LAKE KARIBA: A Man-Made Tropical Ecosystem in Central Africa MONOGRAPHIAE BIOLOGICAE Editor J.ILLIES Schlitz VOLUME 24 DR. W. JUNK b.v. PUBLISHERS THE HAGUE 1974 LAKE KARIBA: A Man-Made Tropical Ecosystem in Central Africa Edited by E. K. BALON & A. G. CaCHE DR. W. JUNK b.v. PUBLISHERS THE HAGUE 1974 ISBN- 13: 978-94-010-2336-8 e-ISBN-13: 978-94-010-2334-4 001: 10.1007/978-94-010-2334-4 © 1974 by Dr. W. Junk b.v., Publishers, The Hague Softcover reprint of the hardcover I st edition Cover design M. Velthuijs, The Hague Zuid-Nederlandsche Drukkerij N.V., 's-Hertogenbosch GENERAL CONTENTS Preface . VII Abstract IX Part I Limnological Study of a Tropical Reservoir by A. G. CacHE Contents of Part I. 3 Introduction and Acknowledgements . 7 Section I The Zambezi catchment above the Kariba Dam: general physical background 11 1. Physiography 13 2. Geology and soils . 18 3. Climate .. 25 4. Flora, fauna and human population. 41 Section II The rivers and their characteristics. 49 5. The Zambezi River . 51 6. Secondary rivers in the lake catchment 65 Section III Lake Kariba physico-chemical characteristics 75 7. Hydrology .. 77 8. Morphometry and morphology . 84 9. Sampling methodology 102 10. Optical properties . 108 11. Thermal properties 131 12. Dissolved gases 164 13. Mineral content 183 Section IV Conclusions . 231 14. General trophic status of Lake Kariba with particular reference to fish production. 233 Literature cited. 236 Annex I 244 Annex II .. 246 V Part II Fish Production of a Tropical Ecosystem by E. K. BALON 1 249 Contents of Part II . 253 A Parable . 255 1. Introduction and acknowledgements 257 2. Methods............. 265 3. Age and growth studies by E. K. BALON, J. HOLC~IK, S. FRANK, I. BASTL, K. CERNY, K. CHITRAV ADIVELU, A. KIRK A, I. KRUPKA, E. D. MUYANGA & K. PIVNICKA . 280 4. Total production, available production and yield of major fish taxa from Lake Kariba . 428 5. The eels . 446 6. Fish production of the drainage area and the influence of eco­ system changes on fish distribution (with a section by J. M. KAPETSKY) . 459 7. The success and failure of the clupeid introduction (with a section by J. G. WOODWARD) 524 Concluding discussion . 542 Epilogue ..... 554 Literature cited. 558 Appendix A. List of symbols used . 574 Appendix B. Efficiency of rotenone-cove samples by G. P. BAZIGOS . 575 Appendix C. Tables of mean sizes of individual species, life intervals and growth intensity . 595 Appendix D. Tables of individual species and single sample production computations . 603 Appendix E. Morphometry of sampling sites and standing crop tables. 625 Appendix F. Time of annulus inception: a pond experiment by E. K. BALON & E. M. CHADWICK . 643 Appendix G. Lepidological study: Key scales of Lake Kariba fishes . 647 Part III Plates: An Annotated Photographic Summary of the Lake Kariba Ecosystem by E. K. BALON 677 General Index . 749 1 Sections written by other authors bear their names after each title (see also Contents of Part II, p. 253-254). VI PREFACE In 1964 the Lake Kariba Fisheries Research Institute (LKFRI) was created in Kariba, Rhodesia as a United Nations Development Program Project, and executed by the Food and Agriculture Organization (FAD) in cooperation with the Governments of Rhodesia and Zambia. Dr. A. G. COCHE took charge of the Limnological Section and conducted research on the entire lake between January 1965 and January 1966. In 1966 the Central Fisheries Research Institute (CFRI) was created in Chilanga, Zambia by the Department of Wildlife, Fisheries, and National Parks. It was also supported by a UNDP Project executed by FAD. Between 1967 and 1971 Dr. E. K. BALON & Dr. A. G. COCHE were in charge of the Sections of Ichthyobiology and of Limnology respectively. The results of their FAD research activities on Lake Kariba are united in this volume. In the first part A. G. COCHE presents a limnological synthesis. In the second part E. K. BALON studies in detail the fish production and succession. The views expressed are those of the authors and do not necessarily coincide with those of the Food and Agriculture Organization of the United Nations. EUGENE K. BALON ANDRE G. COCHE Department of Zoology Lake Kossou Fishery Development University of Guelph Project (FAD) Guelph, Ontario, Canada Kossou, Ivory Coast, W. Africa The associate authors I. BASTL (Bratislava), G. P. BAZIGOS (Rome), K. CERNY (Prague), E. M. CHADWICK (Guelph), K. CHITRAVADIVELU (Ceylon), S. FRANK (Prague), J. Hod~:IK (Bratislava), J. M. KAPETSKY (Ann Arbor), A. KIRKA (Bratislava), I. KRUPKA (Bratislava), E. D. MUYANGA (Zambia), K. PIVNICKA (Prague) and J. WOODWARD (England) are not responsible for the arrangement of their contributions or for the final format of the book. VII ABSTRACT Kariba Dam was built to harness the Zambezi River