22ND IAHR-APD CONGRESS 2020, SAPPORO, JAPAN )ON THE DISTRIBUTION OF SALINITY AND FISH SPECIES IN A CHANNEL NETWORK IN THE MEKONG DELTA, VIETNAM
Shinji Fukuda (Tokyo University of Agriculture and Technology, Japan), MASAOMI KIMURA (Kindai University, Japan)
INTRODUCTION: The Mekong Delta is a large flat low-lying area with complex channel
networks for irrigation and transportation by ship. The area is well-known as a major
food production region in Vietnam but is under the threat of saline water intrusion. Manygates have been and are being built to prevent or minimize the impact of saline water
intrusion. However, the importance of ecosystems and ecosystem services gained lessattention irrespective of their capacity to provide necessary goods and services forhuman well-being. Therefore a series of surveys on the distribution of salinity and fish
species in Ben Tre, Vietnam were conducted.

Figure 1. Distribution ofsalinity and selected fish
species in the sampling
locations:(a) Salinity,(b) Rasbora sp.,(c) Oryzias sp.,(d) Oreochromis sp., and(e) Ambassis vachellii.
● species’ presences● its absences

METHODS: We conducted a series of surveys on salinity and fish fauna at 81 points from October 2017 to October
2019 (Figure 1). Salinity (pss) was measured using different salinity and conductivity sensors according tosampling sites. Missing data on salinity was filled using the salinity-conductivity relationship (SAL=0.0006COND;
R² = 0.996) obtained in the target region. Relationship between salinity and fish abundance was analyzed andmapped over Ben Tre province in order to understand the spatial distributions of salinity and fish species.
RESULTS & DISCUSSION: Salinity in the surveyed sites ranged between 0.04 and 23.33 pss with mean salinity being
2.6 pss (Figure 1a), which can be categorized into fresh- and brackish water. We observed 79 species including
freshwater, brackish water and marine species (Figure 1b-1e). Rasbora sp. was found in various salinity conditions(up to 14 pss)(Figure 1b), while other species occurred in a specific range (Figure 1c-ie). Oryzias sp. occurred
widely but close to main channels. This may partly because of its low swimming ability against fast flows. Incontrast, Ambassis vachellii was found mainly in the coastal area with salinity over 10 pss. These results reflect theeco-physiological characteristics (e.g., salinity tolerance and swimming ability) of the target species. Suchbiological parameters should be studied for a deeper understanding of species’ response to anthropogenic impacts
such as gate construction.
CONCLUSIONS: Considering the impact of tidal gates and its operation, ecohydraulic conditions (i.e., water flow andsalinity) around an irrigation facility needs to be considered in future research. For this to achieve, numericalsimulations of water flow and salinity in small- to large-scale channels and rivers are essential, for whichlongitudinal and cross-sectional profiles of the rivers and channels are required.
ACKNOWLEDGMENTS: This study was supported in part by the Grants-in-Aid for Scientific Research (Grant number: 17H04631,17H03886). We thank Vu Hai Son, Dr. Trieu Anh Ngoc, Yohei Asada, Yuki Matsuzawa, Seiya Aihara, Grant Thompson, andDaisuke Yamada for their assistance in the data collection.

Shinji Fukuda (Ph.D.)
Institute of Agriculture, Tokyo University of Agriculture and Technology, Japan
Website: http://shinjifukuda-medaka.com/ e-mail: shinji-f@cc.tuat.ac.jp