According to Rainboth (1996), this species bears a strong resemblance to Pangasius djambal (see below), a fact that was also observed by some of the fishermen contributing to the present survey. A fisherman from Chiang Khong, Thailand, stated that the two species are the same, whereas a fisherman from Nakhon Phanom, Thailand, mentioned that the two species are very similar in appearance and behaviour.

However, some problems still exist with the taxonomy of this species, particularly in the case of the upper stretch of the survey area along the Lao-Thai border. Rainboth suggested that the species currently believed to be Pangasius bocourti along that stretch could in fact be another, so far undescribed, species. As with many other Mekong fish, there is an urgent need to clarify the taxonomy of this species in the Mekong context (i.e., many type specimens on which identifications are made, originate from relatively few sites within the Mekong). This problem should be kept in mind when analysing the data for this species.

According to the present survey, Pangasius bocourti occurs throughout the survey range, from Chiang Saen in the north to the Mekong delta.

Again, the area around the Khone Falls appears to be a borderline between two different migration patterns. Below the Khone Falls, an upstream migration begins in November when the water level within the river decreases and continues well into the dry season, at least until February. This migration was reported by all stations, from the Cambodia-Viet Nam border to just south of the Khone Falls. In the late dry season, or the beginning of the monsoon season, a downstream migration occurs along the same stretch, from the Khone Falls to the Mekong delta.

From just below the Khone Falls, an upstream migration begins at the onset of the flood season. This migration was reported by eight stations, from just below the Khone Falls all the way to Chiang Saen in the north. From Boulikhamxay province and upstream, the migration occurs a little earlier, from April to June; however, nearer the Khone Falls, in Mukdahan and Ubon Ratchatani provinces, it occurs in July-August. The migration appears to be triggered by a combination of rising water levels, strong rain and increased turbidity.

Observations on mature eggs were scattered evenly over a long period, from March to August. This pattern may indicate different populations, separated genetically by the timing of their reproductive behaviour. Considering the taxonomic confusion that still exists over Pangasius bocourti, it could also reflect the fact that two different species are involved.

Previous studies at the Khone Falls identified Pangasius bocourti as one of the migratory pangasiid catfishes that migrate over the Khone Falls (Baird, 1998; Singanouvong et al., 1996b).

Hypothesis:
There are two different populations of Pangasius bocourti in the Mekong River. One occurs from the Mekong delta to the Mukdahan-Savannakhet area upstream from the Khone Falls. This population may consist of two sub-populations with some degree of genetic overlap.

Another population occurs from the area around Boulikhamxay-Nong Khai provinces to the area around Chiang Rai-Bokeo provinces in the north. This population may constitute a separate species.

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