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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. , . Mr. :N. Odero, Director, FIsheries Division, Mr. V. O. Sagua, Director, Kainji Lake Nairobi, . 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.

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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, , 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 , 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. Physico-chemical Data for Nyumba Ya Mungu (inflow area) (samples of surface water above 1-2 m depth) 1973 Date 1972 1973 4 Apr. 4th June 1st Jan. 3 April 12.45 '7 ' Time 13.30 12.00 14.45 L_.) Secchi disc (em) 45 70 Temperature "'C 22.8 26.8 31. 1 27.3 ocygen mg/l 6.0 7.0 15 Free C02 mg/l 20 40 40 810 conductivity K20 Jl?nhos/cm 520 550 480 8.4 pH 7.2 7.2 6.8 13 NO"N mg/I 8 9 13 0.02 o N02-N mg/I 0.01 0 0 P04.P mg/l 0.3 30.0 0.5 0.6 Si mg/l 30.0 30.0 Na mg/1 41.6 Kmg/I 4.8 o Ca (mg/l CaCO,) 60 9 380 S04 mg/l 9 13 12 Total alkalinity (mg/I as CaC03) 185 150 0.02 200 o Fe mg/I o 05 005 0 Mn mg/l o 1 0.2 0.2 Total dissolved ~oljds mgjl 285 Total solids O1g/1 305 Suspended solids O1gJl 2.0 Organic matter mg/1 110 42 T. PElR

Table J, Physico-chemical data for the Ruvu River inflow area has its 1 1972 1973 end of the dry seas< Dale 5th June 30th Dec. SLh April the Ruvu River t] Time 08.45 '5.30 j 1.30 u Temperature DC 22.6 24.5 25.3 constant througho 1 Oxygen mg!l 2.5 5.0 1.5 area of Nyumba ), Free C02 rngil 6 10 44 alkalinity during the conductivity K20 ).lrnhos/cm 470 550 450 (December. \972) th: pll 6.8 6.7 6.8 (June. 1972). Botto] NO,-N mg/l 4.5 - 8.0 NO,-N mgll 0 0 0 alkalinity than sur PO,.P mg/l 0.3 0.4 0.3 speaking, the alka} Si mg/I 60 30 60 Mungu and Ruvu Na mgJ1 42,9 about three times h K mg/I 4.9 Great Ruaha Rive Ca (mg!l CaC0 } 60 90 3 60 (PETR \973). The hil S04 mg/1 4 5 2 Ya M ungu may fae Total alkalinity (mg/l a~ CaC03) 225 220 215 Fe mg/! 0.1 0.05 0.1 plankton production 'Mn mgfl o. t 0.1 • 0.1 pH values of 8.4­ Total dissoht:l1 solids mg/1 305 the dam area during Total solids mgf J 307 high pH results in a Sw;pended solids mg/1 2.0 Organic matter mg/l J4,5 In the area of tbe Ri the River Ruvu itse from the intake. This may indicate that the conductivity values (450-550) ~mhos in the was always present continuou& removal of water for the turbines Ruvu River and inflow areas as compared from 6.7 to 7.2, bein disturbs the stratification only in the im­ with 800-960 ~mhos in tbe dam area). side in the river, but mcdiate vicinity of the intake. It is probable that the high sodium content in the inflow area, Most of the physico-chemical analyses of the water is accompanied by a high riverine water may l:x were done· with the Hach model DR-FL chloride content, but the latter was not humic acids, released portable water engineer's laboratory. Tn determined. It was suggested by BAILEY swamp through wi before reaching the 1. considering the results for P04 - P, N03 -N, (1965) that tbe salt pans on the flood plain NOz-N. Fe and Mn limitations inherent would affect tbe chemistry of the lake. phos-phates shOW in the use of this equipment should be borne But the fairly high conductivity of water river than for tbe lal in mind. Total dissolved solids, suspended in the Ruvu River, compared with some seems to be higber solids, total solids, and organic matter other rivers of Tanzania, such as the Great the readings for I content of stored samples were carried out Ruaha and its tributaries (PETR 1973), bighly accurate. B by tbe Chief Government's Chemist labora­ indicates that the lake receives a considerable lake have similar tory in Kampala. The same samples wt;rt; amount of ions through this inl1ow. These indicates that this used for potassium and sodium determina· are evidently carried from , which in the growth of die tions, which were carried out on a flame is considered to be slightly salty (BROWN difference between regards their iron spectrophotometer in the Botany Department 1971) with a conductivity K ZIl ~, 834 ~mhosf of Makerere University, Kampala. cm (KILHAM 1971). It is not known how The total disso Conductivity increases in the foilowing the salinity is influenced by passage through suspended solids, I:i sequence: river Ruvu < inflow area < dam the swamp. situated between Lake Jipe and are higher in the d' area (Table 1-3). The Ruvu River and the Nyumba Ya Mungu. and this sbould be and the Ruvu Rive inftow area have only one third the sodium investigated. content at the dan content (41.& 42.0 mg/I) of the dam area The total alkalinity of the Ruvu River is sence ofthe ricb po (122.2 mg!I), and this agrees with tbc close to that of the lake (Tables 1-3). The The ratio of to NYUMBA YA MUNGU. TANZANIA 43

1973 inflow area has its highest alkalinity at the mean depth gives a high morphoedaphic ~ April end of the dry season (Apnl 1973), but in index of 6.0. This index is higher than those 11.30 the Ruvu River the alkalinity is fairly calculated for the large African man-made 2l.3 constant throughout the year. The darn lakes [Volta ~ 3.2-4.5 (REYNOLDS 1973), 1.l 14 area of Nyumba Ya Mungu has higher Kainji ~ 3.2 and Kariba ~ 1.69 (REGIER :0 alkalinity during the peak of the dry season 1971)]. 6.8 (December, 1972) than at the end of floods 8.0 The Ner Plankton and Aquatic Macrophytes o (June, 1972). Bottom waters have higher alkalinity than surface water. Generally The net plankton of the dam area. where U ) speaking, the alkalinity of Nyumba Ya three samples (April, June and December) Mungu and Ruvu River is high, in fact were collected. was always dominated by about three times higher than that of the Aficrocystis. }"felosira was also fairly com­ and its tributaries mon, but the diatom Nitzschia and Nal'icula (PETR 1973). The high alkalinity of Nyumba were found in very small numbers. The .1 Ya Mungu may facilitate the high phyto­ zooplankton in all three samples included I plankton production in the lake. nauplii, metanauplii and adult copepods pH values of 8.4-8.9 were measured in (Cyclopidae). but in April 1973 nauplii the dam area during the day. This relatively were very scarce. No Diaptomidae were high pH results in an absence of free CO2• found. The December. 1972 sample contained In the area of the River Ruvu inflow and in a certain number of cyc10pid females with the River Ruvu itself, free carbon dioxide eggs, which were absent from the other ~mhos 50) in the was always present and pH values ranged two samples. Cladocera were also found in reas as compared from 6.7 to 7.2, being always on the acidic all three samples, but they were ver:)' scarce the darn area). side in the river, but usually slightly alkaline in the December sample. Rotifers were 1 sodium content in the inflow area. The lower pH of the represented mainly by Brachionus, which nied by a high riverine water may be due to the presence of were present in all three samples. The latter was not humic acids, released into the water from the December, 1972 sample contained two :ed by BAILEY swamp through which the river passes additional species of rotifers. , the flood plain before reaching the lake. The plankton was richest in December y of the lake. Phosphates show higher values for the 1972 at the peak of the dry season. and ::tivity of water river than for the lake, while nitrate-nitrogen poorest in April, 1973 at the beginning of ~red with Some seems to be higher in the-lake. However, rains, when the water level was at its lowest. ;h as the Great the readings for nitrate-nitrogen are not But in general, plankton seems' to be ab­ (PETR 1973), highly accurate. Both the river and the undant in the main lake throughout the ; a considerable lake have similar silica contents, which year, and it should be able to support a : inflow. These indicates that this is not a limiting factor large