ISSN 0859-290X, Vol. 7, No. 1 – September 2001 DEEP POOLS IN THE MEKONG RIVER 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, Deep pools may be spawning Micronema apogon, Pangasianodon hypoph- thalmus, Cirrhinus microlepis, Probarbus jullieni, Catlocarpio grounds for some species. 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 Further north in Luapg Prabang Province, 37 % of the surveyed villages reported that they have a conservation zone nearby the village. These conservation zones were mainly associated with deep pools within the river, which were believed to be important breeding grounds for fish (Sjorslev, 2000). Different pools, different fish The surveys revealed that different fish use different pools. For instance, two pools which are close to each other (Ban Khanh Gneir , Phonthong District and Ban Seine Tay, Muang Khong District) in Champassak Province of Lao PDR have no species overlap. This lack of overlap could be attributed to a combination of the geographical position of the pools and differences in environmental parameters associated with each pool. These parameters include depth, current speed, substrate type, slope, proximity to wetland forest and occurrence of objects (e.g. logs, rocks) inside the habitat (Baird et al., 1998). It is noteworthy that species with very different ecological habits make use of deep pools during the dry season. Many highly migratory species, such as most members of the family Pangasiidae and cyprinids such as Probarbus spp., Catlocarpio siamensis, Cirrhinus microlepis and Cyclocheilichthys enoplos, as well as relatively sedentary species such as Chitala ornata, Villagers understand the value of deep pools as conservation zones Wallago at tu, Mastacembelus armatus and Bagarius yarelli, use the pools during the dry season. Dolphins use them too The Mekong River Dolphin (Orcaella brevirostris) is one of the most eye-catching mammals in the Mekong and at the same time, one of the most threatened. Its ecology is deeply dependant on the existence of deep pool areas (Baird and Mounsouphum, 1997). The distribution of dolphins in the Mekong is restricted to the area from Kratie in the south to the Khone Falls in the north, and includes the lower stretches of the Sesan sub-catchment. This area is also the most important in terms of deep pool fish habitats in the Mekong. This is not a coincidence since dolphins are known to spend most of their time in deep pools, from where they frequently undertake "hunting" migrations following groups of migratory fishes, which constitute their prey (Baird and Mounsouphum, 1997). Without deep pool habitats, and the fishes they sustain, it is likely that dolphins would disappear from the Mekong River. Siltation destroys deep pools In the Sesan River, some important deep pool areas have apparently become shallower during the past few years due to increased silt deposition resulting from altered flow regimes after construction of the Yali Dam in the upper part of the catchment (Fisheries Office of Ratanakiti Province, Cambodia, 2000). As an example, one deep pool in Voen Say District was reported to have gone from 7-8 meters deep to just half a meter deep within the last three years. As a consequence, the abundance of many fish species has been dramatically reduced. Such species include Pangasius krempfi, Pangasius conchophilus, Pan- gas ius polyuranodon, Pangasius larnaudiei, Belodonthichthys dinema and Micronema micronema (Fisheries Office of Ratanakiri Province, Cambodia, 2000). A similar problem has been reported from the Theun River in Lao PDR resulting from the Theun- Hinboun Dam (Terry Warren, personal communication). Deep pools and fish migrations Three main migration systems for fish have been identified within the Mekong mainstream (Coates et. at, 2000). The geographic position of stretches within the mainstream containing a relatively high number of deep pools appears to correlate with these three migration systems. The migration system of the lower basin (i.e. below the Khone Falls) is basically a migration between the important dry season habitats in deep-pool-stretches of the north (Kratie-Stung Treng) to important flood-season feeding habitats of the south (Southern Cambodia- Mekong Delta- Tonle Sap ) Deep pools in the middle Mekong (i.e. from Khone Falls to about Loei) mainly serve as dry season habitats for fishes that spend the flood season on floodplains associated with major tributaries. The relative lack of deep pool areas along the stretch between Thakhek and Loei may be one of the main reasons for the apparent separation between the upper migration system above Loei and the middle migration system between Khone Falls and Loei. This correlation between deep pool areas and migration patterns emphasizes the importance of managing deep pools as an integral part of a broader ecological system. Mekong Fisheries Network Newsletter References Baird, I. G. and B. Mounsouphum (1997): Distribution, mortality, diet and conservation of Irrawaddy Dolphins ( Orcaella brevirostris Gray) in Lao PDR. In: Asian Marine Biology 14: 41-48. Baird, I. G.; P. Kisouvannalath, v. Inthaphaysi and B. Phylaivanh (1998): The potential for ecological classification as a tool for establishing and monitoring fish conservation zones in the Mekong River. Environmental Protection and Community Development in the Siphandone Wetland, Champasak Province, Lao PDR. Technical Report No.2. Chomchanta, P.; P. Vongphasouk, S. Chanrya, C. Soulignavong, B. Saadsy and T. J. Warren (2000): A preliminary assessment of the Mekong Fishery Conservation Zones in the Siphandone area of Southern Lao PDR, and recommendations for further evaluation and monitoring. The Living Aquatic Resources and Research Center, Vientiane, Lao PDR, LARReC Technical Paper No.0001. Coates, D.; A. F. Poulsen and S. Viravong (2000): Governance and trans-boundary fish stocks in the Mekong River Basin. Paper presented at the 3rd Technical Symposium of the MRC Fisheries Programme, Phnom Penh. Poulsen, A F and J .Valbo- ]orgensen (editors) (2000): Fish migrations and spawning habits in the Mekong mainstream -a survey using local knowledge. AMFC Technical Report, Mekong River Commission.