According to Rainboth (1996), Pangasius polyuranodon occurs in the Mekong as far upstream as Stung Treng. The present survey indicates that the species can be found all the way to Xayaboury in the Lao PDR. However, none of the stations in Thailand reported the species, indicating that it is rare in the middle Mekong, from the Khone Falls and upstream.

Below the Khone Falls, the migration pattern of Pangasius polyuranodon is similar to other migratory fishes. It undertakes upstream migrations from Kandal province in Cambodia to the Khone Falls, mainly triggered by receding water levels at the end of the flood season. This migration may take place all the way from the Mekong delta; however, only one station reported this movement in Viet Nam (My Tho town, Tien Giang province). Although the species appears to be common in the Mekong delta, (i.e., it was reported by most stations in Viet Nam), Vietnamese fishermen do not consider it to be a long-distance migratory species.

Downstream migrations during the onset of the flood season, triggered by a combination of early rainfall and rising water levels, were reported by stations from Thalabowirrat in Stung Treng province, to the Mekong delta. These are reproductive migrations, i.e., the reports of eggs indicate a spawning period from May to August.

Above the Khone Falls, the species migrates upstream at the onset of the flood season. This migration is also triggered by a combination of water level changes, the first rainfall and changes in water colour (turbidity). Many fishermen along that stretch use the appearance of certain insects, especially dragonflies, as an indication that migrations are underway.

One fisherman in Khanthaboury, Savannakhet province, reported that Pangasius polyuranodon migrates upstream in schools together with other species. However, the downstream migrations are more sporadic and not with other species.

Pangasius polyuranodon, of a sizes within the range of 20 to 345 g, have previously been reported in the dry season gillnet fisheries at Ban Hang Khone, just below the Khone Falls, during December-February (Baird, 1998). Onle few specimen were reported by the same author in the 'tone' fishery during the beginning of flood season (Baird, 1998).

 

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