plankton feeding fish population. ke Jipe, which in the growth of diatoms. There is very little The area of the Ruvu inflow into the lake alty (BROWN difference between the river and the lake as is much poorer in plankton than the dam ~ ~mhos/ 834 regards their iron and manganese contents. area. The water there is fairly turbid. which )1 known how The total dissolved solids, total solids, by itself may prevent the development of .ssage through suspended solids, and organic matter content phytoplankton. Although there is some Lake Jipe and are higher in the dam area than in the inflow plankton present in the water, its composition is should be and the Ruvu River. The high organic matter is different from that in the dam area. content at the dam evic'ently reflects the pre­ There is a conspicuous absence of Melosira ~uvu River is sence ofthe rich population in the water there. and of the blue-green MicrocySlis. The les 1-3). The The ratio of total dissolved solids to the phytoplankton is represented mainly by 44 T. PhlR various species of diatoms. Closteriurn was of nurnerou& plant islands, mainly around Table 4. List of Fish S also regularly found, evidently washed into dead trees. At that time the quantity of the lake from the swamp through which aquatic plants was considerably higher the Ruvu River passes further upstream. than ill June 1972, with the commonest Characi< Small numbers of Oscillatoria. Dietyo­ plants being Pislia, Ceralophyllwn, and Azalia Rhaba sphaerium and Chrysophyta were present nilulica. The islanels consisted predominantly Cyprinic Barbu, in both samples. The zooplankton contained of Typha dominguensis, Cyperus spp., Phrag­ Barbu. several species of rotifers. but none were mites maurilianus and Ludwigia ereeta, all Barb/,{ abundant. The January sample also contained ofthem flourishing. It is apparent that aquatic Barbu nauplius larvae and Copepoda, suggesting and wetland macrophytes are well developed Barbu that shortly before that time the cyclopid during the lowest water level, and they are Labeo Clariida population was breeding throughout the especially numerous on the recently exposed Clarj~ lake, as large numbers of nauplii were also margins of the lake. Mocho.;; present in the dam. area in January. In Along the Ruvu River, Pahrgmiles and SY1f()(j the April sample only rotifers were found. papyrus grow, and in the water itself there Anguilli The poorconcentration of plankton in the are fairly large £reas with dense populations AngUI Cichlid~ inflow area evidently kd to l:hange in feeding of Ceratophyllum demersum. In quiet sections Tilapl habits for some fish, especi" lIy Tilapia of the river Pislia stratiotes is present in Tiiapi pangani and T. jipe. In the darn area they small amounts. Tt evidently came into this SarOl both fed on phytoplankton, but in the inflow area from a swamp situated a few kilometres Haph area their guts contained bottom deposits. above the site. Before the closure of the dam, a slight • Num' Fishes of Ihe Lake ~r Ihe Ruvu River tOne, marginal reed swamp was present in parts 1972. of the Pangani and Ruvu ri""ers and papyrus A set of monofilament gill-nets of mesh and water lily were recorded from the swamp sizes ranging from 20 mm to 100 mm near the railway bridge in the north-east (stretched) were used to capture as wide of the present lake (BAILEY 1%5). Bailey a range of fish species in the lake and the also found Pistfa and Ceratophyllum in the river as possible. The dam area was fished Ruvu River. on two occasions for 24 hours, with the aim During the present survey Phragmites of obtaining data on diel changes in fish was found fringing the bays of tht: dam area, activity and on relative abundance in Table 5. Total Nwnl and it was especially abundam along the different seasons. In the upper part of the northern shoreline. In general, however, lake and in the Ruvu River only daylight Date the dam area was found to be very poor fishing was carried out, and in the river Time in other species of aquatic and wetland only three different mesh size nets were Number plants. set. viz. 20. 