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/ ,~. ~I' •.• View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Aquatic Commons SPONSORS Hydrobiologists from East, Central and West Mrica with substantial support from other Mrican countries, Fishery Scientists in the United States, Canada, U.K., Europe and the Soviet Union. EDITOR Dr. John Okedi, Director E.A.F.F.R.O., Jinja, Uganda. SUB-EDITOR S. WanaIl'ibwa (Mrs.) EDITORIAL BOARD Dr. L. Obeng, Director, Institute ofAquatic Mr. W. A. Sichone, Fisheries Department, Biology, Achimota, Ghana. Dar es Salaam, Tanzania. Mr. :N. Odero, Director, FIsheries Division, Mr. V. O. Sagua, Director, Kainji Lake Nairobi, Kenya. Research Project, Nigeria. Mr. S. N. Semakula; p,ermanent Secretary, Mr. R. E. Morris, Director, EAMFRO Ministry of Animal Resources, Kampala, Zanzibar. Uganda. Professor A. F. De Bont, Universite de Professor Mohamed Hyder, University of Kinshasa, Republique Democratique du Nairobi, Kenya. Zaire. Professor W. B. Banage, University of Mr. G. E. B. Kitaka, Deputy Director zambia, Lusaka. EAMFRO, Zanzibar. Professor A. M. A. Imevbore, University of Ife, Ile-Ife, Nigeria. PROGRAMME The Mrican Journal of Tropical Hydrobiology and Fisheries will only accept original and well supported ideas on techniques, methodology and research findings frorp. scientists, fishery officers, fishery economists and sociologists. The Journal will therefore strengthen the Mrican research scientist by making research ;vaterial available and also increasing the awareness and utility of a.quatiQ resources. ~ts quality will conform to International standards, and will be published in English and French. ~USCRIPT ADDRESS Manuscripts should be addressed to E.A.F.F.R.O., East African Freshwater Fisheries Research Organi~on, East African Community, Box 343, Jinja, Uganda. REPRINTS Authors will receive 60 reprints free of charge. Extra reprints may be procured on cost. PUBLISHER East Mrican Literature Bureau, P.O. Box 30022, Nairobi, Kenya. ISSUES The Journal consists of one volume a yeaf, consisting of two issues with approximately eighty pages each. SUBSCRIPTION Annual subscription within East Africa Shs. 35. Outside East Mrica, East Mrican Shs. 70, US $ 10.00. LIMNOLOGY AND FISHERIES OF THE NYUMBA YA MUNGU, A MAN-MADE LAKE IN TANZANIA T.PETR Department ofZoology, Monash University, Clayton, Victoria 3168, Australia INTRODUCTION RESULTS AND DISCUSSION After the closure of the large dam on the N}'umba ya Mungu Zambezi River in 1958, which resulted in the Location: Tanzania, Kilimanjaro Region, 3' 40'S: 37' 25'E formation of Lake Kariba, much attention Altitude: 663 m a.s.l. has been paid to this and to similar large Climate: dry, with one rainy 'season (April- man-made lakes, with surface areas more June). than 1,000 km 2 , which were formed later on surrounding vegetation cover: savanna, with various other African rivers. Because of semi-arid scru b. their large size they have attracted the Dam closed (on the river Pangani) on attention of fishery scientists, who antici 1.12.1965. pated large fish catches. Considerably less Dam full on 29.4.1968. attention has been paid to the lesser lakes Surface area when full: 150 km' which appeared on smaller African rivers. Maximum length: 32 km These lakes, such as Lake Ayame in Ivory Maximum width: 8 km Coast, and Lake Nyumba Ya Mungu in Maximum depth: 48 m Tanzania, are less than 200 km' and usually Volume: 9 X 10' m'. do not appear even on more detai led regional Period of investigations: June, 1972-April, maps. However, the fish density explosion 1973 (3 visits). in some of them soon after their formation Physico-chemical Characlerisrics of the Lake was no less spectacular than that in some of and the Rm'u River the large African man-made lakes. The longitudinal dilferences in water transparency reflect the amount of suspended The present paper deals with the limnology particles, ineluding plankton, in the water. of Nyumba Ya Mungu, a man-made lake The upper part of the lake, infl uenced by the in Tanzania (Kilimanjaro Region), during Ruvu River, had more turbid water during the 7th and 8th years after the elosure of the floods (June, 1972), than during the dry Ruvu dam. The area investigated included