The sheatfishes are found in Europe and Asia. There are about 11 genera and about 97 species (Nelson, 2006) with 2 reported from Iran. The phylogenetic relationships within the family are examined by Bornbusch (1995).

This family is characterised by a scaleless and elongate body; a moderately compressed head; a non-protractile mouth; teeth on the jaws and palate; 1-4 pairs of barbels (nasal barbels usually absent; maxillary barbels 1-2 pairs, sometimes vestigial or absent); nostrils separate, anterior ones tubular; 4-21 branchiostegal rays; gill openings very wide; dorsal fin short and spineless (usually fewer than 7 rays and sometimes absent); anal fin very long (41 or more rays) and may be confluent with the caudal fin; adipose fin absent; pectoral fin with a spine, often serrated; and pelvic fins small to absent. The largest species is found in Iran (Silurus glanis).

Genus Silurus
Linnaeus, 1758

These catfishes comprise about 5 species found from Europe to China and India.

This genus is characterised by an elongate body, rounded anteriorly but compressed posteriorly; a depressed head; 2-3 pairs of barbels, the maxillary barbels well-developed and often as long as, or longer than, the head; a large and terminal or superior mouth; teeth in bands on the jaws and roof of the mouth; nostrils well separated; eyes small and not visible from the underside of the head; a very short and spineless dorsal fin; no adipose fin; anal fin very long and united to the rounded caudal fin; pectoral fin with a strong serrated spine; and branchiostegal rays 12-15. The genus has been revised by Kobayakawa (1989).

Krieg et al. (1999) isolated microsatellite loci in both S. glanis and S. triostegus and found that the species diverged less than 20MYA and/or high levels of genomic conservation. Krieg et al. (2000) investigated mitochondrial DNA in S. glanis but found no consistent pattern of geographic structuring in European populations, evidence that gene flow and migration between populations were possible until quite recently. Their study also included S. triostegus and the data was diagnostic for the two species.

A general Farsi name for these fishes is گربه ماهي (= gorbeh mahi, meaning cat fish).

Silurus glanis
Linnaeus, 1758

Aras River Dam, courtesy of Asghar Abdoli (measuring fish)
2.25 m, ca. 90 kg, 5 October 1994

Safid River, 2 km west of Astaneh, 4 June 1978, caught in a large dip-net

Common names

mahi-e sebili, esbele or esbeleh in Gilaki (probably derived from sibil meaning moustache in reference to the barbels), esbele-ye orupaiye, Urupai or Europaiye; گربه ماهي روگاهي or gorbeh mahi rogahee, nake or naque in the Lake Orumiyeh basin, where they are known as "whales".

[naxa, nagka or nakki in Azerbaijan; loko in Armenia; som in Russian; European catfish, sheatfish, wels, wels catfish, Danube catfish].

Systematics

Silurus Glanis was originally described from lakes of Europe, Sweden. A skin is a syntype in the Natural History Museum, London under BM(NH) 1853.11.12:168 (Eschmeyer et al., 1996). An image of this type is available at http://acsi.acnatsci.org/base/image_show_wrapper.html?target=230943. Silurus chantrei Sauvage, 1882 is possibly a synonym (see under Silurus triostegus).

Key characters

This species differs from S. triostegus by having weaker and shorter teeth, the upper and lower jaws meet at an antero-dorsal position (dorsal and superior position in S. triostegus), a less serrate pectoral fin spine posteriorly, and a darker colour. Maxillary barbel length is much longer on average, although there is some overlap. Anterior mandibular barbels are alwayslonger than posterior mandibular barbels while in S. triostegus the posterior mandibular barbels are always longer (Ünlü and Bozkurt, 1996).

Morphology

The lower jaw is longer than the upper. Adults have one pair of maxillary barbels and two pairs of mandibular barbels, for a total of 6 barbels. The maxillary barbel is much longer than the head (equal to head length in Silurus triostegus). The pectoral fin spine is finely serrated or smooth on its inner surface and smooth on its outer surface. Vomerine teeth form a single broad patch, not two as in S. triostegus (Kobayakawa, 1989).

