Helicophagus hypophthalmus Sauvage, 1878
The iridescent shark (Pangasianodon hypophthalmus) is a species of shark catfish (family Pangasiidae) native to the rivers of Southeast Asia. It is not a shark. It is found in the Mekong basin as well as the Chao Phraya River, and is heavily cultivated for food there. The meat is often marketed under the common name swai. It has also been introduced into other river basins as a food source, and its striking appearance and iridescence have made it popular with fishkeeping hobbyists. The swai's omnivorous diet consists of crustaceans, other fish, and plant matter.
The fish is named for the glow or iridescence exhibited in juveniles, as well as the shark-like appearance of this and other shark catfish. It is also known as Siamese shark or sutchi catfish in the aquarium hobby.
Distribution and habitat
Iridescent sharks originate from the large rivers Chao Phraya and Mekong in Asia, though they have been introduced into other rivers for aquaculture. They are a freshwater fish that natively live in a tropical climate and prefer water with a 6.5–7.5 pH, a water hardness of 2.0–29.0 dGH, and a temperature range of 22–26 °C (72–79 °F). They prefer large bodies of water similar to the deep waters of their native Mekong river basin.
The iridescent shark is a migratory fish that moves upstream to spawn during the flood season while the waters are high and returns downstream to seek rearing habitats when the river water levels recede. Upstream migration in this species appears to be triggered by receding waters. At the end of the flood season, the fish migrate back downstream away from flooded waters. The dates of the migrations vary depending on the river system. In the Mekong river basin, they migrate upstream in May to July and return downstream during September through December. South of the Khone Falls, upstream migration occurs in October to February, with its peak in November to December.
The fins are dark grey or black. Juveniles have a black stripe along the lateral line and a second black stripe below the lateral line; they have a shiny, iridescent color that gives these fish their name. However, large adults are uniformly grey and lack the striping. Adults reach up to 130 cm (4 ft) in length and can weigh up to a maximum of 44.0 kg (97 lb).
Despite being classified as endangered, iridescent shark is sold cheaply as swai (pronounced /swaɪ/) in the United States or panga in Spain, the second largest importer after Russia. It has a milder flavor and more delicate texture than the U.S.'s native channel catfish. Swai have moist, sweet, mild flavored flesh with a beige color which turns white after cooking. In the U.S. it is often sold as frozen skin-off fillets weighing from 2 oz to 11 oz each. Typical grading sizes are: 3-5 oz, 5-7 oz, and 7-9 oz. 
In the aquarium
Juvenile iridescent sharks are often sold as pets for home aquariums. However, they are not easy fish to keep, and are not recommended for home aquariums. Iridescent sharks are schooling fish that prefer to be kept in groups of five or more. Accustomed to living in rivers, they are very active fish that require a lot of space. They have very poor eyesight, so any foreign movement they detect outside of their habitat, they will see as an utter threat. If stressed, their first instinct is to flee, and such a blind dash can result in injuries, especially in an aquarium environment. These flights may be terminated by the fish sinking to the bottom, where it may lie on its side or back until it recovers.
Many fish owners are unaware of the enormous size an iridescent shark can reach and also that they have a very harsh bite. If given enough room and fed adequately, an individual of this species can reach 1 m (3 feet) in length. In most home aquariums, the amount of space an iridescent shark has severely stunts its growth. For this reason, most iridescent sharks kept in home aquaria grow to only 15 to 30 cm (6 to 12 inches) in length and die prematurely from organ failure.[dubious ] As a rule of thumb, an iridescent shark requires a minimum tank size of 12 m (40 feet) to develop naturally and live a long, healthy life. Schools require even larger tanks. When provided with adequate sized aquaria and proper husbandry, an iridescent shark may live well into its teens and grow to full size.
- Vidthayanon, C. & Hogan, Z. (2011). "Pangasianodon hypophthalmus". IUCN Red List of Threatened Species. Version 2012.2. International Union for Conservation of Nature. Retrieved 24 November 2012.
- "Pangasius hypophthalmus". Integrated Taxonomic Information System. Retrieved February 7, 2009.
- Froese, Rainer and Pauly, Daniel, eds. (2012). "Pangasianodon hypophthalmus" in FishBase. February 2012 version.
- Axelrod, Herbert, R. (1996). Exotic Tropical Fishes. T.F.H. Publications. ISBN 0-87666-543-1.
- and N. Van Zalinge, Lieng Sopha, Ngor Peng Bun, Heng Kong, J. Jørgensen: Status of the Mekong Pangasianodon hypophthalmus resources, with special reference to the stock shared between Cambodia and Viet Nam. In: MRC Technical Paper. Nr. 1, Mekong River Commission, Phnom Penh 2002, ISSN 1683-148
- "Chef's Resources - Swai Fish Profile". Chefs-resources.com. Retrieved 2012-06-22.
- Aqualand Pets
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