Pangasius larnaudiei occurs throughout the Mekong mainstream, from the Mekong delta to Chiang Saen near the border between the Lao PDR, Thailand and Myanmar.

Together with Pangasius conchophilus, this species is the most important in the wet season fishery (May to June) during its migration through Hoo Som Yai, a steep-gradient channel at the fault line of the Khone Falls (Singanouvong et al., 1996b). Unlike Pangasius conchophilus, however, these upstream migrations by Pangasius larnaudiei are more protracted, with regular smaller movements taking place over a number of weeks. Singanouvong et al. (1996b) considered the ultimate purpose of the migrations to be reproduction. In the tone trap and lee trap fisheries at Ban Hang Khone (Khone Falls), Pangasius larnaudiei ranked third after Pangasius conchophilus and Pangasius krempfi (Baird, 1998).

Again, for this species, the Khone Falls forms a borderline between two different migration patterns.

Below the Khone Falls, the species migrates upstream at the beginning of the dry season, triggered by receding water levels. This migration was reported by all the stations from Dong Thap province in Viet Nam to the Khone Falls. At the onset of the monsoon season, the species migrates in the opposite direction, downstream from the Khone Falls. The downstream migration was also reported by all stations except those in Chloung and Prek Pra Soup districts of Kratie Province, Cambodia.

Above the Khone Falls, the species migrates upstream at the onset of the monsoon season (reported by seven stations). Only one station (Loei, Thailand) reported a downstream migration, which occurs during August-September. Two Thai stations, in Mukdahan and Ubon Ratchatani provinces, respectively, reported that Pangasius larnaudiei is the last species to undertake the upstream migrations at the onset of the monsoon season. A fisherman at Ubon Ratchatani mentioned that Pangasius larnaudiei migrates just after the upstream migration of Pangasius krempfi.

The main trigger of these migrations appeared to be a combination of changes in water levels (12 reports) and changes in water colour (seven reports).

Pangasius larnaudiei is apparently one of the few species of the pangasid family that can be seen near the water surface. This fact was confirmed by at least two stations, located at Chiang Saen and Ubon Ratchatani, where fishermen reported observing them at the surface. Four stations (three in Thailand, one in Cambodia) specifically reported that the species lives in deep pools in the river.

From Kratie and downstream, Pangasius larnaudiei was reported moving between the Mekong River and smaller streams. However, at Loei, the species apparently remains in the Mekong throughout the year.

Eggs and/or milt were reported by 12 stations throughout the survey area during the following months: March (2 reports), April (1), May (6), June (7), July (4), August (1) and September (1). This pattern indicates a peak spawning time from May to August.

Below the Khone Falls, juveniles of 2-4 cm were reported by six stations, from Kompong Cham downstream to the Mekong delta (four stations in Cambodia and two in Viet Nam). All the stations reported observing juveniles from June to October, peaking between June and August. Above the Khone Falls, the smallest sizes were recorded by one station in Xayabouri, which reported juveniles of 6 cm from June to August. At Ubon Ratchatani (Klong Kaem), juveniles of 10-14 cm were reported during June-July. Apart from those stations, only large fish (30 to 90 cm) were reported from upstream of the Khone Falls.

Hypothesis:
Pangasius larnaudiei spends the dry season in deep pools at several places within the Mekong, both upstream and downstream from the Khone Falls. Below the Khone Falls, the stretch down to Kompong Cham constitutes a particularly important dry-season habitat.

The onset of the monsoon season and associated rises in the water level in the Mekong trigger the migration of Pangasius larnaudiei to as yet unidentified spawning grounds where it spawns from May to August. The occurrence of 6-cm juveniles indicates that spawning takes place somewhere further upstream. Below the Khone Falls, juveniles are only recorded from Kompong Cham to the Mekong delta, indicating that spawning takes places somewhere further upstream, above and/or just below the Khone Falls (e.g., from Kratie to Stung Treng).

Juveniles and adults move into the flooded areas and return to the Mekong River when water levels start to recede at the end of the monsoon season. Some of the bigger individuals, especially in the upper sections of the Mekong, may remain in the main channel during the monsoon season.

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