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. The secondary rivers contributed at least 400 thousand tons and rain 42 thousand tons. Most chemical properties point to a low productivity potential. In particular the average quantity of non-carbonated salts available for biological production was very low. Both the morphoedaphic index and its newly proposed form point to a very low potential of fish production. This conclusion was reinforced by other limnological characteristics which may adversely affect the fish production such as timing of water level fluctuations, shallow depth of the trophogenic zone, seasonally unfavourable oxygenation conditions, and general chemical characteristics of the water mass. The biological evolution of Lake Kariba appears to have followed a sequence similar to that experienced under temperate climates. The first phase of maturation characterized by maximum biological productions probably lasted until 1964. The second phase of depressed productivity may last until 1974. A slight increase in productivity should normally take place then. But in view of its inherent characteristics of low productivity it is believed that in the future Lake Kariba, relatively to other tropical water bodies, will retain its general oligotrophic status. The second part of this monograph is devoted entirely to fish fauna, its succession, densities and ecological production, as these relate to limnological, social and en• vironmental issues. Exact usage and definition of abundance, density, standing crop and stock, biomass and production are given; values of total and available production are introduced and terms such as harvest or catch are used to designate that part of production which is above the minimum harvestable size and used by man. Age, absolute and relative growth values were examined by ten authors on 21 species of Lake Kariba fishes. The average density of the fish population was estimated x as 97,000 per hectare. For the entire area of the lake inhabited by fish (determined by echo-sounding), it amounted to 19.6 x 109 specimens. The initial biomass is 2,830 kg/ha, which for the whole lake gives a value 5.7 x 108 kg; the mean biomass was estimated as 3,855 kg/ha or 7.8 X 108 kg. The total production is 3,468 kg/hafyr, or over 700 thousand metric tons for the whole lake. The estimated available produc• tion is 682 kg/ha/yr or 140 thousand metric tons. Yield, the harvestable part of production, was determined according to the minimum harvestable sizes interpolated graphically. The total yield is divided between natural mortality, actual catch and the available yield (i.e. the remainder). It amounted to 599 kg/ha/yr, while the assessed actual catch was 14.8 kg/ha/yr. Since the available yield was estimated to be 189 kg/ha/yr, 395.2 kg/ha/yr of the total yield is attributable to natural mortality. Fishery could increase its exploitation by 32 % of the total yield, while 66 % is lost through natural mortality and 2.5 % forms the actual catch. The 'maximum sustained yield' is thus in the vicinity of 40 thousand metric tons per year, of which actual fishing has been exploiting only 3 thousand tons:
in kg/hafyr in% Total production (A = OR) 3,468 100 Natural mortality (M) ~ 2,786 80.3 Final production (P = P' + E') ~ 697 20.1 Available production (P' = N'i(w - WI-l» 682 19.7 Total yield (YA = A above minimum harvestable size) 599 17.3 Natural mortality of harvestable part (M') ~ 395 11.4 Sustained yield (Y P' + E') ~ 204 5.9 Available yield (YP' = P' above minimum harvestable size) 189 5.4 Actual catch (E') 15 0.4
However a substantial stock of the catadromous eel Anguilla nebulosa labiata, sub• sequently discovered and evaluated as well as the introduced Tanganyikan anchoveta Limnothrissa miodon have to be added to the above values. It has been predicted that eels will disappear from Lake Kariba because the juveniles will be unable to surmount the dam. This prediction has been proved wrong. An abundant population of eels was discovered in the lake at a depth of 25 to 40 m. The age structure of the eel cohorts suggests that juveniles surmount the dam in their second year of life and then spend approximately seven years in streams of the lake drainage. The faster maturing individuals whicli are in better condition emigrate into the ocean, the slower maturing eels remain in the lake longer. The oldest eel in the samples was 18 years old. The catch per unit of effort for hoopnets was 2.35 kg in the upper part of the lake and 0.41 kg for the lower part of the lake. The density at Namazambwe was assessed at 46 eels per 1 ha. The unexploited eel population of Lake Kariba could form a valuable resource. Several rivers ofthe Lake Kariba drainage were investigated as sources of comparative pre-impoudment production values, as well as habitats enabling sustainment of original riverine fish fauna, and as sources from which fish invaded the new lake. The fishes of the Zambezi River and drainage streams fed the local people for many centuries. The whole system formed a valuable source of sustained fish protein harvest.
XI The original 28 fish species of Lake Kariba increased from 1963 to 1971 to 41. Most of these were from the Upper Zambezi. Samples of fishes from the edge of Victoria Falls were utilized to explain the history of exchanges between two fish faunas and the invasion of Lake Kariba. Fishes of the pre-Upper Zambezi River are of a different origin from those in the pre-Middle and Lower Zambezi. When the two separate river systems which now make up the Zambezi were united as a result of a tectonic upwarp, the point of unification became the Victoria Falls. The rushing waters of the river cut steep gorges along the fault lines in the basalt lava. The Falls were considered a physical barrier separating fishes of the Upper and Middle Zambezi. Recent environmental changes which accompanied the creation of man-made Lake Kariba, together with the invasion of the Lake by Upper Zambezi fishes demonstrate the ineffectiveness of the Falls as a downstream barrier. A theory of an ecological barrier caused by differences in habitat and by differences in number of available niches is presented here. This ecological barrier was functional for 500,000 years, when the separate rivers first united. Creation of Lake Kariba changed the character of this ecological barrier so that lentic fishes flushed over the edge of Victoria Falls could invade the lake. The successful introduction into Lake Kariba of the Lake Tanganyika clupeid, when evaluated from its density point of view reveals yet another useless deed of fishery practitioners. Catch survey as well as echo-sounding records indicate that this fish may not attain densities worthwhile for commercial exploitation and that it will have to share the available nutrients with other indigenous fishes probably more desirable for human consumption. If the fish production values of Lake Kariba are recalculated for the whole lake area instead for the 38 % of the total area inhabited by fish it certainly will reveal a picture of low fishery potential. Fishery potential presented in this manner (E' 5.59 kg/ha/yr) will support the conclusions that Lake Kariba is an oligotrophic impound• ment of low productivity; it may, however, have little value in terms of energy dis• tribution and utilization. The relationship of nutrients locked in inshore waters to those of open waters is an unknown quantity. In order to draw valid conclusions on the ecological production potential of Lake Kariba these phenomena will have to be understood much better than they are now. The data obtained are examined with regard to the fish population stability and to the nutrient contents. It is concluded that in terms of production the population as a whole is already stable, but that the single taxa are still in the process of adaptation. Due to the fact that one third of the lake water is replaced each year and some nutrients are brought in with inflowing river waters, the loss of nutrients, as a con• sequence of removal of the sustained yield of fish, could be negligible. Other variables, in which the turnover of nutrients is reflected, are discussed as well as some estimates of the lake's fish potential predicted in the past. Finally, some conclusions concerning fishery policies are given and an attempt is made to debunk man's justification for such environmental changes.
XII