Dorsal fin branched rays 3-5, usually 4, anal fin branched rays 70-108, pectoral rays 12-18 with 1 spine (generally higher on average than in S. triostegus but still overlapping), and pelvic rays 1 unbranched followed by 9-14 branched rays (Coad and Holčík, 2000; Reshetnikov, 2002). Vertebrae 70-76 and total gill rakers 9-17 (counts of 9 and 10 may be lower arch rakers only). Total vertebrae 67-74. In specimens examined by me dorsal fin branched rays 3-4, anal fin branched rays 83-87, pectoral rays 15-16 with 1 spine, and pelvic rays 1 unbranched followed by 11-12 branched rays. Vertebrae 18-19 + 54-56 = 72-74. Total gill rakers 12, reaching the raker below when appressed. The gut has a large stomach and an intestine with 3 about loops. Chromosome number is 2n=60 (Ráb et al., 1994; Klinkhardt et al., 1995).

Sexual dimorphism

Abdurakhmanov (1962) reports that females have longer maxillary barbels, a longer postorbital length and a greater caudal peduncle depth than males in Azerbaijan.

Colour

The body is mottled with brown, green or dark grey, even ventrally, over the base colour. The back is dark, from olive-brown to a blue-black, the sides lighter and the belly greyish-white with bluish speckles. Fins are a dark red-brown to brown-violet. Paired fins have a yellowish streak in the middle. Iris yellowish with black speckles. This species can blend its colour with any bottom on which it lies in wait for prey (Fortunatova, 1961).

Size

Reaches legendary sizes of 5 m and 336 kg but most are much smaller. In the Volga Delta females reach 1.75 m and 31 kg and males 1.95 m and 41 kg (Orlova, 1988); in Dagestan specimens up to 1.93 m and 41.3 kg are recorded although fish weighing 3.2-4.8 kg predominate (Shikhshabekov, 1978).

In the Caspian Sea commercial fishery of Iran, this species ranges in size from 41 to 186 cm and 0.6 to 42 kg (Farid-Pak, no date). Sohrabi (1996a) reports the larger fish in Iranian waters usually weigh 10-40 kg and depicts two fish from Gilan. One caught in 1995 weighed 27 kg, the other caught in 1994 weighed 62 kg. The record fish from Iran, caught by a Mr. Haratonian weighed 120 kg and was 2.2 m long. Eastwick (1864) bought a specimen 4.5 feet long (1.37 m) in the Safid River which had a 7 lb (3.2 kg) fish in its stomach. De Mecquenem (1908) reported that they reached 2 m in the Lake Orumiyeh basin and Anonymous (1977) reports fish from there at 400 lbs (= ca. 182 kg). Asghar Abdoli of the Agricultural and Natural Resources University, Gorgan kindly sent me a photograph of a specimen from the Aras Dam caught 5 October 1994 which was 2.25 m long and weighed about 90 kg.

Distribution

This species is found in Europe, Central Asia and Southwest Asia. In Iran it is found along the whole Caspian coast from the Aras River and Dam, and from the Astara to the Atrak rivers including the Anzali Mordab and the Manjil Dam on the Safid River, from the southwest Caspian Sea and south-central Caspian Sea, and in the Lake Orumiyeh basin (Günther, 1899; Nedoshivin and Il'in, 1929; Berg, 1936; Nümann, 1966; Holčík and Oláh, 1992; Nejatsanatee, 1994; Riazi, 1996; Abbasi et al., 1999; Kiabi et al., 1999; Jolodar and Abdoli, 2004). Also reported from the Karakum Canal and Kopetdag Reservoir in Turkmenistan (Aliev et al., 1988; Shakirova and Sukhanova, 1994; Sal'nikov, 1995) and may enter Iranian waters of the Tedzhen (= Hari) River basin.

Abdoli (2000) maps this species from lower Gorgan, Neka, Babol, Heraz, Chalus, Tonekabon, and Safid rivers, the Anzali Talab, along the Caspian coast, and in the middle Aras River, and in the lower Talkheh and Zarrineh rivers of the Lake Orumiyeh basin.