40 and 60 mm. Wet weight (g) In the north of the lake, near the inflow Out of the total 14 species recorded from Overall mean biom of the Ruvu River, a great variety of mano­ experimental catches and commercial fish phytes was present both in the water and landings (TanIc 4), 7 spedes are of potential Date along the shores. In June 1972, during the commercial importance. with 4 of these Time high water level, Ceratophyllum demersum being exploited \'iz. two Tilapia, Sar01hero­ Number was common, and a small number cf Pistia don esculentus and Cfarias mossambicus. Wet "''eight (g) stratiofes and Ludwigia erecta were also In the dam area the smallest mesh size Wet weight (g{hr) Overall mean bion present. Typha was the dominant plant of nets of 20 mm caught predominantly Hap/o­ the shoreline. In April, 1973 the low water chromis (June, 1972) or Rhahdalesles (April, INets applied: 2' level in the lake resulted in the formation 1973). These were followed by Barbus 2Nets applied; 2 NYUMBA YA MUNGU. TANZANIA 45 nly around Table 4. List of Fish Species in Nyumba Ya Mungu and in the Ruvu River iuantity of bly higher Ruvu NYM NYM commonest River in~ow dam Characidae and Azoll a Rhabdalestes tangens;s (Lonnberg, 1907) !ominantly Cyprinidae + + 'p., Phrag­ Barbus paludinoslls Peters, 1852 + + +" erecta, all Barbus kerstenii Peters, 1868 + + tat aquatic Barbus oxyrthynchlls Pfeffer, 1889 + + Barbus jacksonii GUnther, 1889 .;- developed + + Barbus !ineomaculafus Boulenger, 1903 + + I they are Labeo cylindricus Peters, 1852 + -j + y exposed Clariidae Clarias mossambicus Peters, 1852 + '1ifes and Mochocidae self there Synodonlis punctulatus Gunther, 1889 + Anguillidae pulations Anguilla nebulosa labiata Peters, 1852 ( +)* t sections Cichlidae 'esent in Tilapiajipe Lowe, 1955 + + + into this Tilapia pangani Lowe, 1955 + + + lometres Sarorherodon esculemus (Graham, 1929) + Haplochromis gr. "bloyeri" Sauv., 1883 + .;- +

* Numerous specimens were seen in conunercial fish catehes. t One specimen over 100 cm tolal length was eaptured by commercial fishermen in December, )f mesh 1972. 00 mm .s wide md the fished he aim In fish ce in Table 5. Total Number and Weight for 24-hoe.r Experimental Fishing in Nyumba Ya Mungu of the Iylight 1972 1 river Date 2-3 June were Time 15.00-18.00 18.00-10.00 10.00-15.00 24 hour weight Number 139 278 101 Wet weight (g) 4,792.1 26,444.7 9.813.2 41,050.0 from OveraU mean biomass 1,710.4 g/hr fish mtial 1973' these Date 4-5 April Time 12.00-17 .00 17.00-11.00 24 hour weight 'fero- Number 77 334 Wet weight (g) 7,046.9 6,885.8 14,538.0 size Wet weight (g/hr) 1,409.4 382.5 ,plo­ Overall mean biomass 605.8 g/hr pri!, rhus tNets applied : 20,30,40, 50, 60, 80, 100 mm (stretched), one of each size; each net 1.5 m deep, 33 m long. 2Nets applied: 20,40, 50, 70, 100 nun (stretched), one of cach size; each net 1.5 m deep, 33 m long. 46 T. PETR of the man-mac pa/udinosus and B. lineomaculatus; Tilapia were very common in 80 mm nets (about Coast which is 0 formed only a minute proportion of the catch 3-!- inch), which is very close to the 31 inch In spite of th< from these nets. In 60 to 100 mm nets the mesh size gill nets whieh are the most used in the RUVl - catch was dominated by Tilapia. Tilapia common nets used (ANNUAL REPORT in the inflow an dominated the total catch of all nets together, 1970). Ripe females of Sarotherodon escu­ with the excepti and they were followed by either Labeo lentus (TL 17.1-185 em) were collected - B. Iineomacu/atu. or Synodontis. in April, 1973 [rom 70 mm nets (:1 inch). In June 1972 there was little difference Ripe females of Labeo cy/indricus were Table 6. The Troph in the nocturnal and day-light catches in round in the river and the lake in April, in experim weight and numbers of fish. However, but at the end of rains they were absent. s in April 1973, the night catch was con­ This may indicate that Labeo breeds at the siderably poorer than the day catch. The end of the dry season or the beginning of RhabdaJ ....