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FLOODPLAIN RIVER FOOD WEBS IN THE LOWER MEKONG BASIN A Dissertation by CHOULY OU Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chair of Committee, Kirk O. Winemiller Committee Members, Masami Fujiwara Thomas D. Olszewski Daniel L. Roelke Head of Department, Michael Masser December 2013 Major Subject: Wildlife and Fisheries Sciences Copyright 2013 Chouly Ou ABSTRACT The Mekong River is one of the world’s most important rivers in terms of its size, economic importance, cultural significance, productivity, and biodiversity. The Mekong River’s fisheries and biodiversity are threatened by major hydropower development and over-exploitation. Knowledge of river food web ecology is essential for management of the impacts created by anthropogenic activities on plant and animal populations and ecosystems. In the present study, I surveyed four tropical rivers in Cambodia within the Mekong River Basin. I examined the basal production sources supporting fish biomass in the four rivers during the dry and wet seasons and explored the relationship between trophic position and body size of fish at various taxonomic levels, among local species assemblages, and across trophic guilds. I used stable isotopes of carbon and nitrogen to estimate fish trophic levels and the principal primary production sources supporting fishes. My study provides evidence that food web dynamics in tropical rivers undergo significant seasonal shifts and emphasizes that river food webs are altered by dams and flow regulation. Seston and benthic algae were the most important production sources supporting fish biomass during the dry season, and riparian macrophytes appeared to be the most important production source supporting fishes during the wet season. In the river with strong flow regulation from an upstream impoundment, seston and benthic algae were even more important production sources supporting fishes during the dry season. My findings challenge the Eltonian theory of size-based trophic structure in food webs and also contradict the broadly accepted ii prediction of the fishing-down-the-food-web concept. Eltonian and fishing-down-the- food-web concepts propose that trophic level is strongly correlated with body size, but I found no significant correlation between body size and trophic position for fish assemblages. Results suggest that body size distributions are not useful for prediction the trophic structure of communities with diverse detritivores, omnivores and insectivores, but that it is a good predictor of trophic position among piscivorous fishes. iii To my grandmother Sun Lach. iv ACKNOWLEDGEMENTS I received help from many people and agencies to pursue this degree. First of all, I would like to express my gratitude my advisor Kirk Winemiller who has been a great and understanding advisor. I am so glad and fortunate that I chose to come to work with him. He has been providing me with invaluable advice, guidance and constant encouragement throughout the course of this degree. I would also like to thank my committee members: Masami Fujiwara, Thomas Olszewski and Daniel Roelke for constructive comments on my dissertation. Second, I am very grateful to local villagers of Thmei, Seim Pang, Veurn Sai, and Lomphat for their help in catching and buying fish. I also would like to thank collaborators at the Inland Fisheries Research and Development Institute of Cambodia; Dr. Nam So for providing the permits and Putrea Solyda for help in the field. Third, I would also like to thank both current and previous members of the Winemiller lab including Carmen Montaña, Katie Roach, Andrew Jackson, Daniel Fitzgerald, Caroline Arantes, Rabecca Pizano, Bibiana Correa, Allison Pease, Crystal Watkins, and Nathan Lujan. Special thanks to Carmen and Katie, with whom I shared an office, for reading and giving feedback that improved my dissertation. Carmen and I have also shared many life experiences as international students from developing countries. Thanks also go to Michael Treglia, Nicole Smolensky and Christopher Schalk of Fitzgerald lab. v Fourth, I am very thankful to financial sources from the Fulbright Fellowship program, Carolyn and George Kelso via the International Sportfish Fund, and Texas A&M University via teaching assistant positions. I am also grateful to the Writing Center at Texas A&M for their help to improve my writing. Finally, I would like to thank my family members for their constant support and love. My husband is the driving force that fuels my energy getting through this journey toward my degree, especially through the tough time during the preliminary exam. He has not only endured hardship of living apart from me while fulfilling his duties as an assistant professor, but also has been very patient and loving, I cannot ask for more. My grandmother, who is illiterate, has always believed in me and that I can be a great achiever in whatever I do. She has taught me to become an independent woman in a male-dominated society. She is my role model and true inspiration. My parents have provided me with endless love despite the fact they did not want me to go to college. They finally are convinced that it was the best decision that they let me attend college in the capital Phnom Penh, far away from home. vi TABLE OF CONTENTS Page ABSTRACT ...................................................................................................................... ii ACKNOWLEDGEMENTS .............................................................................................. v TABLE OF CONTENTS ................................................................................................ vii LIST OF FIGURES .......................................................................................................... ix LIST OF TABLES ........................................................................................................... xi CHAPTER I GENERAL INTRODUCTION ................................................................... 1 The Mekong River Basin ............................................................................... 1 Food web ecology ......................................................................................... 4 CHAPTER II SEASONAL HYDROLOGY DRIVES SHIFTS IN PRODUCTION SOURCES SUPPORTING FISHES IN THE LOWER MEKONG RIVER BASIN ....... 9 Introduction .................................................................................................... 9 Materials and methods .................................................................................. 13 Results ........................................................................................................ 26 Discussion .................................................................................................... 39 Conclusion .................................................................................................... 50 CHAPTER III RELATIONSHIP BETWEEN BODY SIZE AND TROPHIC POSITION IN FISH ASSEMBLAGES OF TROPICAL RIVERS IN THE LOWER MEKONG BASIN .......................................................................................................... 52 Introduction .................................................................................................. 52 Materials and methods .................................................................................. 56 Results .......................................................................................................... 61 Discussion .................................................................................................... 73 Conclusion .................................................................................................... 80 CHAPTER IV GENERAL CONCLUSIONS ................................................................. 82 LITERATURE CITED .................................................................................................. 86 APPENDIX 1 ............................................................................................................... 112 vii APPENDIX 2 ................................................................................................................ 134 APPENDIX 3 ................................................................................................................ 147 viii LIST OF FIGURES Page Figure 1. Map of the study sites ...................................................................................... 14 Figure 2. Mekong River during the dry season and wet season ...................................... 18 Figure 3. Sekong Rriver during the dry season and wet season ...................................... 19 Figure 4. Sesan River during the dry season and wet season .......................................... 20 Figure 5. Srepok River during the dry season and wet season ........................................ 21 Figure 6. Biplot of δ13C and δ15N of tissues from fishes and basal production sources collected from the Mekong River ........................................................ 30 Figure 7. Biplot of δ13C and δ15N of tissues from fishes and basal production sources collected from the Sekong River ......................................................... 31 Figure 8. Biplot of δ13C and δ15N of tissues from fishes and basal production sources collected from the Sesan River ............................................................ 32 Figure 9. Biplot of δ13C and δ15N of tissues