Anders F. Poulsen and John Valbo-Jorgensen* Recent studies have indicated the very important role that deep pools have in the ecology of the Mekong River. They serve as dry season refuges for many species of fish, as permanent habitats for others, and they form an important link between habitats for migrating fishes. Many local communities have recognized their importance in fisheries terms, and have established management measures to protect them. Some deep pools downstream of dams have been lost in just a few years as a consequence of changes in flow regimes and siltation. One of the driving forces for fisheries ecological processes in the Mekong is the physical separation of important habitats (feeding, spawning and nursery areas, and dry season refuges). The vast floodplains associated with the lower Mekong are the feeding and rearing habitat for the majority of commercially important fish species. As floodwaters recede at the end of the monsoon season, it is crucial that fishes can seek refuge in dry season habitats. In general, dry season habitats are associated with the main river channels and with permanent water bodies on the floodplain. Within the main river channels, deeper sections of the river are used by a large number of species during the dry season. These are known as deep pools. Most of the information reported here relates to deep pools on the main river, but deep pools on tributaries are equally important. What is a deep pool? A deep pool is a confined, relatively deep area within a river channel, which acts as a dry season refuge for a number of important fish species, and as a permanent habitat for other species. Deep pools have been recognized elsewhere as important dry season refuges for fish in floodplain rivers (Welcomme 1985) but there is little documented information regarding their ecological function in major rivers. Much of the information we present here is from recent local knowledge surveys, in which more than 400 fishers were interviewed at 51 sites along the Mekong mainstream (see detailed report in Poulsen and Valbo-Jorgensen 2001). Deep pools in the Mekong In the Kratie-Stung Treng region of northern Cambodia, 58 deep pools have recently been documented (Vannaren and Kin in press). These pool habitats are critical for sustaining fisheries within the two provinces and for the whole of Cambodia and the Mekong Delta in Vietnam. Many species spawn in association with these pools and larvae and juveniles move downstream to the floodplain areas in Southern Cambodia and Vietnam. Some of the species using these habitats include Boesemania microlepis, Mystus wyckioides, Chitala ornata, Micronema apogon, Pangasianodon hypoph- thalmus, Cirrhinus microlepis, Probarbus jullieni, Catlocarpio siamensis and even the famous Mekong giant catfish Pangasianodon gigas. Further upstream, around the Khone Falls in southern Lao, there are several deep pools actually named by locals after the main species living in them (Roberts and Baird 1995). These pools are important feeding and spawning grounds for several species. For instance, Boong Pba Gooawng (literally Boesemania microlepis pool is an important feeding ground for the Irrawaddy dolphin ( Orcaella brevir9stris), a spawning ground for Boesemania microlepis and also is reported to have been a spawning ground for Catlocarpio siamensis in the past. Fish Conservation Zones (FCZs) were established around many of these deep pools by villagers in the period 1993-97 (Baird et. al. 1998). In recent follow-up surveys most of the villagers reported that one of the main reasons for the perceived increase in fish populations was that the FCZs act as a dry season, deep water habitat for many species (Chomchanta et. al. , 2000). Altogether, 24 species of fish were reported to have increased in numbers as a result of the FCZs, including both relatively sedentary species (such as Boesemania microlepis, Chitala blanchi and Chitala ornata) as well as migratory species (such as Pangasius conchophilus, Pangasius macronema, Probarbus jullieni and Cirrhinus microlepis). * Assessment of Mekong Fisheries Component, PO Box 7980, Vientiane, Lao PDR, email: mekongfisheries @usa.net Mekong Fisheries Network Newsletter ISSN 0859-290X, Vol. 7, No. 1 – September 2001 DEEP POOLS IN THE MEKONG RIVER Deep pools may be spawning grounds for some species.