According to the findings of the present survey, the maximum size of Pangasius macronema is about 28 cm, which is somewhat larger than 18 and 20 cm reported by Rainboth (1996) and Roberts and Vidthayanon (1991), respectively. In spite of being one of the smallest pangasiid species, it is very important to local fisheries, a fact that is reflected in the number of fishermen that were able to provide information about the habits of the species. Pangasius macronema, however, is very similar to several other schilbeid and pangasiid species, and cases of misidentification may have occurred.

The principal habitat of this species appears to be the Mekong mainstream; only one fisherman, in Khammouan province of the Lao PDR, reported that it migrates up into small streams. The distribution range covers most of the Mekong except for a short stretch covering Nakhon Phanom and Mukdahan provinces in Thailand.

In Cambodia and Viet Nam, the species is generally present throughout the year, although in varying degrees of abundance. During the period when it is migrating it is more abundant.

Northwards from Boulikhamxay province in the Lao PDR, the species migrates upstream during April to August. The survey data indicate, however, that May and June comprise the main period of upstream migration in that region. Fishermen in Loei province of Thailand and Xayaboury province in the Lao PDR concurrently reported downstream migration during October-November (another fisherman in Xayaboury, however, reported upstream migration during the same period).

In Cambodia, an upstream migration starts in November and continues until January/February. One fisherman in Stung Treng province also reported that Pangasius macronema migrates upstream during May-June, while fishermen at four stations reported the fish moving downstream during the same period.

Very little information was provided on the direction of the migrations in Viet Nam, but at the two lowermost stations in the Mekong delta, Pangasius macronema were reported to migrate downstream from July to November.

Pangasius macronema has previously been reported to have passed the Khone Falls in southern Lao PDR during April-May, when it forms the basis for one of the most important fisheries in that region (Roberts and Baird, 1995; Baird et. al., 1999).

Limited information only is available about the breeding habits of this species. Only one fisherman from Tien Giang province in Viet Nam reported that the species spawns in the Mekong mainstream during August-September. This was later than the period reported by Bardach (1959), who stated that Pangasius macronema spawned in Cambodia in June.

Eggs were observed in the abdomen of this species throughout the year except in February. The largest number of reports was for April to June. One fisherman in Cambodia reported that Pangasius macronema carried eggs twice a year, during May-June and November-December. Since he was the only person to have reported that fact, it is possible that there was confusion between two species, and further verification is needed. One fisher in Loei province of Thailand reported that the fish migrates downstream during October-November.

Juveniles sized 2-4 cm were observed throughout the year in all countries except the Lao PDR, where juveniles smaller than 6 cm were not reported. The average of all the minimum sizes reported for all countries, month for month, indicates that spawning mainly takes place between August and October.

Many fishermen provided information about natural events that coincides with the migrations of Pangasius macronema. Changes in water level appear to have an influence on the movements of this species.. Seven Lao fishermen mentioned changes in the colour of the water as an important indicator of peak occurrence.. Eight fishermen in the Lao PDR and two in Thailand affirmed that the peak occurrence of this species coincided with the emergence of insects. Most of the fishermen identified dragonflies, while one mentioned termites and stoneflies.

Hypothesis:
Three populations of Pangasius macronema exist.

The distribution range of the first population is from Thakhek, Khammouan province of the Lao PDR to Chiang Saen, in Chiang Rai province of Thailand. This population migrates upstream during May-June at the onset of the flood period, to spawn in the upper reaches of the Mekong River. After spawning, adults and juveniles spread out through the area.

The second population is distributed from Khemmaratch, Ubon Ratchathani province of Thailand in the north, down to Kratie province in Cambodia. This population breeds above the Khone Falls. After spawning, juveniles and adult fish disperse throughout the area. When the water level starts decreasing in November, Pangasius macronema migrates to some of the deep pools (e.g., near Kratie), where they stay until the next flood.

The third population partly overlaps with the second one. It is distributed from Stung Treng in Cambodia, down to the Bassac River in Can Tho province and to the Mekong delta. This population spawns in the area around Quatre Bras at the confluence of the Tonle Sap and Mekong rivers. These fish migrate to, and remain in, the same area as the second population during the dry season.

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