• overall results of the diel fishing, when both rains, and this coincides with the breeding Barbus j catches are compared, seem to suggest that of Labeo in the Great Ruaha River (RPETR Barbus, the lake fish catches decreased from June 1973). MatuLity coeRicients calculated from Barbus, Labeo c 1972 to April 1973 (d. 41 in June 1972 the body and gonad's weight for Barbus Synodo. compared with 15 kg in April 1973) (Table line omaculatus, B. pa/udinosus, Rhabdalestes Tilapia 5). However, these data are not exactly tangen.'lis, and Synodontis punetulalu.r were Tilapia comparable for three reasons: the fishing higher at the end or floods than at the onset Sarothl in 1973 was carried out about 1 km distant of rains, indicating that these species, and Haplol from that in 1972, in a different season, possibly some others, for which not enough and the set of nets applied in 1973 differed material was available, spawn predominantly slighily from that of 1972. Nevertheless, towards the end of Roods. it is possible that a decrease in fish abundance Bascd on food analysis of 538 fish be­ may be at least partly responsible for the longing to 10 species and captured during difference between the 1972 and 1973 the two die I experimental fishings, the trophic catches as indicated by the continuous structure of the fish community in the dam decline in commercial catches since J970. area was calculated. Barbus jacksonii and such as Tilapi< The females or the majority of species C/arias mossambicus, on which no or in debris from s respond to the onset or rains (April) by adequate information is available, are not weed beds, , developing gonads and spawning. Large included. Primary consumers, i.e. phyto­ from the su numbers of the small Barbus lineomaculatus plankton feeders and deposit feeders (Tilapia, phytoplankto and Rhabdalestes tangensis (97 to 100 %) Sarotherodon and Labeo), form 82.3 ~~ of were found to be ripe females both in April the total biomass of the fish capture. Omni­ piscivores WI mossambicus, and in June. i.e. at the end of floods. Mature vores (all Barblls species and Synodontis) probably inh females of Synodontis formed 43 to 69 ~~, form 14.4 o~ and carnivores (Haplochromis Differentia Haplochromis gr. 'bloyeti' remales formed and Rhabdalestes) 3.3 '; of the total weight 24 to 38 % and Tilapia females 25 to 57 % (Table 6). The carnivores and most of the was most tangensis ar of the total adult specimens at the onset omnivorous fish in the lake are small in They are b, of rains and at the end or floods. The smallest size, but appear in large numbers. The river, feediJ ripe Tilapia pangan; was 20.0 cm (total absence of large predators of commercial in the infl( length), and it occurred in a 40 mm net, importance in Nyumba Va Mungu is in l this corresponding to about l~ inch mesh sharp contrast with their abundance in the vorous. fe size. This size of net is not used by the large man-made lakes Volta, Kariba and filamento on aquatic commercial fishermen. Ripe females of Tilapia Kainji. The trophic structure of Nyumba jipe and T. pangani of 22.5-25.0 cm size Ya Mungu seems to be much closer to that dam they NYl1MIlA YA MUNGll, TANZANIA 47

