Aaptosyax grypus
Aaptosyax grypus has previously been reported to be a rare endemic species of the Mekong region with distribution limited to large rivers in the middle Mekong (Rainboth, 1996). It is, furthermore, listed as "Data deficient" on the IUCN List of Endangered Animals.
The survey indicated a distribution range within the Mekong mainstream from Sambor in Cambodia to Loei in Thailand. However, one of the reports from Loei stated that the species was no longer found there, indicating that present-day distribution may have become significantly reduced.
Detailed migration information was obtained from four stations from O Krieng in Sambor district, Cambodia, to Klong Kaem in Ubon Ratchatani province, Thailand.
Aaptosyax grypus is reported to be a pelagic species that migrates at the same time as Probarbus, i.e., from December to February. This movement coincides with the timing of the reported upstream migration of small cyprinids in the same stretch of the Mekong River. Since Aaptosyax grypus is a predatory species, it could be hypothesised that it migrates upstream in order to follow its prey comprising smaller migratory fishes.
Roberts (1993) suggested that the upstream migration starting from late December might be a spawning migration. According to Dr. Chavalit Vidthayanon of the Department of Fisheries in Thailand, the species spawns during the dry season (April) in pools near rapids in the Mekong mainstream. However, no information was obtained on spawning during the survey.
Migrating Aaptosyax are all big fish and no information has been obtained on smaller specimens. The only existing report of juveniles is that of two specimens weighing 100 g, which were caught by gill nets in June 1996 at Ban Hang Khone, southern Lao PDR, just south of the Khone Falls (Baird 1998). Specimens have also been caught in the Songkhram and Moon rivers.
The survey confirmed that Aaptosyax grypus is an extremely rare species. All the interviewed fishermen agreed on this conclusion and one of them even reported that it had been three years since he had last seen any of this species.