Wallago attu

Wallago attu is a freshwater catfish of the family Siluridae, native to South and Southeast Asia. It is commonly known as helicopter catfish or wallago catfish. Some regional designations, such as the Indian Sareng, the Bengal Boal, the Sylheti Gual or the Malaysian and Indonesian Tapah are also occasionally used in English. W. attu is found in large rivers and lakes in two geographically disconnected regions (disjunct distribution), with one population living over much of the Indian Subcontinent and the other in parts of Southeast Asia. The species can reach a total length of 1 m (3 ft 3 in). It shares parts of its native range with the externally similar, but much larger Wallagonia leerii, and is subsequently often confused for it. This catfish is one of the fish species that has been used as food in Southeast Asia since ancient times. In many areas, Wallago attu is found alongside the externally similar and related catfish species Wallagonia leerii. In Indonesia and Malaysia, both species are referred to as ikan tapah, and in English, both are sometimes called helicopter catfish. Popular accounts such as media reports, claims by fishermen, or local folklore stories, and even scientific publications, often confuse the two or are altogether unaware of the difference. Therefore, claims exist that Wallago attu reaches lengths of more than 1.8 m (5 ft 11 in) and weights of more than 45 kg (99 lb). Biologists, however, are firm that it does not grow beyond a length of roughly 1 m (3 ft 3 in). The current rod-and-reel angling record for a Wallago attu caught in the wild and authenticated by the International Game Fish Association is a specimen of 18 kg (40 lb) from the Vajiralongkorn dam reservoir in Thailand, while some specialized recreational catch-and-release breeding ponds in the region claim to harbour specimens in the 20–30 kg (44–66 lb) range. It is thus assumed that reports about specimens even larger than that actually refer to Wallagonia leerii, which can grow to twice the length and several times the weight of Wallago attu. Wallago attu lives through large parts of South and Southeast Asia. Its range, however, seems discontiguous with a significant gap between the population inhabiting the Indian subcontinent and the one found across mainland and insular Southeast Asia. W. attu thus stands as an example for a species with a disjunct distribution. On the Indian subcontinent, its range includes all the major rivers of India, Pakistan, Bhutan, Nepal and Bangladesh, such as the Ganges, Indus, Narmada, Godavari, Krishna and Mahanadi as well as the island of Sri Lanka. To the Northwest, its range extends beyond Pakistan into Iran and Afghanistan. To the East, it can be found as far as the Irrawaddy river basin in Myanmar. The second population occurs in Southeast Asia and encompasses Thailand, Laos, Cambodia, Vietnam, Malaysia and Indonesia. Here it inhabits the Mae Klong, Chao Phraya and Mekong drainages, as well as those of the Malayan peninsula and the islands of Java and Sumatra. It is absent from Borneo, which ichthyologist Tyson R. Roberts considers 'surprising'. The gap between the two populations is largely formed by the Salween and Tenasserim river drainages in Burma, where W. attu is not found. The reasons for this disjunct distribution are unknown.