Cirrhinus microlepis can be found throughout the survey area, from the Mekong delta to Chiang Saen near the border between the Lao PDR, Thailand and Myanmar.

Its migration patterns are markedly different above and below the Khone Falls. Below the Falls, an upstream migration from Phnom Penh to the Khone Falls occurs between November and February, consisting mainly of sub-adult fish of sizes from 10 to 50 cm. From April to July, Cirrhinus microlepis migrate in the opposite direction, downstream from the Khone Falls. That migration, which also comprises mainly sub-adults up to about 50 cm in size, appears to be less conspicuous as no reports of downstream migration were made by some stations along that stretch of the river.

In the Mekong delta, only downstream migrations were reported, comprising mainly juveniles sized between 2 and 20 cm. The smallest fish were mainly reported from June-July, whereas Cirrhinus microlepis sized between 10 and 20 cm were mainly reported from September to November.

The migration pattern above the Khone Falls is less clear. From Klong Kaem district, Ubon Ratchatani, Cirrhinus microlepis migrate upstream in February, while further upstream in Ubon Ratchatani, at Khemmaratch, they migrate upstream during March-April. At Mukdahan, they migrate upstream in May. However, during June-July, they migrate downstream at Klong Kaem. During that period, fish in reproductive condition are reported.

Along the river stretch from Savannakhet to Xayaboury, no migration was reported. At Loei, the species is found throughout the year. From Xayaboury to Chiang Saen, upstream migrations occur from March to August. Again, there appear to be two distinct migrations, one of sub-adults (ranging from 15 to 50 cm) during March and April, and a second migration during June and July that comprises large Cirrhinus microlepis within the range of 40 to 90 cm. The latter movement appears to be a spawning migration as mature fish bearing eggs were reported during July-August.

Previous studies at the Khone Falls have indicated that Cirrhinus microlepis is one of the most important fish for the local fisheries during the dry season from January to March, when it migrates upstream (Warren et al, 1998; Baird, 1998). From December to February it is also an important species for the dai fisheries in the Tonle Sap River when it migrates downstream into the Mekong River (Lieng et al., 1995).
Artificial spawning has been induced in Cirrhinus microlepis at Pakse, in Champassak province, the Lao PDR (Bouakhamvongsa et al., 1994). Local fishermen reported that in June they would catch broodstock 52 km south of Pakse town at what they believed to be a spawning site. They reported the spawning migration to be a downstream one and indicated that its origin might be as far upstream as Khammouan.

Hypothesis:
There are at least two populations of Cirrhinus microlepis:

(a) One population from Loei to Chiang Saen undertakes upstream spawning migrations from May to August to spawning grounds within the main river channel, where spawning occurs in June-July.

(b) The second population occurs in the stretch of the river from Boulikhamxay in the north to the Mekong delta. In fact, it may be two populations with some degree of overlap, since spawning was reported to occur in the Mekong mainstream, both above and below the Khone Falls (July-August). The eggs and larvae drift downstream and out into the flooded areas. Juveniles and adults also move downstream and out into the floodplains during the flood season, particularly in southern Cambodia and Viet Nam, and up through the Tonle Sap River system. When the water begins to recede at the end of the flood season, the fish move back into rivers where they start a non-reproductive upstream dispersal migration.

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