SPONSORS Hydrobiologists from East, Central and West Africa with substantial support from other African countries, Fishery Scientists in the United States, Canada, U.K., Europe, Soviet Union and Israel. OBSERVATIONS EDITOR SIZE ON BOTTO Dr. J. Okedi, Director, E.A.F.F.R.O., Jinja, Uganda. WITH EMPHASI: EDITORIAL BOARD Mr. M. Abolarin, Co-Manager, Kainji Lake Professor W.B. Banage, Makelere Univer­ Project, Lagos, Nigeria. sity, Kampala, Uganda. ALMO J. CORDONE Al UNDP(SF)/ FAO Lake Vici Mr. J. Kambona, Chief Fisheries Officer, Mr. R.E. Morris, Director, E.A.M.F.R.O., Freshwater Fisheries Resel Dar es Salaam, Tanzania. Zanzibar. Mr. J. Mubanga, Director, Fisheries Division, Dr. T. Petr, Senior Lecturer in Hydrobiology, Chilanga, Zambia.. Makerere University, Kaflpala, Uganda. Dr. L. Obeng, Director, Institute of Aquatic Professor Mohamed Hyder, University of Biology, Achimota, Ghana. Nairobi, Kenya. Mr. N. Odero, Director ofFisheries, Nairobi, Professor, A. F. De Bont, Universite de Kenya. Kinshasa, Kinshasa XI, Republique Demo­ Mr. S.N. Semakula, Chief Fisheries Officer, cratique du Zaire Entebbe, Uganda. PROGRAMME The African Jout-nal of Tropical Hydrobiology and Fisheries will only accept original and well supported ideas on techniques, methodology and research findings from scientists, fishery officers,. fishery economists and sociologists. The Journal will therefore strengthen the African lesearch scientist by making research material available and also increasing the awareness and utility of aquatic resources. Its quality will conform to International standards, and will be published in English and French. MANUSCRIPT ADDRESS Manuscripts should be addressed to E.A.F.F. R.O., East African Freshwater Fisheries, Research Organisation, East African Community, Box 343, Jinja, Uganda. REPRINTS The Lake Victoria Fishe Authors will receive 60 reprints frce of charge. Extra reprints may be procured on cost. ject and the East A PUBLISHER Fisheries Research Orga East African LiteratllIc Bureau. P.O. Box 30022. Nairobi. Kenya. on a co-operative explora ing survey of Lake Vic1 ISSUES objectives of this progran The Journal consists of one volume a year, consisting of two issues with approximately tain the relative abundan eighty pages each. of the major commerci evaluate bottom trawlin~ SUBSCRIPTION fishing technique. The j Annual subscription within East Africa Sh. 35. Outside East Africa. East African Sh. 70, bottom trawling survey h US $ IO,(Xl 1968, and terminated in F A .a ._,,~ .. ;' RELATIONSHIP BETWEEN THE WATER LEVELS AND THE FISH CATCHES IN LAKES MWERU AND MWERU WA NTIPA, ZAMBIA R. WILLIAMS Department of Wildlife, Fisheries and National Parks, Nchelenge, Zambia The relationships between water level and catch per effort in two Zambian lakes are compared. In the relatively stable Lake M weru, a positive correla­ tion exists which can be used, with certain reservations, to prcdict the state of the fishery two years in advance. The cause of the relationship is probably the effect of water level on the marshy and swampy breeding areas, where at least the most common species in the commercial catch (Tilapia macruchir) has definite limits for the depth of water in which it will breed. For Mweru wa Ntipa, a consistant definite relationship does not exist, probably because the water level and extent of the lake fluctuate widely. Present address: R. Williams, 7 Pollock 51., N. Plympton 5038, Adelaide,S. Australia. In several of the Zambian fisheries (e.g. a generally muddy bottom, the depth in­ Kafue Flats and Lake Mweru) there appears creasing from 2 m at the southern end to an to be a correlation between water level and average of 10 m in the north, with isolated the landings of fish, especially cichlids, about "holes" up to 27 m deep. There are two two seasons later. If this relationship could major affluent rivers; the Luapula, which be precisely determined, it would be a valu­ drains from Lake Bangweulu and enters at able tool for fisheries management, enabling the southern end; and the Kalungwishi, which prediction of the state of the fishery. The drains from the highlands to the east and effect of water level regimes on the respective enters half way along the eastem shore (Fig. fish catches will be evaluated. As the statis­ 1). Both affluents, especially the Luapula, tical data have several shortcomings, they have extensive marshes at their mouths. The can be used only to indicate general trends, only effluent is the Luvua River at the and the conclusions given are preliminary. north-western corner. The water level is 2 Lake Mweru has an area of 4,070 km , and fairly stable, with a mean annual variation an altitude of approximately 920 m, and is of 1.5 m, and an extreme of 5 m (DE bounded along the eastern and