There appears to be some taxonomic confusion regarding the genus Henicorhynchus. In a recent taxonomic revision of the cyprinid genus, Cirrhinus, Henicorhynchus was treated as a junior synonym of Cirrhinus (Roberts, 1997). We choose to follow the nomenclature of Rainboth (1996) and therefore, at least for the time being, use the name Henicorhynchus siamensis.

Together with another species from the same genus, Henicorhynchus lobatus, Henicorhynchus siamensis may be the most abundant fish species in the middle and lower Mekong (Roberts, 1997). It has been suggested that Henicorhynchus lobatus is an ecological keystone species (Roberts and Baird, 1995; Roberts, 1997). The information provided by fishermen during the AMFC survey was based on a photograph of Henicorhynchus siamensis. Roberts (1997) indicated that it might be difficult to distinguish between specimens of the two species and, therefore, some of the information provided by fishers may possibly be related to either, or both, species.

According to the present survey, the distribution of Henicorhynchus siamensis ranges from the Mekong delta all the way along the Mekong River to Chiang Khong, near the border between the Lao PDR, Thailand and Myanmar.

This species is well known for lateral migrations into the floodplains during the flood season and then returning to the rivers when the floodwater begins to recede (Rainboth, 1996). The genus Henicorhynchus is the most important group of fish in the dai fisheries of the Tonle Sap River, constituting more than 60 per cent of the catch from November to February (Lieng et al., 1995). During that period, the fish move into the Tonle Sap River from flooded areas along the river as well as the Great Lake. After reaching the Tonle Sap, they migrate down to the Mekong River and continue their journey upstream, at least until they reach the Khone Falls (Lieng et al., 1995; Baird 1998).

The results of the present survey support the above findings. All the stations from just upstream of Phnom Penh to the Khone Falls reported that Henicorhynchus siamensis migrated upstream between October and February. At Muk Kompul district in Kandal province of Cambodia, the species was reported to migrate upstream just before the full moon. Further upstream, at two stations near Kratie, it was reported to migrate during full moon and at Sambor, a little further upstream, they were reported to migrate immediately after the full moon. Some fishers exploit that migration by following the fish all the way to Stung Treng.

Near the Khone Falls, the migration pattern appears to be less conspicuous. Upstream movements continue through March, but in April the fish apparently move in both directions. From May to July, with the onset of the monsoon season, the fish migrate downstream from the Khone Falls, a movement reported by all the stations down to the Mekong delta. Here, the fish migrate into canals and flooded areas during August-September (e.g., as reported from Cay Lai, Tien Giang province, in Viet Nam). This migration includes the movement of very small fish, (young-of-the-year) as well as mature fish measuring about 20 cm. As the water recedes (November-December), the fish migrate back to the Mekong.

Upstream of the Khone Falls, migratory movements are also less conspicuous. Near Ubon Ratchatani, Henicorhynchus siamensis migrate upstream from February to June. In February and March, that movement consists mainly of juveniles, whereas from April to June, it comprises adults (15-20 cm). Further north, from Xayaboury to Chiang Khong, upstream migrations occur from March to July, first by juveniles and later by adults.

Observations of mature eggs were reported from April to July, with a strong peak during May and June. Nearly all stations downstream from Savannakhet-Mukdahan reported that May and June was the spawning period. At Sambor, one fisherman reported observing mature females "releasing eggs that then flowed downstream" during May.

At Chiang Khong, the fish were reported as migrating into tributaries to spawn from May to July. At Loei, spawning was reported to occur in July and August in a Mekong tributary (Loei River) in a small pool with a slow current. In general, spawning apparently occurs over a longer period and extends into August and September from Loei and upstream.

The limited information obtained for the stretch from Savannakhet-Mukdahan to Loei may indicate the occurrence of two different populations, or even species, above and belowthat stretch, respectively.

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