcoides, is also eaten (Mageramov and Zarbalieva, 1989).

Reproduction

Roux (1961a) maintained that this species did not reproduce in Iranian rivers but Rudin (1966) said that they inhabited the Safid and Gorgan rivers. The main spawning river was the Volga as 90% of the Caspian stock reproduced there, travelling as far up as the Moskva River. Males arrive at spawning sites before females. Despite their size, these sturgeons may leap out of the water on the spawning run and possibly during spawning. Adhesive eggs are deposited on sandy substrates, with rocky and gravelly bottoms near the bank, in the strong current of mid-river (1.5-2.0 m/sec.). Water temperatures are 9-17°C and eggs develop in 9-10 days (Novikova, 1994; Vecsei et al., 2002). Spawning usually takes place at a depth of 4-15 m, sometimes as deep as 40 m. Weight loss after spawning may reach 50% and females are only ready to spawn again after 5-6 years and males after 3-4 years (4-8 and 4-7 years in Speer et al., 2000). The migration in the Volga River occurs year-round with peaks in spring (<30% of the stock) and autumn. The spring race reach the spawning beds in the same year, reproduce and return to the sea. The winter race, migrating in summer and fall, overwinter in the river and reproduce the following spring. The spring run is in March and April and the winter run in September and October. The chief spawning period in the Kura River is from the end of May to the beginning of June (Zakharyan, 1972) and fish were found as far up as Tbilisi (= Tiflis).

Fecundity reaches, exceptionally, 7,729,700 eggs but does not increase with age for fish of equal length and weight (Raspopov, 1987; Raspopov and Dubinin, 1990). Mean fecundity for the Volga stock was 531,600 eggs. Normal deposition of eggs is 500/sq m in the Volga but densities fell below 5/sq m in the 1980s, as low as 0.2/sq m and with an average of 1.5/sq m (Novikova, 1994). Kura River sturgeon are less fecund than Volga sturgeon. Egg diameter reaches 4.3 mm. Eggs are a dark silver and oval. Larvae hatch in 10-14 days, the yolk sac is absorbed in 10-14 days and feeding larvae move downstream at up to 60 km/day (Vecsei et al., 2002).

Parasites and predators

Niak et al. (1970) report infestations of the ciliate Trichodina sp. in sturgeons (species unspecified) in breeding ponds in Iran. Golvan and Mokhayer (1973) describe a new species of acanthocephalan, Corynosoma caspicum, and also Leptorhynchoides plagicephalus from this sturgeon in Iran. Mokhayer and Anwar (1973) report on sturgeon parasites in general (see under Acipenser gueldenstaedtii). Mokhayer (1976b) reports gas bubble disease in Iranian sturgeons without specifying the species of sturgeon as well as the monogenetic trematodes Diclobothrium armatum and Nitzschia sturionis. Pourgholam (1994) reports the coelenterate Polypodium hydriforme from this species caught on the Babol Sar and Bandar-e Torkeman fishing grounds in Mazandaran. Larvae of the nematode Anisakis simplex and the acanthocephalan Corynosoma strumosum are also reported from this species (Annual Bulletin 1993-94, Iranian Fisheries Research and Training Organization, Tehran, p. 48-49, 1995). Gorogi (2006b) recorded the nematodes Cucullanus sphaerocephalus and Anisakis schupakovi, the cestode Eubothrium acipsenserinum and the acanthocephalans Leptorhynchoides plagicephalus and Corynosoma strumosum from Iranian waters. Sattari and Mokhayer (2005a; 2005b) recorded the occurrence of parasites in this species from the Iranian southwestern and central coast of the Caspian Sea. The species found were the nematodes Cucullanus sphaerocephalus, Eustrongyloides excisus and Anisakis sp., the cestode Eubothrium acipenserinum, the acanthocephalan Corynosoma strumosum, the digenean trematode Skrjabinopsolus semiarmatus. General conclusions were that the diversity of parasites was less in Iranian waters than in the northern Caspian Sea, perhaps a reflection of the more varied habitat, its productivity and the carbonate ions differing between the two regions. The diversity of parasite seems to have declined over time also, perhaps as a result of unfavourable environmental conditions, particularly in the freshwater ecosystem which limits the waters available for spawning and parasite acquisition. Shenavar Masouleh et al. (2006) found hatchery fingerlings to harbour Diplostomum spathaceum and Trichodina sp.