Also aid to occur in the Tigris-Euphrates basin in Iraq but needs specimens for confirmation (see Freshwater Fishes of Iraq).

Zoogeography

The isolated populations of this wide-ranging species have not been studied. It may well have an origin in the Ponto-Caspian basin from which it has dispersed.

Habitat

This large species is found in the larger water bodies over soft bottoms and can tolerate brackish water (even for spawning) and moderately low oxygen levels. Warm, deep waters with slow current are preferred. It is active at night. Adults are solitary and found under overhanging banks or submerged trees. This catfish overwinters in aggregations on river beds. Resumed activity in spring depends on the local water temperature regime, probably as early as March in Iran. Riazi (1996) reports that this species is native (resident) to the Siah-Keshim Protected Region of the Anzali Mordab. Knipovich (1921) reports this species from depths of 23.8-25.6 m in the Iranian Caspian Sea.

Movements, such as hunting for food, are stimulated by such environmental factors as temperature, solar radiation, air pressure and turbidity after rain. It is sensitive to extra-aquatic sounds. The head canal system is very sensitive and can track prey over distances up to 55 times the length of the prey and follow signals up to 10 seconds old (Kottelat and Freyhof, 2007).

Age and growth

Life span is 22 years for males and 16 years for females in the Volga Delta. Growth is most intensive in the first years of life. After maturity the annual increase in length is 5-7 cm. Relative weight increase is very high (30%) while length increase is 6-10%. June-July is the period of greatest length increase while weight increase takes place mainly in autumn. Maturity begins at 3-4 years, 57-66 cm and 1.3-2.3 kg but may be as early as 2 years, 51-52 cm and 1.2-2.2 kg or as late as the sixth year (Orlova, 1988). Maximum life span is 80 years.

Abbasi and Valipour (2006) found 9 age groups in the Anzali Lagoon with females making up 68.5% of fish caught. Total length of 95 fish was 19.8-186.5 cm and weight was 47.7-30,000 g.

Food

Food in the Volga Delta includes such fishes as Cyprinus carpio, Abramis brama, Scardinius erythrophthalmus and, prior to the regulation of the Volga, Rutilus rutilus and herrings (Clupeidae). Crustaceans now form part of the diet there (Orlova, 1988). At one time this catfish in the Volga delta ate 62-68% of its annual food in one month in spring when the Caspian roach (Rutilus rutilus) arrived on its spawning run. In the Kura region, commercial fishes such as Cyprinus carpio, Abramis brama, Rutilus rutilus, Aspius aspius, Sander lucioperca, Chalcalburnus (= Alburnus) chalcoides, Barbus brachycephalus or Barbus capito, and Silurus glanis make up 30.27% by frequency and 20.18% by weight, non-commercial species such as Scardinius erythrophthalmus, Tinca tinca, Cobitis spp., Rhodeus amarus, Pungitius platygaster, Atherina boyeri, Alburnus alburnus, Blicca bjoerkna, and Caspiomyzon wagneri make up 50.68% and 33.79% respectively, marine fishes such as Clupeidae, Mugilidae, and Gobiidae make up 12.90% and 8.60% respectively, and crustaceans 25.64% and 17.09% respectively (Mamedov and Abbasov, 1990). In Azerbaijan, Abdurakhmanov (1962) reports Gobio gobio to comprise 22.4% of the diet, Caspiomyzon wagneri 15.7% and eggs 2.2%, Chalcalburnus (= Alburnus) chalcoides 10.2%, Alburnus alburnus 10.2%, Cobitis taenia 9%, Barbus lacerta 7.8%, Barbus capito 6.7%, Chondrostoma oxyrhynchum 5.6%, Capoeta capoeta 3.4%, Blicca bjoerkna 2.2%, and loaches 4.5%.