for the production of hydro­ electric power. Its closure in December 1958 created Lake Kariba, which to-day is one of the largest reservoirs in the world. Background information is presented about the physiography of the Zambezi catchment above Kariba, its geology and soils, and the climate and the biology of the lake area. The rivers are studied, in particular the Zambezi River. In the chapter about hydrology, the water budget components are discussed. It is concluded that the Zambezi River contributes on an average 77 percent of the inflow, other rivers 16 percent, and rainfall 7 percent. Evaporation accounts for 14 percent of the water losses. The theoretical renewal time of the water mass is close to three years. Lake Kariba is naturally subdivided into four distinct basins. Within the lower one several sub-basins can be defined mainly on the basis of the local influence of affluents and submerged topography. Morphometric parameters which characterize in detail the planimetry and the bathymetry of the lake are given with reference to the normal operating water level of 485 m. The physico-chemical limnology of the water mass was studied in 1965 and in 1968/69. It is compared to data compiled for other African lakes and reservoirs. The changes which have taken place along the longitudinal axis of the reservoir between its upper and lower ends were especially put into evidence in the optical, thermal, and chemical properties. The depth of visibility generally ranged from 50 to 1060 cm, averaging 405 cm. It is equivalent to the depth at which about 22.7 percent of the incident solar light was found. The depth of the euphotic zone could be estimated as 3.54 DV. The penetration of the visible light changes markedly with time. The average vertical attenuation coefficient for total light varied from 0.20 to 2.20 according to the basin considered. The study of the penetration of spectral light blocks (445, 530, and 630 nm) showed that the green light was generally the most penetrating. The depth of the euphotic zone was relatively small, averaging 10 m in the upper basins and 16 m in the lower ones. It could be estimated as equal to 4.2/AVAC min. From the thermal point of view, the two lower thirds of the reservoir presented all the characteristics of a warm monomictic lake. Upstream, riverine conditions pre­ vailed. Total circulation occurred in the lacustrine part during the cool dry season around 22 a C. During stratification, a tilted thermocline existed mostly around the 20-25 m depth. The annual heat budget approximated 15000 cal. cm - 2yr - 1. It represented only 13 percent of the global radiation delivered to the surface of the lake. Residual heat was about 40000 cal. cm - 2. To better compare lakes among themselves new indices were defined relating heat contents to mean depth. On such a basis, the Annual Heat Index for L. Kariba was greater than that of any other lake studied. The Tropicality Index (1400 cal. cm - 2 m - 1) characterized the lake as truly tropical in nature. Estimated IX energy contents in the lower basin pointed to a relatively high work of the wind with a very low efficiency and to a stability representing 37 percent of the total work. The lake surface waters were well supplied with dissolved oxygen. The latter's depth distribution was typically clinograde outside the total circulation period. The oxycycle was closely correlated to the thermal cycle, a strong oxycline developing at the thermocline and the DO content drastically dropping below. Relatively small hypo­ limnetic area deficits pointed to low productivity in tropical waters. The average rate of hypolimnetic oxygen depletion varied around 0.100 to 0.150 mg.cm- 2 day-1 during the stagnation period. Considering that 2 mg. L - 1 of dissolved oxygen represented the lower average limit for fish occurrence in tropical waters, the seasonal depth variation of this index showed that unfavourable oxygenation conditions existed in the eastern part of the lake for at least five months of the year. The total mineral content was small. In 1965 the lake's annual average for total solids was 58 mg. L - 1, for salinity 42.4 mg. L - 1, for conductivity (20 0 C) 73 micromhos. Waters belonged to the calcico-carbonate type. The average ionic composition closely resembled that most currently encountered in the world. Water was very soft, total alkalinity varying between 0.42 and 0.88 meq. L - 1. Mineral replenishment mainly took place through river inflows but thermal stratification and density currents contributed to the delay in recycling processes of minerals brought into the lake's hypolimnion with the river floods. Quantitative evaluation of the annual chemical budget showed that the Zambezi River imported more than two million tons of minerals per year.