Bangana behri is found from Sambor, Cambodia, in the south to Chiang Khong, Thailand, in the north. According to Rainboth (1996), this species occursin rocky stretches of the mainstream during the dry season and moves to tributary streams during the high water period. During the present survey, several fishermen reported that the fish lives in association with rocky parts of the Mekong River. Since rocky stretches do not occur downstream of Sambor, the distribution pattern does, in fact, appear to be determined by the presence of rocky habitats. However, records from two stations in the Mekong delta in Viet Nam suggest that the species occurs there, thus contradicting the above theory. From the records of the two stations, Bangana behri are reported to be non-migratory, which is also contradictory to the above theory and to previous studies. These reports may be the result of misidentification and therefore need re-checking.

The migration pattern below the Khone Falls appears to be somewhat contradictory. At two stations in Stung Treng province, fishermen reported that the species migrates upstream at the onset of the monsoon season (May-June) and downstream in the dry season from November to February. At other stations in Sambor and Kratie, as well as two stations just south of the Khone Falls, the species was reported to migrate downstream at the onset of the monsoon season and upstream in the dry season. The reason for that movement appears to be the presence of the important tributary system comprising the Sekong-Sesan-Srepok rivers. Fishermen at stations near the tributary system reported that fish migrate from this system into the Mekong River during periods of receding water, and then migrate upstream in the tributaries during the monsoon season, possibly to spawn.

Upstream of the Khone Falls, this species begins migrating upstream in the dry season (February to May) and continues into the beginning of the monsoon season. This movement may, in fact, be two separate migrations: a dry season, non-reproductive migration of smaller fish, and an early monsoon season migration of larger fish in spawning condition.

Non-reproductive, dry-season migrations of Bangana behri have previously been reported from just above the Khone Falls between December and February, at which time it becomes one of the most important fish in the fisheries (Warren et al., 1998). These are mainly small specimens with a mean body weight of 275 g. Juveniles of the species, with mean sizes around 100 to 150 g (Baird, 1998), have also been reported by dry season fisheries at Ban Hang Khone, immediately downstream of the Khone Falls.

There were no reports during the two studies, both above and below the Khone Falls, on upstream migrations of this species during the early monsoon season.

Several accounts received during the survey indicated that the upstream migrations occurring from the Khone Falls all the way to Chiang Khong in northern Thailand are triggered by increasing water levels and changes in water colour (from clear to red-brown). The species was reported to migrate upstream in schools together with other cyprinids such as Labeo cf. pierry, Cirrhinus microlepis, Morulius chrysophekadion and Cyclocheilichthys enoplos, as well as the loach, Botia modesta.

These monsoon season migrations appear to comprise larger fish compared to the dry season migrations, and at Paksan and Xayaboury, in the Lao PDR, the fish were reported to have mature eggs during June-July migrations.

Hypothesis:
The distribution of Bangana behri is determined by the occurrence of rocks and boulders in the Mekong River.

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