In the Anzali Lagoon of Iran according to Abbasi and Valipour (2006), this species ate 78.6% bony fishes, 15.8% crustaceans, 4.13% insects, 0.9% amphibians and 0.5% bivalves. Carassius auratus dominated at 33.9%, followed by the crustacean Macrobrachium spp., at 14.22%, Neogobius kessleri at 4.59% and Proterorhinus marmoratus at 2.75%. C. auratus dominated in spring and autumn, Macrobrachium spp. in summer and N. kessleri in winter. Consumption of fish increased with size, being 44.2% at 20-55 cm and 94.5% at 91-125 cm, Cannibalism was not observed and commercial fish stocks were not consumed.

Generally a wide variety of fishes is taken along with crayfish, frogs and even birds and small aquatic mammals. It is a voracious predator but stories of attacks on dogs and small children are more legendary than factual although human remains may be scavenged (Gudger, 1945b). Active feeding occurs at water temperatures above 8°C so winter feeding is minimal or absent. While feeding often occurs at night, catfish can be heard feeding in the evenings by the snapping of the mouth and tail strikes on the water. In cloudy water conditions they come into shallow water to take earthworms, grasshoppers and frogs washed in from nearby fields (Mihálik, 1982).

Young catfish feed on plankton, particularly Cladocera such as Daphnia, Chydorus, Alona and Bosmina among others. Later the diet involves mosquito larvae, larger crustaceans, organisms associated with the river bank, worms, snails and young fishes. Cannibalism occurs if food is short (Mihálik, 1982).

Reproduction

Non-intermittent spawning in Dagestan takes place in late May and continues to the middle of July when the eggs of females mature at a water temperature of 20-22°C. Spawning rarely occurs below 20°C. Males may actually have running milt 30-40 days earlier than this and also later, a longer potential spawning season. Adhesive yellow eggs are laid in depressions in weed beds, formed by the male pressing on the plants. Fecundity is up to 285,000 eggs with diameters around 2-3 mm (to 467,000 elsewhere). Elsewhere spawning may be intermittent (Shikhshabekov, 1978).

The male guards the incubating eggs, even during the day, moving his tail fin every 3-5 minutes to ensure adequate oxygen supplies. Nests in Europe may be on the fine roots of plants which hang freely in the water. The nest is in shallow, 40-60 cm, water. Males pursue females just under the water surface, an indication spawning will occur the same evening or the next day. Spawning usually occurs in the evening, often before a thunderstorm on warm and stifling days. The male nudges the female in the anal region, swims under her and may lift her so that her back is above water, the male wraps himself around the female for 10-12 seconds, the male and female separate and the female sinks slowly to the bottom and discharges eggs, the male following to release milt. This process can be repeated several times over 1.5-2.0 hours and the water around the nest is milky from sexual products. Eggs hatch after 2.5-3.0 days at 23-25°C. Larvae are light sensitive and die in direct sunlight ands also if water temperature falls below 13-14°C (Mihálik, 1982).

Parasites and predators

Mokhayer (1976b) records the digenetic trematodes Aphanurus stossichi and Bunocotyle cingulata, the nematode larvae Anisakis sp. and the nematode adults Cucullanus sphaerocephala, and the acanthocephalan Corynosoma caspicum. Ataee and Eslami (1999, www.mondialvet99.com, downloaded 31 May 2000) report the helminth Mazocea alaosa from the gastro-intetsinal tract of fish from the Anzali wetland. Masoumian et al. (2005) recorded the protozoan parasite Trichodina perforata from this species in the Aras Dam in West Azarbayjan. Khara et al. (2006a) record the eye fluke Diplostomum spathaceum for this fish in the Amirkalayeh Wetland in Gilan. Sattari et al. (2005) surveyed this species in the Anzali and Amirkelayeh wetlands, recording Raphidascaris acus, Raphidascaroides sp. and Eustrongyloides excisus.

The Caspian seal, Pusa caspica, is a predator on this species (Krylov, 1984).

Economic importance

There is some opportunity for sport fishing for this species in the Anzali Mordab and Lake Orumiyeh basin where it will take spinners and spoons as well as frog live bait. It reputedly puts up a tremendous fight (Anonymous, 1977).

