Chitala blanchi
The distribution range of Chitala blanchi has previously been stated to be between Khemmaratch in Thailand, and Kratie in Cambodia (Rainboth, 1996). The restricted range, caused by the preference of this fish for mainstream habitats containing rocks and boulders, has resulted in the species being included in the IUCN Red List of Threatened Animals (listed as "lower risk/near threatened"). The species is endemic to the Mekong River.
The present survey indicates that the distribution range is somewhat larger, from Xayaboury in the Lao PDR to Kompong Cham in Cambodia. All three stations in Kompong Cham province reported that the species is rare, confirming the preference of this species for habitats associated with rocky substrates.
There is broad consensus among fishermen throughout the survey area that Chitala blanchi only undertakes short, local migrations. From Kratie downstream to Kompong Cham, the species was reported to migrate into smaller tributaries when the water level starts to rise, returning to the Mekong mainstream when water level begin to recede. At several places, from Kratie to the Khone Falls, the species was reported as common throughout the year. Further upstream (e.g., Xayaboury and Thakhek), water level changes were reported to be the main trigger of the local migrations.
Most fishermen reported that the species spends the entire year in deep pools in the main river channel. However, at a few places (e.g., Klong Kaem in Ubon Ratchatani province in Thailand), the fish were reported to enter tributaries during the flood season.
Eggs were observed in the fish during February (1 report), March (4), April (14), May (8), June (2), July (2), August (1), September (2), October (2), November (1) and December (1). This pattern indicates that spawning occurs over an extended period from the late dry season to the early flood season (March to June). According to one fisherman in Nakhon Phanom (Thailand), he had observed spawning activity of Chitala blanchi at several places in the Mekong during March and April in the main river channel in areas containing submerged wood and rocks. He said the female guards the fry.
Juveniles measuring 2-6 cm were reported at several places, e.g., Stung Treng, Kratie and Kompong Cham provinces in Cambodia, and Ubon Ratchatani and Nakhon Phanom provinces in Thailand. The smallest sizes, around 2 cm, were observed from March to July, again an indication of an extended spawning period from March to June.
(b) Chitala ornata
Within the Mekong River, Chitala ornata has a larger distribution range than Chitala blanchi, from Chiang Khong near the border between the Lao PDR, Thailand and Myanmar, to the Mekong delta. In Viet Nam, it was recorded by all stations except those in My Tho, Tien Giang province, and Cau Ke and Tieu Can districts, both in Tra Vinh province. Furthermore, as opposed to Chitala blanchi, this species is not endemic to the Mekong River, but is found throughout mainland South-East Asia (Rainboth, 1996).
Throughout its range, Chitala ornata was reported to migrate locally into smaller tributaries and flooded areas during the flood season, returning to the main river channel when water levels start to recede. Within the Mekong mainstream, it lives in pools. Several fishermen mentioned that the habits of Chitala ornata and Chitala blanchi were the same within the mainstream, including spawning habits. One fisherman in Mukdahan was able to distinguish between the two species by observing them when they surfaced, e.g., during spawning. There were more direct observations of spawning among this species, i.e., from Loei, Paksan, Nakhon Phanom and Ubon Ratchatani provinces. As with Chitala blanchi, eggs were laid on submerged wood from March to July, with the female guarding the fry. Smith (1945) stated that after spawning the female departs and the male guards the eggs.
(c) Notopterus notopterus
Notopterus notopterus is distributed throughout the area of the present survey, from the Mekong delta to Chiang Saen in Chiang Rai province, Thailand. Fishermen throughout its distribution range reported that the species only undertakes localised lateral migrations from the Mekong River to floodplains during the flood season, and back to the main river or other permanent water bodies during the dry season. Many fishermen, especially in Cambodia, stated that the species was more common in permanent lakes than in the main river channel.
Further upstream, in the Lao PDR and Thailand, the species is common in the Mekong mainstream as well as its tributaries, living in pools with submerged wood and shrubs. At several places, it was reported to move into tributaries during the flood season, where it apparently spawns.
Reports on eggs showed a strong peak in May (14 reports) and June (11 reports). Two fishermen in Viet Nam also reported observing eggs from September to November. Furthermore, two fishermen (one from Khemmaratch in Thailand, and one from Sambor in Cambodia), reported separately that the species spawns twice per year.
Three fishermen reported spawning behaviour. Two fishermen in Viet Nam observed spawning in flood-plain habitats (flood-plain grassland and flood-irrigated ricefields, respectively). One fisherman in Khemmaratch observed spawning in mainstream pool habitats, with the eggs attached to submerged vegetation.