Morulius chrysophekadion is one of the large cyprinids. The survey confirmed its distribution throughout the basin, from the northernmost stations in the Lao PDR and Thailand to the southernmost stations in Bassac and the Mekong delta in Viet Nam.

There is some variation in the maximum size attained by this fish in the four countries. In Viet Nam, maximum reported size of Morulius chrysophekadion was 40 cm, while in Cambodia and the Lao PDR it was 70 cm. In Thailand, fishes of 90 cm were reported (about 7 kg), which is considerably higher than the maximum size of 60 cm, reported by Rainboth (1996).

Spawning behaviour had not been directly observed by any of the interviewed fishermen. Although eggs were reported to have been observed in the abdomen of the species between February and October, there are significantly more reports on such observations from April to July. It thus appears that the species has a relatively long spawning season. This assumption is supported by the fact that small juveniles (2-4 cm) were reported year-round, with the highest number of reports being made for the October-November period. This indicates a main spawning period around August. Bardach (1959) reported the occurrence of spawning in Cambodia during June-July.
Juvenile Morulius chrysophekadion were also recorded in small numbers during the juvenile study carried out by AMFC in An Giang province, Viet Nam, in June-July 1999.

One fisherman in Viet Nam suggested that Morulius chrysophekadion spawns in flooded ricefields and grasslands. That suggestion is consistent with Smith (1945), who indicated that the species had an important spawning ground in the Bung Borapet swamp in the Chao Phraya catchment in Thailand.

Fishermen in the Lao PDR and Thailand agreed that Morulius chrysophekadion migrates upstream from March to August. Downstream migration in October was only reported by one fisherman in northern Thailand.

In Cambodia, upstream migration was reported from October to March, while the downstream migration was observed from March to August. In Viet Nam, there is very little information on migrations in the mainstream of the Mekong and Bassac rivers. The information that does exist is contradictory.

There were several reports of the species migrating into tributaries, small streams and canals. In the Lao PDR and Thailand, the species apparently starts migrating when the water changes from being clear to a reddish-brown colour (at the start of the monsoon season) or when the water level begins to rise.

Hypothesis:
Multiple populations of Morulius chrysophekadion exist along the Mekong River. In the Lao PDR and Thailand, the species starts its upstream migration at the onset of the monsoon season, and continues to move into tributaries and flood-plain areas to spawn. In northern Cambodia, the species migrates downstream at the same time and moves into the floodplains to spawn further downstream, with some specimens even travelling all the way to the Great Lake.

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