d o w n s t r e a m d r i f t o f invertebrates in n a r o m o r u r i v e r , A TROPICAL RIVER IN CENTRAL KENYA BY \m ATHOOKO, JUDE MUTUKU; B . ED-Sc i . (lions ) - t TJ h >• A THESIS SUBMITTED IN PART FULFILMENT FOR THE DEGREE OF MASTER OF SCIENCE IN ZOOLOGY(HYDROBIOLOGY) OF THE UNIVERSITY OF NAIROBI. Nairobi-Kenya 1988 UNIVEP5ITY OF NAIROBI 11 DECLARATION I, Jude Mutuku Mathooko, hereby declare that this thesis is my own original work and has not been presented for a degree in any other University. Date: • « / * / ? £ Signed Jude M. Mathooko This thesis has been submitted for examination for the degree of Master of Science of the University of Nairobi with my approval as the University Supervisor. Date: Signed: Dr. Kenneth M. Mavuti Supervisor and Senior Lecturer Department of Zoology University of Nairobi -lii- DEDICATION To my sons Muthiani, Mathooko and Ndundu -IV- ACKNOWLEDGEMENTS I am greatly indebted to my supervisor, Dr. Kenneth R.M. Mavuti for his invaluable advice and encouragement in the course of preparing this thesis. Sincere thanks are due to Dr. Macolm R. Litterick for his ingenuous advice and discussions during the planning stages of this research project. His assistance in designing some of the equipment is gratefully acknowledged. To Mr. Jamhuri N. Mathooko, I give thanks for his inestimable assistance and encouragement during the field work. Special thanks go to the manager, Naro Moru River Lodge, for generous and unrestricted access to the Naro Moru River and to the watchmen for offering security on numerous occasions without which some night samplings would have been intolerable. The assistance of the following is also gratefully acknowledged: Ministry of Water Development, Nairobi, for providing me with the discharge data on Naro Moru River, to its laboratory in Industrial Area, Nairobi} for the calibration of the simple turbidimeter used during the present study and Hydrology Office, Nyeri for allowing me to use its flowmeter; the Meteorological Department, Dagoretti Corner, for rainfall information on Mount Kenya region and the Kenya National Council for Science and Technology for its financial support. -V- Finally, I am thankful to my wife, Jane for her patience during the entire course and to all those who gave moral support during the course of preparing this thesis. -vir TABLE OF CONTENTS TITLE ........................................................ i DECLARATION.................................................. ii DEDICATION .......................... ■...................... i i i ACKNOWLEDGEMENTS ........................................... iv LIST OF CONTENTS ........................................... vi LIST OF FIGURES ............................. ,.............. ix LIST OF TABLES ............................................. xi ABSTRACT ....................................................xiv CHAPTER 1. GENERAL INTRODUCTION, AIMS AND OBJECTIVES, STUDY SITE AND LITERATURE REVIEW 1.1 General introduction ............................... 1 1.2 Aims and objectives ............................... 3 1.3 Naro Moru River and study site..................... 5 1.3.1 General characteristics of Naro Moru River ........ 5 1.3.2 The study site ..................................... 11 1.4 Literature review .................................. 13 CHAPTER 2. MATERIALS AND METHODS 2.1 Pilot survey ........................................ 30 2.2 Routine collection of drift samples............... 30 2.3 Factors that affect diel and seasonal fluctuations of the drift fauna ................... 33 2.3.1 Light intensity^sunlight and moonlight ........... 33 2.3.2 Estimation of the water velocity at net mouths and river water velocity........................... 35 2.3.3 River discharge and the volume sampled by the drift nets .......................................... 36 2.3.4 Turbidity .......................................... 37 2.3.5 Benthic fauna abundance ............................... 3g 2.3.6 Dissolved oxygen concentration..................... 41 2.3.7 Water and air temperatures ........................ 43 2.3.8 Conductivity ....................................... 44 2.3.9 pH ... •.... ............ 44 2.4 Methods of assessment of the diet of trout ......... 44 -vii- CHAPTER 3. THE CATCHMENT AND PHYSICO-CHEMICAL CHARACTERISTICS OF NARO MORU RIVER 3.1 The catchment..................................... 47 3.2 Physico-chemical characteristics of Naro Moru River ........................................ 49 3.2.1 River discharge and rainfall.....................' 49 3.2.2 Water and air temperatures ...................... 55 3.2.3 pH and conductivity ............................. 59 3.2.4 Turbi d i t y ........................ ................. 62 3.2.5 Dissolved oxygen concentration .................. 