in 80 mm nets (about of the man-made lake Ayame in Ivory the Ruvu River. The Ruvu River seems to ry close to the 3t inch Coast which i' of a ,imdar size (186 km'). have two distinct trophic groups: a group which are the most In spite of the limited number of nets of small species such as Barbus, Rhabda­ (ANNUAL REPORT used in the Ruvu River, all species present lesfes and Haplochromis, which all feed of Sarofherodon escu- in the inflow area of Nyumba Ya Mungu. mainly on chironomids, which they evidently .5 em) were collected with the exception of B. o:ryrhynchus and obtain predominantly from aufwuchs, and 70 mm nets ('1 inch). B. lineomaculafus, were also captured from a group of deposit and aufwllchs feeders, ..abeo cylindricus were >od the lake in April, Table 6. The Trophic Struclure of Fishes Taken in the NYL'mta Ya Mungu dam area (based on fish capcured ams they were absent. in experimental nets during 24-hour fishing in June, 1972 and Apd], 1973) at Lobea breeds at the Species Feeding No. % No. Weight Weight ~n or the beginning of category (g) ( /'~) Ides with the breeding Rhabda/eSfes fange1lSi.~ 4 157 14.5 679 1.2 ;Ruaha River (RPETR Barbus paludinosus 3 135 12.4 690 1.4 Barbus lineomanulafUS 3 80 7.3 441 0.8 lcients calculated from Barbus oxyrhynchus 3 9 0.8 421 0.8 's weight for Barbus Labeo eylindricus 2 109 10.0 8.206 15.2 lUdinosus, Rhabdalesfes S.vnodontis plme/ulatus 3 114 10.4 6.216 11.4 antis puncfu!afus were Tilapia jipe 1 172 159 34.113 62.5 oods than at the onset Tilapia pangani Sorotherodon esculentus 1 32 2.9 2.511 4.6 lat these species, and Haploehromis gr. "bloyefi"' 4 281 25.8 1,131 2.1 for which not enough spawn predominantly Total 1,089 100.0 54,439 100.0 Ids. Feeding calegories lysis of 538 fish be­ %Number % Weight and captured during I Fhytoplanklon feeders 18.8 67.1 al fishings, the trophic 2 Bottom deposit feeders 10.0 15.2 3 Omnivores 30.8 14.4 Immunity in the dam 4 Carnivores 40.2 3.3 rJarbus jacksonh and on which no or in such as Tilapia and Labeo, feeding on organic jipe and T. pangani also have a different is available, are not debris from sediments deposited in aquatic diet in different areas, being bottom and ISumers, i,e. phyto­ weed beds, and on debris and aufwuchs aufwuchs feeders in the river and in the pOsit feeders (Tilapia from the surfaces of aquatic plants. No inflow area of the lake, but plankton feeders '0), form 82.3 ~~ of phytoplankton or tripton feeders and no at the dam (Table 7). fish capture. Omni­ piscivores were recorded, although Clarias LOWE (1955) noticed that one species es alld Synodon/is) mossambicus, belonging to the latter group, of Tilapia in Lake Jipe was feeding on a 'OTes (Haplochromis probably inhabits the river. variety of food, such as plant material and of the total weight Differential feeding in different habitats bottom debris. This ability to adapt to " and most or the was most conspicuous in Rlwbdalestes various kinds of food indicates the high de­ Jake are small in Tangensis and Haplochromis gr. "bloyefj", gree of adaptability of Tilapia, and possibly rge numbers. The They are both strictly insectivorous in the also other species, to new environments, ors of commercial river, feeding on chironomid larvae; but and seems to contribute to their successful Ya Mungu is in in the inflow area Haplochrornis is omni­ and rapid colonization of ne'.'! reservoirs. abundance in the vorous, feeding on chironomids and green When the fish statistics were collected Tolta, Kariba and filamentous algae, while Rhabdalesres feeds in 1970, the predicted 500-1,000 tons of fish cture of Nyumba on aquatic and terrestrial insects: and at the in c:ommercial catches per annum (BAILEY mch closer to that dam they both feed on zooplankton. Tilapia 1965) for Nyumba Ya Mungu proved to 48 T. "ETR

Table 7. Feeding Habits of Fish in the Dam and Inflow areas of Nyumba Ya Mungu SUMMARY I. The NyUlnba Species Dam area Inflow area lake on the PangaJ Rhabdaleste,f iaflgensis zooplankton feeder insectivore Region, Tanzania) w Barbus paludinosus omnivore omnivore 1965, and the filling \ B. IineOmaCIJlnllfS omnivore omnivore B. axyrhyndms omnivore Discivore 196R, resulting in the B. jacksonii omnivore about 150 km' surfa! Lobeo cylindricus bottom deposits bottom deposits lake is situated in Svnodontis punrllliafus ommvort: present limnological Tilapia jipe phytoplankton feeder bottom deposits on this lake in the G. pangani