Nevraev (1929) reports on catches in various regions of Iran in the early years of the twentieth century. There were no evident trends of increase or decrease. In the Astara region from 1901-1902 to 1913-1914 the catch varied irregularly from 699 to 4031 fish, in the Anzali region from 1901-1902 to 1918-1919 the catch varied from 18,177 to 206,485 fish, in the Safid River region from 1899-1900 to 1917-1918 the catch varied from 3290 to 43,835 fish, in the Mazandaran region from 1906-1907 to 1913-1914 the catch varied from 5282 to 11,283 fish, and in the Astrabad region from 1902-1903 to 1912-1913 the catch varied from 3500 to 26,200 fish. The commercial catch in Iran from 1956/1957 to 1961/1962 varied between 4,913 kg and 37,630 kg (Vladykov, 1964) and from 1965/66 to 1968/69 varied from 11 to 31 tonnes (Andersskog, 1970) but in the 6 years from 1980 to 1985 catches were recorded by the Food and Agriculture Organization, Rome as respectively 2, 2, 0, 2, 3, and 0 tonnes. The catch has been as high as 107,593 kg for the Anzali region alone in 1934/1935 (Vladykov, 1964). Holčík and Oláh (1992) report a catch of 2663 kg in the Anzali Mordab in 1990, at 3.6% of the catch the sixth most important fish there, and from 1932-1964 reported catches varied from none to 12.6 tonnes annually. The Iranian fishery peaks in October and in April (Farid-Pak, no date). This species is of great commercial importance in Dagestan (Shikhshabekov, 1978). The roe has been used as a form of caviar and glue has been made from the swimbladder and bones. In the Lake Orumiyeh basin this fish was used as fertiliser since, being scaleless, it could not be eaten for religious reasons (De Mecquenem, 1908).

Robins et al. (1991) list this species as important to North Americans. Importance is based on its use as food, for industrial processes (such as fertilizers, fish meal, pet food, novelty products although not specified for this species), in sport, and in textbooks. The swimbladders were used for isinglass in the Caspian Sea basin (Lönnberg, 1900a).

This species has been implicated in ichthyootoxism, the symptoms of which are summarised under the genus Schizothorax. The presence of venom associated with the pectoral fins (Coad, 1979b) needs definitive examination.

Conservation

Lelek (1987) classifies this species as rare to vulnerable in Europe because of habitat changes and angling pressure. Kiabi et al. (1999) consider this species to be of least concern in the south Caspian Sea basin according to IUCN criteria. Criteria include commercial fishing, sport fishing, medium numbers, habitat destruction, widespread range (75% of water bodies), present in other water bodies in Iran, and present outside the Caspian Sea basin. Vulnerable in Turkey (Fricke et al., 2007).

Further work

Population numbers need to be monitored carefully as it is fished for although it lacks scales. Populations in isolated basins have not been examined in detail for their relationships to see if they are distinct.

Sources

Kobayakawa (1989) revised the genus Silurus and his data are incorporated here. Mihálik (1982) reviewed the biology of this catfish.

Iranian material: CMNFI 1970-0509, 1, 131.2 mm standard length, Gilan, Safid River at Hasan Kiadeh (37º24'N, 49º58'E); CMNFI 1979-0685, 2, 63.9-107.7 mm standard length, Gilan, Safid River below Dehcha (ca. 37º22'N, ca. 50º06'E); CMNFI 1979-0788, 2, 293.5-335.6 mm standard length, Mazandaran, Gorgan River at Khvajeh Nafas (37º00'N, 54º07'E); CMNFI 1979-1236, 1, 269.0 mm standard length, Mazandaran, Gorgan River at Khvajeh Nafas (37º00'N, 54º07'E); CMNFI 1980-0123, 1, 225.3 mm standard length, Gilan, Safid River around Dehcha (37º22'N, 50º06'E); CMNFI 1980-0905, 1, 208.0 mm standard length, Mazandaran, Gorgan River at Khvajeh Nafas (37º00'N, 54º07'E); OSU 4278, 2, 227.2-280.2 mm standard length, Azarbayjan-e Bakhtari, Zarineh River (no other locality data).