seasons. This was due not only to the the River, which is one of the two rivers increased concentration of particles brought entering the lake. in by the river, but also to the stirring up of 40 T. PETR Table 1 (Cominued).. sediments by wind action on the ~haHow surface and 15 m depth was 2.3°C (Table I). water. In the dam area high turbidity of In this month there was little oxygen present water ""as recufiJed ilt the peak of the dry beluw 7 m depth. Both the temperature At the intake-3rd 11 season in December, 1972, but this can be and the oxygen gradients indicate that the Depth (m) Ter Surface attributed to dense phytuplankton. The lake, at least in the dam area, was s.tratified 1 turbidity of the: Ruyu River is higher than at that time, and als.o in April 1973, when in 3 that of the dam area hut lower than that of 22.5 m depth only 1.5 mg.!] O2 was recorded. 5 the inflow area. Secchi disc readings sho",' It appears. that even 7 years after the 7 that during the rainy and flood seasons water completion of the dam, the lake chemically 10 13 is about four times Jes~ transparent in the stratifies. although it is pmsible that this 14 inflow area than in the dam area. 1 awards 5tratification is limited to the dam area, t he end of the dry 'icason this difference is which is narrow and somewhat protected less than during floods. from wind and currents by the land con The surface water t,mpcratur~ is higher figllration. As no mea~urements were carried during the dry season than during the floods, out elst:wht::re in the main lake. it is not und the difference is morc pronounced in kno\\'n whethe; the deoxygenated water the shallDw area of the lake where the river forms only pockeb in the deepest part!> enters. In the dam area thc.:re is only a small such as ubserved tn Lake Kariba (COCHE tt-mpcrature gradient, with the difference 1963), or whether the wbolc lake is stratified be:t\1ieen the surface and 2:l.5 m depth being during the dry season, with a continuous only O.SoC at the beginning of rains (April, layer of deoxygenated deep water, as for 1973). lim'ever, 3t the peak of the dry example in Lake Volta (CZERNlN-CHU season (December, 1972), in the same area, DENITZ 1971). It is noteworthy that the the temperature difTeren(.;e between the water is stratified in the dam area only 300 m Table 2. Plwsico-cher Table 1. F~ysko-ch~mkal Data for Nyumba Ya Mungu (dam area) 1972 1973 Date 2 Jun. :\ Jun. 31 Dec. 4 Apr. 4 Apr. Time 16.':;:0 time i2.3J II.CO 12.30 1245 Seec ~2.5 deoth (m) S 10.0 S S Terr Secchi disc (cml 167 - - 141 - OCYI temperature "C 25 6 25.1 284 27.8 "27.3 Fro oxygen mgtl R.9 7.3 9.1 8.5 1.5 con, ~W conductivity K2 u pmhos/cm 020 8CO 960 810 pH pH 8.9 8.8 8.9 8.7 B.4 NC N03-N mgiI 10 7 0 12 il NC NOz-N mg/} 0 0 0 0.07 0.02 PO P04.P mg!l 0.02 0.05 0.04 0 0 Si Si mg/l - 11.5 28.0 30.0 122.2 N' Na mg/l 1274 - - - K 1>.0 K mg/l 12.7 - - - 0 Ca (mg/l CaC03) 35 35 - 0 0 SC S04 mg/l 7 8 12 14 9 T, total alkalinity (mg/I as CaC03) 200 ~CO 340 )05 380 F' Fe mg/l 0.05 0 O.OJ 0.01 0.02 ~ VJn mg/l 0 0 0 0 0 T total dissoh"ed solids mgJI 508 475 - - - T total solids mgjl 513 485 - - - S suspended solids mg/l 4.5 10.0 - - C organie Ir.atter mg/l 2:0 198 NYUMBA YA MUNGU, TANZANIA 41 I las 2.3'C (Table l), Table J (Comjnued). Vertical Pronles of Temperature and Oxygen (dam area) ~tle oxygen present ~ the temperature June 1972 , indicate that the At the intake-3ed June, 1972-12.25 hours 300 m from the intake-Jed June, 1972--16.00 hours ~ea, Depth (ill) Temp. cC ~.~ sat. 02 Depth (m) Temp. 0C ~~ saL. Oz was stratified Surface 25.6 112 Surface 27.6 117 pril 1973, when in I 25.5 112 1 27.0 121 IO2 Was recorded 3 25.1 96 2 25.5 118 , years after th~ 5 25.1 92 3 25.2 103 e Jake chemically 7 25.1 92 5 25.1 82 10 ~.1 92 6 25.0 73 "'$SIble that this 13 25.1 85 8 25.0 70 the dam area 14 25.1 85 10 25.0 48 Icwhat protected y the land Con De~ember, 1972-300 ill from the intake-I 1.00 hours ~nts were carried Depth (m) Temp. 'C ~~ sat. O lake. it is nol 2 Surface 28.4 139 ygenated v.ater I 28.5 107 , deepest parts 3 28.2 83 ,ariba (CaCHE 5 27.3 49 lake is stratified 7 27.2 40 9 26.4 16 L a Continuous II 26.2 8 water, as for 13 26.2 7 ERNl N-CH lJ 15 26.1 5 orthy that the rea only 300 m Table 2.