The fil mahi is so large that its predators are only effective on young fish. They include Sander lucioperca and Silurus glanis and, needless to say at all sizes, mankind.

Economic importance

This species provides the best caviar according to Borodin (1930). The large eggs fetch a higher price on the American market. Up to 80% (3000 kg in 2002) of the legal beluga caviar export is consumed in the U.S.A. (Hamilton, 2002). A 1227 kg specimen caught in Russian waters in 1924 gave 245 kg of caviar worth £189,350. In the 1990s, a 225 kg fil mahi could yield 22 kg of caviar worth $120,000 (Trickey, 1995). Catches in the Volga region in the 1970s were in the range 740-2650 tonnes and in the 1980s 460-900 t comprising 4.4-12.2% and 3.7-4.4% respectively by weight of the total catch of all sturgeons there. The highest catch in the Caspian Sea was in 1902-1907 (Birstein, 1993). Khodorevskaya et al. (1997) and Khodorevskaya (1999) summarise the decline in catches and make the startling observation that 96.3% of all fil mahi in the Volga River are hatchery reared.

Fil mahi were fished intensively off the Iranian coast in the southeastern Caspian and in 1950 amounted to 38.6% of the total sturgeon catch. During the five-year period 1957/1958 to 1961/1962 fil mahi catches in the Gorgan Division of the Iranian fishery varied between 86-90% of total Iranian catches. The Atrak River estuary area was particularly important for this species. Catches of the oldest age groups has declined and the proportion of young and immature fish has increased. Iranian rivers suitable for this sturgeon were the Safid and the Gorgan but both are now regulated so Iranian stocks are probably maintained by fish reproducing in the rivers of the former U.S.S.R. (Filippov, 1976). Fil mahi cannot be managed by Iranian authorities therefore. However the "Gharasoo" Research Station in Mazandaran is researching the culture and release of fil mahi up to 1 kg (Madbaygi, 1993b) and farming through pen culture in Gorgan Bay (Iranian Fisheries Research and Training Organization Newsletter, 11:6, 1996). Two million "roes" (presumably young fish) were released into the Caspian Sea from Mazandaran prior to 1 June 1995 with a further 2 million to be released later in the year (http://netiran.com/news/IRNA/html/950701IRGG08.html). In 1997, 852 fishermen were fishing for fil mahi on the northern Iranian coast (Anonymous, 1997c).

Farid-Pak (no date) gives the months of September-October and March-April as the most important for the fisheries of this species. Nevraev (1929) gives catch ranges of 109-3100 fil mahi individuals for the Astara region of Iran over the period from 1901-1902 to 1913-1914, for the Safid Rud region 104 to 730 individuals for the period 1899-1900 to 1913-1914, for the Mazandaran region 31 to 491 individuals for 1906-1907 to 1913-1914, and for the Astrabad (= Gorgan) region 688 to 1764 individuals for 1902-1903 to 1913-1914. Vladykov (1964) records average yearly catches in Iran of this species from 1927/28-1931/32 to 1957/58-1961/62 with ranges of 57,820-418,059 kg body weight (5.4-33.0% of the total sturgeon catch) and 2038-32,873 kg caviar (2.6-20.4%). There was an upward trend in caviar production from this species in the 1950s (Vladykov, 1964). RaLonde and Walczak (1970b) summarise yields for the years 1963 to 1967 in Iran of meat and caviar as 572.3 tonnes (40.1 tonnes), 583.5 (47.3), 575.8 (39.1), 458.1 (29.5), and 507.2 (30.0) respectively. A commercial house maintains (1995) that caviar from this species comprises only 3% of the total catch. Taghavi Motlagh (2001) noted a decline in the share of Iranian caviar production from 18% in 1971 to 4% in 2000.

