According to the present survey, the distribution range for Pangasius krempfi is from the Mekong delta and along the Mekong to Chiang Saen, near the border between Thailand, the Lao PDR and Myanmar.

Upstream of the Khone Falls, Pangasius krempfi is mainly seen during its upstream migration from May to September (12 out of 15 stations reported that migration; the remaining three stations did not report on this species). The migration is dominated by fish in spawning condition. Seven stations above the Khone Falls reported eggs occurring during the migration in June-September, with a peak in July. One station (Sungkom district, Nong Khai in Thailand) reported that fish releasing eggs were caught during July-August near Pa Sak Neua village. At the same village, the species was also reported during October-November. At that time, the fish do not have eggs.

The upstream migrations occur several times, in peaks of 3-5 days, during the migration period.

Only two reports of downstream movement were received from above the Khone Falls, both during October. According to the fishermen, downstream movements are much less conspicuous because, unlike upstream migrations, they are not undertaken in big schools within short peak periods.

Below the Khone Falls, eggs were observed (six reports) in the abdomen of Pangasius krempfi during May-August, with a peak in June-July. However, the migration pattern during that period is less clear. At Thalabovirrat, near the Khone Falls, the fish migrate upstream in May-June, in line with the reported migration further upstream.

From Stung Treng to Kompong Cham provinces in Cambodia, the fish migrate downstream during the same period, although less conspicuously (several stations did not report downstream migrations). At Sambor, one fisherman reported catching fish that were releasing eggs in June-July.

In Kandal province, near Phnom Penh, Pangasius krempfi was reported to migrate upstream from Viet Nam, from March to May. This migration occurs in peaks of up to five days and involves sizes between 40 to 90 cm. There were no reports on spawning condition.

Almost all the stations above the Khone Falls reported that most fish are within the range of 40 to 90 cm, with the majority measuring above 70 cm. At Chiang Khong in the north, fish weighing 7-10 kg were reported during the monsoon season migration. Most of the stations reported that they had never seen juveniles of this species. However, one station (Mukdahan, Thailand) reported seeing juveniles as small as 4 cm during May-June. Juveniles of such a size would appear to be at least two months old, which would thus imply a spawning time during March-April. As this information does not concur with the other reports on spawning for this species, it could be a case of misidentification and should be verified.

South of the Khone Falls, in Cambodia and Viet Nam, several reports on juveniles were received. Thalabovirrat, near the Khone Falls, reported fish of 6 cm, while Sambor reported sizes of 10 cm. In Sray Son Thor district of Kompong Cham province (Cambodia), 2-cm juveniles were reported from August to October.

In Viet Nam, most reports were of fishes of 10 to 70 cm. In Vinh Huu sub-district, Tien Giang province (near the saline intrusion zone), juveniles sized between 6 and 50 cm were reported in February, with sizes increasing as the dry season progresses (e.g., in April, the size range was reported to be between 20 and 50 cm). Only one station in Viet Nam (Cao Lanh, Dong Thap province) reported that the species undertakes migrations (upstream from November to February, and downstream from April to July, with both movements mainly involving large fish of 70 to 90 cm). There were no reports from the Mekong delta on spawning or the occurrence of eggs.

The results of previous studies at the Khone Falls support the findings that Pangasius krempfi spawn during the early flood season from May to July (Roberts and Baird, 1995). At Ban Hang Khone, from May to July, Pangasius krempfi is one of the most important species in the monsoon season for the wing trap fishery (Baird, 1998). All the fish migrating during that period were in reproductive condition, with the smallest weighing about 1.2 kg. The Ban Hang Khone study concluded that only reproductive individuals were thought to make the annual migration. The present survey supports that finding.

According to the Ban Hang Khone study, local fishermen believed that the migrations were triggered by rising water levels in the Mekong River (Baird, 1998). That view is supported by the findings of the present survey for below the Khone Falls, as well as above, in the Lao PDR and Thailand.

Pangasius krempfi is believed to spend part of its life in the sea (Roberts, 1993; Roberts and Baird, 1995). In fact, Roberts and Baird (1995) stated that Pangasius krempfi was an "anadromous fish with a life story resembling that of salmon". The present survey is obviously not able to verify this (i.e., marine fishermen were not interviewed). The occurrence throughout the year in the delta region, as well as several reports that juveniles were observed in the same area, indicate that at least part of the population remains within fresh water and brackish-water for most, if not all, of their lifecycle.

Some doubt remains about the status of the taxonomy of the species. Rainboth (1996) indicated that there might be two different species involved, one in the middle Mekong (based on specimens caught at Nong Khai) and one in the lower Mekong (Cambodia-Viet Nam).

Hypothesis:
There are two populations of Pangasius krempfi in the Mekong River. One population is centered in the upper reaches in Xayaboury and Luang Prabang provinces in Lao PDR. This population migrates upstream during May to September from downstream sites, possibly all the way from just south of the Khone Falls to spawning grounds along the mainstream Mekong all the way to Chiang Khong, near the Lao-Thai-Myanmar border. The larvae drift downstream with the current to unidentified nursing areas.

Another population migrates downstream from around Stung Treng to unidentified spawning grounds somewhere between Stung Treng and Kompong Cham during the spawning season, from May to August. The larvae drift downstream to nursing grounds in the flood plains in the Mekong delta. When the water level begins to recede in October, the fish migrate back to the main river and begin an upstream dispersal migration, reaching the stretch just below the Khone Falls. Pangasius krempfi spend the dry season in deep pools within the mainstream.

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