62 3.2.6 Water current velocity .......................... 65 CHAPTER 4. COMPOSITION AND STRUCTURE OF THE BENTHOS IN NARO MORU RIVER 4.1 Results .......................................... 67 4.1.1 Pilot survey...................................... 67 4.1.2 Colonization experiments at station 3 .......... 75 4.1.3 Composition of the benthos collected at the time of maximum colonization of the artificial substrate baskets ................................ 75 4.1.4 _ Seasonal variations in the benthic abundance of the major taxa ................................ 82 4.2 Discussion ........................................ 86 CHAPTER 5. COMPOSITION, STRUCTURE, SEASONAL AND DIEL FLUCTUATIONS OF DRIFT FAUNA IN NARO ' MORU RIVER 5.1 Results .......................................... 90 5.1.1 Composition and structure of the drift fauna .... 90 5.1.2 Seasonal fluctuations of the drift fauna ....... 94 5.1.3 Diel fluctuations of the drift fauna ............ 101 5.2 Discussion ......................................... 112 5.2.1 Composition and structure of the drift fauna ..... 112 5.2.2 Seasonal fluctuations of the drift fauna ........ 115 5.2.3 Diel fluctuations of the drift fauna ............. 119 -Vlll’- CHAPTER 6. FACTORS THAT AFFECT DIEL AND SEASONAL FLUCTUATIONS OF THE DRIFT FAUNA 6.1 Results ........................................... 122 6.1.1 Light intensity-sunlight and moonlight .......... 122 6.1.2 River discharge and the volume sampled by the drift nets ........................................ 126 6.1.3 Dissolved oxygen concentration ................... 132 6.1.4 Turbidity ......................................... 134 6.1.5 pH and conductivity ............................... 134 6.1.6 Water temperature ................................ 136 6.1.7 Water current velocity .......................... 137 6.1.8 Benthic fauna abundance ............................. 139 6.2 Discussion ........................................ 144 6.2.1 Light intensity-sunlight and moonlight ........... 144 6.2.2 River discharge and water current velocity ........ 147 6.2.3 Dissolved oxygen concentration.................... 149 6.2.4 Turbidity ......................................... 150 6.2.5 Water temperature ................................ 150 6.2.6 pH and conductivity ............................... 151 6.2.7 Benthic fciuna abundance........................... 153 CHAPTER 7. DRIFT FAUNA AS POTENTIAL FOOD FOR FISH 7.1 Results ............................................ 154 7.1.1 Composition of trout diet ........................ 154 7.1.2 Seasonal variation of aquatic and terrestrial trout diet .................................. 158 7.1.3 Availability and selection of terrestrial and aquatic drift food sources by trout ............. 163 7.2 Discussion ........................................ 170 CHAPTER 8. GENERAL DISCUSSION AND CONCLUDING REMARKS ... 175 r e f e r e n c e s ............................................. .... 178 APPENDICES 191 LIST OF FIGURES Figure Page 1 Map of Kenya showing the location of Naro Moru River 6 2A Naro Moru River and the location of the study site, the river gauging station 5BC2, the Forest Guard Post and the seven pilot survey stations (Sr S7) 7 2 B Naro Moru River drainage area (hatched) and the associated drainage areas 8 3 Cross-sectional profile of Naro Moru River at the study site 12 4 Diagramatic plan of the apparatus for drift and debris separation 32 5 The phases of the moon 34 6 Plan of a simple turbidimeter 38 7 Discharge regime for Naro Moru River, 1984-1987 50 8 Mean monthly rainfall amounts recorded at Naro Moru Forest Guard Post Station 54 9 Cross-sectional profiles of pilot survey stations (S1-S7) 71 10 Mean numbers of benthic animals per basket against exposure period (days) 77 11 Monthly variations in the density of the major groups of the benthos in Naro Moru River, 1986-1987 83 12 Monthly benthic composition (per cent mean numbers of benthic animal^ per basket) and seasonal benthic abundance variation of the dominant taxa in Naro Moru River, 1986-1987 85 13 Seasonal variation in diel drift in Naro Moru River, 1986-1987 97 14 Percentage composition of total drift inverte­ brates in Naro Moru River, 1986-1^87 98 * -x- 15 Diel variation of drift rates in Naro Moru River, 1986-1987 103 16 Diel variation of drift rates in Naro Moru River for 14/15 December 1986 showing an alternans pattern 105 17 Day and night fluctuations of the drift rates in Naro Moru River, 1986-1987 108 18 Effect of sunlight intensity on the total number of drift animals per 2h. on 5/6 June 1987 125 19 Effect of moonlight on the drift animals in Naro Moru River, 1986-1987 127 20 Seasonal fluctuations of the river discharge and the numbers of drift animals in Naro Moru River, 1986-1987 128 21 Thp relationship between