This species has been studied in ponds as breeders are used to produce fingerlings which are then available as experimental fish for chemical and growth studies. Karimzadeh et al. (2005) studied cytochrome P4501A1, a major isoenzyme in the monooxygenase system which can be induced by polycyclic aromatic hydrocarbon pollutants. Khoshbavar Rostami et al. (2006) studied the effects of polyaromatic hydrocarbons from Caspian Sea oil wells on 8.5 g fingerlings and found these chemicals to seriously affect the fish blood and enzyme systems. Khoshbavar Rostami et al. (2004; 2006) studied the organophosphate diazinon and its deleterious effects on haematological parameters in this sturgeon. Sharifpour et al. (2004) studied the effects of the insecticide endosulfan, sturgeon weighing 3-5 g showing irregular swimming, whirling, convulsions, with other conditions, and eventually death. Endosulfan is highly toxic to beluga fingerlings. Sudagar et al. (20050 examined the addition of betaine and methionine (an important nutrient and an enzyme) to the diet of juvenile beluga. The fish showed improved weight gain, weight gain percentage, specific growth rate, protein efficiency ratio, net protein utilisation, condition factor, survival, and price index at enrichment levels of 0.5% betaine and 1% methionine. Ghorbani et al. (2004) examined the influence of a series of microelements (zinc, nickel, cobalt, manganese, iron and copper) on the level of proteolytic enzymes and alkaline phosphatase activity (used for enzyme inmmunoassays) in the digestive tract of juvenile beluga. Most treatments showed the level of enzyme activity was less than the control. Shahsavani (2002) determined blood parameters of fingerlings from fish farms and found the fish to be healthy. Blood parameters are used to indicate physiological condition and sublethal stress due to endogenous and exogenous changes, hence the need to determine normal values. Askarian et al. (2006) looked at serum osmoregulatory parameters under different light regimes, one form of physical stressor in aquaculture of this endangered species. No differences in serum cortisol levels were found between treatments although elevations of serum cortisol, glucose and triglyceride occurred.in a continuous dark regime. Gafarian et al. (2007) used probiotic bacillus in the feeding of larval sturgeon and found that it positively affected feeding efficiency and levels of carcass nutrient composition. Khoshbavar-Rostami et al. (2007) examined the immune response to Aeromonas hydrophila bacterin.

Robins et al. (1991) list this species as important to North Americans. Importance is based on its use in aquaculture and aquaria, as food and in textbooks.

Conservation

See also under A. gueldenstaedtii. Critically endangered in Turkey (Fricke et al., 2007). Despite loss of 99% of the Volga River spawning beds to dam construction, natural reproduction increased over a recent five-year period, but continues to be dependent on the variable flow-regime (Raspopov and Dubinin, 1990). Novikova (1994) estimated the capacity of the Volga spawning grounds to be 9-11,000 fish. A major problem in the 1990s was poaching. Trickey (1995), referring to Russian stocks, expected a legal harvest of 4400 tonnes with poachers taking twice that amount. This legal and illegal catch is still less than catches of 20 years ago, primarily because of pollution. Birstein (1996) records the catch of the Volga delta hatcheries in 1995 to be only 35 fish, insufficient for artificial reproduction. Natural spawners are taken by poachers. The level of poaching in the Ural River is also high, and this was the only river where some natural reproduction was going on. The fil mahi has effectively stopped reproducing in the Caspian Sea.

Moghim et al. (no date) note that juveniles of this species are caught in the beach seine fishery for other species in Mazandaran. During 2001-2002, 23,760 seine hauls had a by-catch of 6% for this species among sturgeons captured.

Khodorevskaya and Novikova (1995) point out that cooperation among all the Caspian Sea states is needed to maintain this species along with an annual release of at least 20 million young from hatcheries. Fingerlings released per year from 1998 to 2002 range from 6.9 to 12.6 million for all Caspian states (CITES website). Spawning migrations are now seen only in the Volga and Ural rivers, the Kura, Terek and Sulak rivers no longer supporting stocks. The Volga migration was 25,500 fish weighing 2600 t in the early 1970s but has fallen to 11,700 fish weighing 750 t. The commercial catch fell from 2000 t to 500 t. In the Volga River 96.3% of the spawning population consists of hatchery fish although the Ural River maintains a naturally reproducing stock.