western shores KIMPE 1964). by cliffs about 50 m high. It is shallow with Mweru wa Ntipa (Fig. 1) is an iI}temal 22 V. WILLiAMS MATERIALS Fisheries sta been publishe Government FISHERIES CENTRAL S to 1969). Pre­ been obtained from SOULSl data have beel MENT OF W BIA for 1956 been obtained Data on wa 9° S for both lakes with total land estimate of to; Mweru as thi based on ex] estimated loea: mate has not whole period Ntipa, the estil KALUNGWISHI R· reliable, being ~. and eatch per catch per effor o 40 measured dire. Relationship Kilometres annual averag 29° E viously are gi' fitted by the Fig 1. Map of Lake Mweru and Mweru wa Ntipa. Cross hatching shows Luapula marsh area; stippling correlations c show pre-1962 extent of Mweru wa Ntipa; I !! IIII ! III indicates escarpments. BAILEY (195 as the depend drainage system, and its area varies widely is bordered by cliffs approximately 70 m high. most reliable : in response to water level fluctuations. Its The rest of the area is marshy, its extent the best one . main inlet is Mofwe Dambo, which diverts varying with water level. water from the Kalungwishi River during of the fisheri( In both lakes, fluctuations in water level tionary" gill . most rainy seasons (BRELSFORD 1955). Its are correlated in time and extent with the maximum area is 1,560 km2 although there "kutuffi9ula" rainfall. The rainy season lasts from October has been no open water on at least six occa­ the nets by btl sions between 1867 and 1964 (BRELSFORD to April and averages 107 cm per annum because they 1955), and it occasionally dries out com­ on Mweru and 132 em per annum on variation depi pletely. The deepest and most permanent Mweru wa Ntipa (METEOROLOGICAL fishermen. Rl1 water is along the south-eastern shore, which DEPARTMENT 1967-68). water levels c RELATIONSHIP BETWEEN THE WATER LEVELS AND THE FISH CATCHES 23 MATERIALS AND METHODS years previously were less significant statis­ tically. Fisheries statistics from ]964 to ]969 have been published for both fisheries by the Government of Zambia (GAME AND RESULTS FISHERIES DEPARTMENT 1964 and Lake Mweru CENTRAL STATISTICAL OFFICE 1965 to ]969). Pre-]964 and post-]968 data have From recorded data, the level of high been obtained from unpublished reports and water appears to follow a four- to six-year from SOULSBY (1959, 1960). Water level cycle. This appears to have also been the data have been published by the DEPART­ case before records started (personal com­ MENT OF WATER AFFAIRS OF ZAM­ munications with local residents). Fig. 2 BIA for ]956 to 1lJ60. Post-1960 data have shows that fluctuations in catch per effort been obtained from unpublished reports. tend to follow the same pattern, and that Data on water level and catch per effort those for the southern camps, typified by for both lakes are plotted in Figs 2 and 3, Isokwe, are more extreme. with total landings for Mweru wa Ntipa. No The formal correlations between water estimate of total landings is given for Lake level and catch per effort averaged for seven Mweru as this is highly unreliable, being major camps is shown in Fig. 4. Table I based on exports from the fishery plus gives the average correlation coefficient and estimated local consumption. The latter esti­ those for the seven camps individually. For mate has not been revised throughout the most camps these are positive, and for those whole period of review. For Mweru wa where it is negative, the correlation is not Ntipa, the estimate of total landings is more significant. The best fit was .obtained by reliable, being based on a regular gear survey splitting the data into two groups and treat­ and catch per effort data. At both lakes, ing each group as a separate regression, viz. catch per effort and species composition are data rdating to water levels from ]956 to 1961 (points 1 to 6 in Fig. 4), and those 40 measured directly at the markets. Relationships between catch per 'ffort and relating to water levels from 1962 to 1967 annual average water level two years pre­ points 7 to 12). This division was consistent viously are given in Fig. 4. The lines were for all camps. fitted by the least iquares method and I area; stippling correlations calculated by the methods of Mweru wa Ntipa BAILEY (1959). Catch per effort was chosen BRELSFORD (1955) describes the ex­ as the dependant variable because it is the ely 70 m high. treme variations in water level before 1954. most reliable statistic for both fisheries, and hy, its extent The recorded data show a rise of 8 m the best one available to indicate the state between the 1961 and ]964 peaks. These of the fisheries. Catch per effort for "sta­ in water level variations have a great effect on the extent tionary" gill nets only was used. Data for of open water, and for this reason Fig.