Wallago attu

Wallago attu, boal, or helicopter catfish, is a species of freshwater catfish of the family Siluridae, native to South and Southeast Asia. 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. This species can reach a length up to 1.1 m (3 ft 7 in).

It shares parts of its native range with the externally similar, but much larger Wallagonia leerii, and is subsequently often confused for it. It can, however, be differentiated by its relatively long and narrower head, as well as its dorsal fin, which is high and sharp, opposed to that of W. leerii, which is lower and rounded. Additionally, the eyes of W. attu lie above the mouth of the fish, whilst they lie on the same level as the mouth in W. leerii.

This catfish is one of the fish species that has been used as food in Southeast Asia since ancient times. In Thailand, its meat is popularly used to make thot man (ทอดมัน, "Thai fish cakes"), similar to clown featherback (Chitala ornata).

Habitat

As a large, predatory fish, W. attu maintains a largely piscivorous diet. Gut content analysis performed on specimens from the Godavari river in India shows that about 90 to 95% of the consumed food consisted of animal matter. Among the prey fishes most commonly found in the stomachs of Godavari river W. attu are razorbelly minnows (Salmophasia phulo), ticto barbs (Pethia ticto) and perchlets (Chanda nama), all of which are small species that reach maximum lengths of about 10–12 cm (3.9–4.7 in).

Distribution

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.