Silurus triostegus
Heckel, 1843

NMW 92345, syntype, Tigris River near Mosul, ca. 582 mm standard length

Common names

esbele, and jirri, yeri, yery, yari or iry (= eel), esbele-ye beinolnahrein (= Mesopotamian catfish); sag mahi (= dog fish) in Khuzestan; gorbeh mahi (= catfish).

[djirri, jirri, girri or yerri (= eel) in Arabic; Mesopotamian catfish, Tigris catfish].

Systematics

Silurus chantrei Sauvage, 1882, a species with 4 barbels, was described from the "Fleuve Koura à Tiflis (= Kura River at Tbilisi, Georgia) but was possibly based on material from the Tigris-Euphrates basin (Berg, 1948-49; Haig, 1952) and may well be a synonym of this species. Two syntypes of Silurus chantrei, 160-170 mm total length, are in the Muséum national d'Histoire naturelle, Paris under MNHN A.3932 (Eschmeyer et al., 1996).

Günther (1899), Banister (1980) and Hora and Misra (1943) considered that S. triostegus may not be distinct from S. glanis but Coad and Holčík (2000) detail differences.

The type locality of Silurus triostegus is the "Tigris bei Mossul" according to Heckel (1843) and the description was based on 4 specimens although the catalogue in Vienna lists 1 specimen in spirits and 2 stuffed specimens. The card index in Vienna in 1997 lists only NMW 92345 as a dried syntype. Coad and Holčík (2000) found only a single stuffed type, ca. 582 mm standard length. Eschmeyer et al. (1996) list 1 dried syntype of triostegus in the Senckenberg Museum Frankfurt (SMF 2623, formerly NMW) and this has a standard length of ca. 710 mm. An image of S. triostegus (SMF 2623) is available at http://acsi.acnatsci.org/base/image_show_wrapper.html?target=221708.

Key characters

This species differs from S. glanis by having robust and longer teeth (snaggly, catching on flesh), the upper and lower jaws meet at a dorsal and superior position (antero-dorsal in S. glanis), a distinctly and coarsely serrate pectoral fin spine posteriorly, and a lighter colour. Maxillary barbel length is about equal to head length while in S. glanis it is much longer on average, although there is some overlap. Anterior mandibular barbels (when present) are always shorter than posterior mandibular barbels while in Silurus glanis the anterior mandibular barbels are always longer (Ünlü and Bozkurt, 1996).

Morphology

Bears 4 barbels in some specimens as opposed to the usual 6 in S. glanis but there is evidence that the four-barbelled Silurus (sometimes placed in a distinct genus Parasilurus Bleeker, 1862, e.g. in Berg (1949), now synonymised with Silurus - see Eschmeyer (1990)) have 6 barbels when young and one pair of mandibular barbels is reabsorbed (see Haig, 1952). Specimens with one pair and with two pairs of mandibular barbels have been reported (Kobayakawa, 1989). Ünlü and Bozkurt (1996) record 4 mandibular barbels (2 pairs) in 3 specimens, 3 barbels in 1 specimen and 1 pair in another specimen for Turkish Euphrates fish. In Silurus triostegus, the adults apparently lose one pair of barbels (F. Krupp, in litt., 1992). Coad and Holčík (2000) found all S. glanis in their study had 4 mandibular barbels (2 pairs) while in S. triostegus 11 fish had 4 mandibular barbels and 12 fish lacked the posterior mandibular pair. There was no apparent trend in barbel loss associated with increase in body size.

The pectoral fin spine is strongly serrated on its inner surface and smooth on its outer surface. Vomerine teeth are in two patches, not one as in Silurus glanis (Kobayakawa, 1989), although Ünlü and Bozkurt (1996) record some specimens of S. triostegus with only one patch. The maxillary barbel reaches only to the end of the head, not much longer as in S. glanis. However, Ünlü and Bozkurt (1996) report that maxillary barbels are longer than the head and later that they reach the end of the head; it is suggested here that barbel length varies individually. The lower jaw is longer than the upper jaw. Teeth in both jaws are recurved, the band of teeth is wider than in S. glanis, and the teeth are stronger and longer. The eye is larger than in S. glanis.