Since stocks are maintained mostly by artificial rearing, this sturgeon has been proposed for inclusion in the "Red Book of the U.S.S.R." which forms the basis for measures to protect species (Pavlov et al., 1985; Mina, 1992). Stocks have been increased through rearing and natural reproduction in the Ural River, the number rising from 9.6 million in 1976 to 15.3 million in 1983, so the status of this species was then regarded as acceptable. However Lelek (1987) and Birstein (1993) list this species as vulnerable to endangered. Kiabi et al. (1999) consider this species to be endangered in the south Caspian Sea basin according to IUCN criteria as does IUCN and CITES (Vecsei et al., 2002). The U. S. Fish and Wildlife Service lists it as threatened under the U.S. Endangered Species Act as of 21 October 2004 (http://news/fws.gov/newsreleases, (dated 20 April 2004) and downloaded 22 April 2004) and the Wildlife Service has been petitioned to make it endangered (Speer et al., 2000). Endangered status would stop importation of flesh and caviar to the United States. Suspension of trade in this species from the Black Sea basin by the U.S. Fish and Wildlife Service was instituted in 2005 (Federal Register, 2005) and imports from Iran are banned for political reasons along with other sturgeons. Criteria for the various status assessments include commercial overfishing (fishermen cannot even catch the set quotas), failure of regulatory oversight, few in numbers, habitat destruction, dams preventing spawning migrations, medium range (25-75% of water bodies), absent in other water bodies in Iran, poaching, pollution, diseases due to pollution, and presence outside the Caspian Sea basin. The World Wildlife Federation (WWF) listed this species as number 4 on the top 10 most endangered species in the world (www.extravalue.com/sturgeon.shtml, downloaded 13 March 2000). The species status may be changed to Appendix I on the CITES listing, when international trade in its caviar would be banned (Vecsei et al., 2002). The export quota for this sturgeon in the Caspian Sea 2004 was reduced to 4425 kg although an illegal harvest was still substantial (www.tehrantimes.com, downloaded 14 October 2004).

Illegal fishing from 1990 onward and cessation of hatchery releases will lead to loss of the stock unless an agreement between Caspian states can be reached to protect this species.

The invasion of the ctenophore Mnemiopsis has led to declines in the kilka (Clupeonella spp.) stocks, a prime food of fil mahi (Kideys, 2002).

Caviar from Russian caught fil mahi bought in New York stores has been examined for pollutant content (Boyle, 1994). Three stores carried caviar with 3.17-3.27 parts per million of DDT plus its metabolites DDD and DDE, 410-640 parts per billion of the PCB Arclor 1254, and 2.1-2.8 parts per million selenium. These values are below the U.S. Food and Drug Administration's action levels of 5 parts per million for DDT, 2 parts per million of PCB and 10-50 parts per billion of selenium in drinking water. Nevertheless they are cause for concern.

Various studies have been carried out on the aquaculture of this valuable sturgeon in Iran. Mohseni et al., (2000) have studied effective stocking density of eggs and larvae in incubators and rearing tanks in order to maximise production and avoid various morphological deformities.

Abdolhay and Tahori (2006) give fingerling production as:-

Process/Year	2000	2001	2002	2003	2004
Female broodstock captured	32	29	29	48	16
Injected broodstock	19	14	21	30	9
Spawning rate * (%)	74	71.4	62	65	77
Fertilisation rate (%)	55	65.5	65	54	65
Survival rate in incubators (%)	62	73.4	62	32	72
Survival rate in tanks (%)	80	62	56	100	79
Stocking density in ponds (fish/ha)	82,100	51,639	51,333	52,359	65,448
Survival rate in ponds (%)	73	51.3	67	43	59
Fingerling production (x 1000)	1900	640	24,037	42	146

* Rate of response to hormone injection