Catlocarpio siamensis is the largest of the Mekong cyprinids. It is said to grow to a length of up to three meters (Smith, 1945; Rainboth, 1996). In this survey the largest reported size weighed 150 kg.

Catlocarpio siamensis is now a rare fish species. However, the species is apparently encountered regularly at several stations ranging from Nong Khai, Thailand, and further north up to Chiang Saen, both on the Thai and the Lao sides of the Mekong River. It has also been recorded at stations in Nakhon Phanom and Ubon Ratchathani, Thailand. It is more common in Cambodia and Viet Nam, where it is encountered all the year at many stations.

Juveniles (2 to 6 cm in length) were reported by three locations in Thailand: Chiang Saen (Chiang Rai province), Tad Phanom (Nakhon Phanom province) and Khemmaratch, (Ubon Ratchathani province). In Cambodia, juveniles of the same size were reported in Sray Son Thor (Kompong Cham province) and Muk Kompul (Kandal province). In Viet Nam, juveniles have been recorded in Can Tho (Can Tho province) and Cao Lanh (Dong Thap province) in the Mekong River and in some canals.

Juveniles of 10-14 cm in length were also reported in the Songkhram River, Thailand, in November during the Phase I (trial) of the survey.

While the adults have a preference for large pools in the Mekong River, at least during part of the year, juveniles are mostly seen in floodplain habitats and small tributaries, from where they are sometimes collected and stocked in ponds. This is consistent with information given by Smith (1945), that "this is a fish of the large streams" but that "it breeds in Bung Borapet and other bungs (swamps) into which the floodwater of the river flows". However, large mature fish have not been observed in floodplain habitats and it is more likely that Catlocarpio spawns in certain habitats within the main river channel, from where eggs and/or larvae reach rearing habitats on the floodplain partly by passive drift.

Eggs are seen from January to August, but most fishermen reported finding eggs from May to July. This is consistent with the presence of juveniles, 2-4 cm in length, from July to November.

Hypothesis:
Above the Khone Falls, there are at least three populations of Catlocarpio siamensis. All three populations migrate into tributaries to spawn either in the tributary itself or within the associated floodplain.

One population occur in the upper part of the survey area and migrate from the Mekong mainstream to tributaries, e.g., in Chiang Rai province.

Further downstream, a population occurs around Nakhon Phanom province from where it migrates into tributaries, e.g., the Songkhram River.

Finally, there appears to be a population in and around Ubon Ratchathani province, which also migrates into tributaries to spawn.

Below the Khone Falls, there appears to be only one population. Spawning occurs mainly in the upper part of the stretch in the Mekong (i.e., upstream from Phnom Penh) and possibly also in the Sesan tributary system.

When the water begins to recede at the end of the flood season, the young-of-the-year and sub-adults migrate from flooded areas back into the main river channels and tributaries. During the dry season, Catlocarpio siamensis lives in deep pools associated with the main river channels.

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