Dorsal fin branched rays 3-4 (counts of 3 rays are more common than in S. glanis; 3 in ten fish and 4 in thirteen fish examined by Coad and Holčík (2000)), pectoral fin branched rays 11-14 with 1 spine, pelvic fin branched rays 8-13 after 1 unbranched ray, anal fin rays 77-94, vertebrae 16-17 + 52-53 = 69-70, and total gill rakers 12-17, reaching the one below when appressed (Coad and Holčík, 2000). The stomach is large, and apparently more elonagte than in S. glanis although this may be distortion due to food content. The intestine has about 3 loops.

Sexual dimorphism

Unknown.

Colour

The upper body is mottled pale yellow-brown and black. Overall colour may appear dark or light and yellowish. Generally much lighter than Silurus glanis. The belly and lower head are white with the belly having black spots. Maxillary barbels and margin of the lower jaw very dark brown.

Size

Reaches 4.5 kg in Iraq (van den Eelaart, 1954; Herzog, 1967), 1.5 m in the Syrian Euphrates (Gruvel, 1931) and to more than 2 m (Krupp, 1992).

Distribution

This species is found in the Tigris-Euphrates basin including its Iranian portion in Khuzestan and such rivers as the Arvand, Bahmanshir, and Jarrahi, and the lower Karun, Karkheh and Dez (Marammazi, 1995; Abdoli, 2000).

Zoogeography

This species is presumably a relative of S. glanis but its closest affinities may lie with species to the east. Differentiation of the two species may have occurred around the Middle to late Miocene but this requires further study (Coad and Holčík, 2000).

Habitat

van den Eelaart (1954) reports this species from open and vegetated lakes and marshes and rivers in Iraq. The larger fish are mostly confined to rivers, entering marshes and lakes only on floods. The young have a greater tolerance of high temperatures and low oxygen.

Age and growth

Al-Abood (1989) found age groups 2 to 7 years for fish from a marsh area north of Basrah, Iraq in 6 weight groups from 300 to 3900 g. Al-Hassan and Al-Sayab (1994) examined 600 specimens from the Al-Hammar Marsh north of Basrah, Iraq for age using vertebrae and eye lens diameter and found 6 age groups. Oymak et al. (2001) describe 11 age groups in Atatürk Dam Lake, Turkey with females having higher L_∞ (202.85 cm versus 113.98) and lower K (0.046871 versus 0.101972) values than males. Males matured at age 3 and females at age 4.

Food

Fish are an important food including Liza abu and Acanthobrama marmid (Al-Shamma'a and Jasim, 1993; Ünlü and Bozkurt, 1996; personal observations). Aquatic insects are also taken but fish predominate.

