Wallago attu
The distribution of Wallago attu is from the Mekong delta up to northern Lao PDR and Thailand. Reported to grow to over 90 cm, it is one of the large Mekong species. Rainboth (1996) stated that the fish reach a size of 200 cm.
There is some confusion as to what degree the species migrates in the Mekong mainstream. However, there appears to be consensus between fishermen in the four survey countries that the species migrates to smaller streams, canals and the floodplain at some stage during the flood season. During the dry season, this species lives in deep pools. The migration appears to have the dual purpose of (a) pursuing food, especially at the time where smaller fishes are migrating, and (b) spawning.
Eggs were reported to be present in the abdomen of Wallago attu from March to October, with most fishermen reporting eggs from May to July. Juveniles smaller than 4 cm are found from June to December, with an apparent peak during October-November. One fisherman in Chiang Khong, northern Thailand, reported that: "In June-July, groups of fish larger than 2 kg spawn in shallow water on flooded grassland. The eggs attach to the substrate and hatch within three days." Another Thai fisherman in Loei province reported personally observing spawning in the Huai Kid reservoir near the mouth of the Huai Kid stream. Small juveniles were also observed in a swamp in Chiang Rai province of Thailand.
One Vietnamese fisherman reported that the fish breed in ricefields. The timing of spawning coincides with that reported by Bardach (1959), who stated that Wallago attu spawns from May to October, with peak activity from July to September.
Hypothesis:
Wallago attu only undertakes short longitudinal migrations to
the nearest stream, as well as some localised movements to pursue
schools of smaller fish on which it preys. During the flood
season it stays in swamps, canals and streams on the flood plain,
where it also spawns. When the water level in the Mekong mainstream
drops and the floodwater recedes, Wallago attu migrates to the
Mekong River or larger tributaries, where it lives in deep pools
until the next inundation period.