According to the present survey, the distribution range for Pangasianodon hypophthalmus is from the Mekong delta all the way up the Mekong to Chiang Khong and Bokeo in the north. However, it was not recognised at Chiang Saen, near the border between Thailand, the Lao PDR and Myanmar.

Four stations in the middle Mekong, from Mukdahan in the south to That Phanom in the north, did not report observing this species. Information from Khammouan and Boulikhamxay provinces in the Lao PDR was also very limited. However, from Sungkom district (Nong Khai province of Thailand) to Chiang Khong, upstream migration was reported from May to July. Two of those stations (Loei and Xayabouri) reported eggs and milt during the migration. At Loei, the fish were reported to "swim upstream near the surface, early in the morning".

In general, this species appears to be very rare above the Khone Falls. Furthermore, in that stretch of the river, mainly large-sized individuals were seen (e.g., 90 cm and above). Juveniles were not observed in that stretch, except at one place (Loei) where sizes of 15 to 30 cm were reported.

South of the Khone Falls, a pronounced upstream migration was reported from October to February (peaking in November-December). All stations from Kandal to Stung Treng (where it extends into April) consistently reported this migration. It is apparently triggered by receding water levels and appears to be a dispersal migration, following the lateral migration from flooded areas back into the Mekong River at the end of the flood season. Two stations (Kratie and Kompong Cham) reported that the migration occurs during the full moon.

From May to August, a migration takes place in the opposite direction, downstream from Stung Treng to Kandal and into the Mekong delta, at least as far as Cai Be. This movement was reported by 13 stations, including a flood-plain station in Tien Giang province of Viet Nam.

Eggs were reported from Stung Treng to Kandal between March and August, with a strong peak during June-July. Thus, the downstream migration is both a spawning migration and a trophic migration that eventually brings the fish into flood-plain areas in Cambodia and Viet Nam during the flood season.

In An Giang and Dong Thap provinces of Viet Nam, larvae of Pangasianodon hypophthalmus are caught every year in June-July during their downstream drift from spawning site(s) somewhere upstream in Cambodia. They are caught in specialised larvae dai nets just south of the Cambodian-Vietnamese border, and are used as stocking material in the cage culture industry in Viet Nam. According to one "larvae" fisherman from the Chao Doc area, at least four species of pangasiids were caught in the larval stages at Chao Doc during June-July until 1998, when the activity was banned at that site. The most important species were Pangasianodon hypophthalmus and, to a lesser extent, Pangasius bocourti (see below). The two other species were not identified. During the two-month period of operation, several (usually three) peaks of Pangasianodon larvae occur.

The occurrence of larvae (or young-of-the-year) was also reported during the present survey. At three stations in Kompong Cham province and one station in Kandal province in Cambodia, as well as two stations in Viet Nam, fish larvae of a size of 2 cm were reported from May to July.

In general, the fish from the Mekong delta are below 50 cm, with the majority being below 30 cm.

Pangasianodon hypophthalmus is one of five pangasiid species considered important in the Khone Falls lee trap fishery industry from May to July each year (Baird, 1998). The wing trap is designed to catch fishes when they migrate over the waterfalls. However, in 1994, the species was not found in lee catches.

The same study described a gill-net fishery operation at Ban Hang Khone, just south of the Khone Falls during the same period, which targeted Pangasius krempfi (see below). This fishery recorded 28 species over a four-year period (1993-1997). However, Pangasianodon hypophthalmus was not among them (Baird, 1998).

Hypothesis:
Pangasianodon hypophthalmus spawns in deep pools in the Mekong mainstream somewhere between Kratie and the Khone Falls at the beginning of the flood season. When the eggs hatch, the larvae drift downstream until they are swept out into flood-plain areas in southern Cambodia and Viet Nam. At that time, the current in the Tonle Sap River reverses, resulting in a proportion of the larvae drifting up the Tonle Sap and out into flooded areas along the Tonle Sap River and the Great Lake. When the water level begins to recede at the end of the monsoon season, the fish return to the Mekong mainstream and begin a dispersal migration upstream through Cambodia to near the Khone Falls.
Therefore, from the Khone Falls and downstream, Pangasianodon hypophthalmus consists of one singlepopulation .

Another distinct population spawns at an, as yet, unidentified spawning ground(s) further upstream, at least as far as Xayabouri. There may be some degree of overlap between the two populations, although the lack of reports from the stretch in the middle Mekong from Mukdahan to Nakhon Phanom suggests that any such overlap is limited.

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