Fish are an important food including Liza abu and Acanthobrama marmid in the Iraqi marshes and presumably those across the border in Iran (Al-Shamma'a and Jasim, 1993; Ünlü and Bozkurt, 1996; Dawood, 1997; personal observations). The food in Hawr al Hammar, Iraq was predominately fish (Liza abu, Aphanius spp., Aspius vorax, Thryssa spp., Acanthobrama marmid, Silurus triostegus (young less than 16 cm were eaten by adults longer than 35 cm), Barbus sharpeyi, Heteropneustes fossilis and Cyprinus carpio) followed by shrimps (mainly Metapenaeus affinis), frogs (Rana esculenta) and crabs (mainly Sesarma boulengeri) with relative importance indices of 70.8, 16.3, 6.4 and 4.9 respectively (Al-Daham and Al-Seyab, 2000). Liza abu was the most important fish through most of the year (except July and August when absent)(relative importance index 42.0, followed by C. carpio at 11.5), in numerical abundance and total weight. During July the prey was B. sharpeyi and C. carpio and in August prey was restricted to C. carpio, presumably opportunistic feeding. Other fish species were mostly young of the year and of minor importance. The diet at Al-Fuhoud in the Hawr al Hammar was 70.7% fish by volume (Al-Shamma'a, 2005). Aquatic insects are also taken but fish predominated. Dawood (1997) also studied diet in the southern Hammar Marsh and found fish to be the most important prey year round while shrimps (Metapenaeus affinis) and molluscs were important  in certain months (mostly absent April-August). The disappearance of shrimps probably relates to their migratory pattern. Aquatic insects were found mostly in the spring. There is a reverse relationship between the presence of fish and shrimps. Fish and shrimps increased in the diet with increase in size while aquatic insects, molluscs and small crustaceans decreased with size. Fish species eaten were Liza abu, Barbus luteus, Alburnus sp., Cyprinus carpio, Thryssa hamiltoni, Heteropneustes fossilis, Aphanius sp., Gambusia holbrooki and Silurus triostegus. Frogs, detritus and aquatic plants were also found in the gut contents. Feeding occurred more at night and with another peak in late afternoon. The index of fullness values increased in April-September when water temperatures and metabolic rate rose.

Reproduction

Spawning takes place in March in Iraq (van den Eelaart, 1954; Al-Hassan et al., 1990) to May and June in Turkey (Oymak et al., 2001). The highest condition factors were found in April in Atatürk Dam Lake, Turkey, the mean egg diameter was greatest in May at 1.937 mm and fecundity attained 120,300 eggs (Oymak et al., 2001).

Parasites and predators

Mortazaei et al. (2000) report an infection rate of 33.3% (22 of 6 fish) with the parasitic worms Proteocephalus sp. in this species from Khuzestan marshes.

Economic importance

This species forms 8.5% of the total catch in Iraq (Das et al., 1978), the total catch in 1976 being 691 t (Petr, 1987), but it is not a popular food. As a scaleless fish it is not eaten by Shi'a Muslims. It was found to have potential for use as a protein concentrate.

Conservation

This species is not commonly collected in Iran but this may be a consequence of habitats sampled and gear used. It may be under some threat as it is fished for, at least in neighbouring Iraqi waters.

Further work

Population numbers have not been examined for fish in Iran nor are its biology or relationships well known.

Sources

Kobayakawa (1989) revised the genus Silurus and his data are incorporated here as is the data of Ünlü and Bozkurt (1996) for Turkish Euphrates specimens.

Type material: See above, Silurus triostegus (NMW 92345).

Iranian material: CMNFI 1993-0133, 1, 192.9 mm standard length, Khuzestan, probably Karun River at Ahvaz (31º19'N, 48º42'E); SNM-YR 6421, 1, 325.6 mm standard length, Khuzestan, Karun River at Ahvaz (31º19'N, 48º42'E); ZSM 21832, 1, 406.0 mm standard length, Khuzestan, Dez River at Harmaleh (31º57'N, 48º34'E); ZSM 21833, 1, 425.4 mm standard length, Khuzestan, Dez River at Harmaleh (31º57'N, 48º34'E); uncatalogued, 1, 317.7 mm standard length, Khuzestan, probably Karun River at Ahvaz (31º19'N, 48º42'E).

Comparative material: BM(NH) 1874.4.28:3-5 and 1875.1.14:8 (same jar), 4, 236.7-511.5 mm standard length, Iraq, Tigris River near Baghdad (ca. 33º21'N, ca. 44º25'E); BM(NH) 1888.5.17:1, 1, 336.2 mm standard length, Iraq, Fao, presumably a fish market (no other locality data); BM(NH) 1893.6.23:26-28, 3, 254.3-425.7 mm standard length, Iraq, Fao (no other locality data); BM(NH) 1892.9.1:26, 1, 270.8 mm standard length, Iraq, Fao (no other locality data); BM(NH) 1920.3.3:167-176, 13, 123.1-490.9 mm standard length, Iraq, Basrah (30°30'N, 47°47'E).

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