phytoplankton feeder aufwuchs feeder S. esculenlus phytoplankton feeder impoundment. The I Haplochramis lJ,,'. "bloyetj" zooplanklon feeder omnivore river, was also inve~ • 2. The lake in the be a considerable underestimate. In 1970, resulted in very heavy fishing pressure on be stratified at the i.e. 5 years after the closure of the dam, this fish population and in the very effective (December) and at the total landings were 28,508.5 metric removal of large numbers of adult Tilapia. (April). An oxygen tons. (ANNUAL REPORT 1970). This It has been observed that also in large man­ 5 and 9 m. depth" corresponds to 312 kg per hectare, which made lakes such as Volta and Kariba, the of the dry season, is higher than the harvest from managed, largest fish catches occurred in the 5th and practically without unfertilized ponds in East and Central 6th years of impoundment and then a decline plankton was abl Africa (BAILEY 1966). This was followed in catches followed (PETR in press). Microcvstis and c, by a decline in landings in the following The application of a particular mesh 3. The conductiv years: in ·1971 catches had decreased to 10 sized gill net for each niche, which is char­ in the sequence Ri 370.5 tons (ANNUAL REPORT 1971), acterized by dominance of a certain fish dam area. At the in 1972 to 7,287.4 tons, and in 1973 it was species, was suggested for Lake Kainji, it is about twke estimated that only 3,000 tons of fish were in order to successfully manage and utilize River (450-550 11· landed (KAYUZA, personal communica­ experimental fishing of Nyumba Ya Mungu water may origin: tion). The 1973 crop corresponds to 33 kg proved that the 31 inch gill net, which is slightly salty. 1 per hel:tare. This value is much closer to the is currently the one most widely used by the is on the aJakaJin( average of 27 kg per hectare per annum commercial fishermen, does not catch im­ that in the infiue calculated for 8 dams of Tanzania with mature Tilapia and allows the Tilapia to (pH 6.8.-7.2), bei surface areas ranging from 1.2 to 1,540 'pawn at least once, and possibly several water (pH 6.7-6.8 hectares (BAILEY 1966). There were 3,161 times, before they reach the size at which the RuYU River n fishermen on the lake in 1970, and 3,386 they are captured. On the other hand, the of humic acids rt in 1971, indicating that in 1971 the catch fishing pressure docs seem to result in a it passes througl per fisherman was only one third of that in very high fish mortality for the adult Tilapia Lake Jipe to Ny 1970. This evidently became even worse in and Sarotherodon above a certain size, 4, The conce the years which followed, with catches in consequently affecting the number of young potassium are h 1973 being only I 0.5~;'; of those in 1970. being produced by them. A thorough survey than in the Ru\ It is almost certain that the lake had the of the effect of various size giU-nets on fish the high sodiulI highest standing crop of fish in 1970 as a populations in various niches of the lake, together with a result of very favourable breeding and indicating surface and deep waters, is the latter was feeding conditions between 1966 and 1969 desirable for establishing the preferred habi­ were reported 1 and small fishing pressure. It is probable tats of particular fish species and their age covered by the: that the increase in number of fishermen who groups, with the aim of exploitation of fish 5. The mor have heen employing more and more gear, species other than Tilapia and Sara/herdan. higher than tb NrUMBA YA MUNGU, TA'lZANIA 49 Mungu SUMMARY made lakes. However, it seems to be pre~ lfiowarea I. The Nyumba Ya Mungu man-made mature to apply this index to a not yet Yore lake on the (Kilimanjaro fully stabilized lake for tbe prediction of 'Ore ore Region, Tanzania) was closed in Dccember polential fish harvest. 'r. 1965, and the filling was completed in April, 6. In the dam area of tbe lake, littoral Jr. 1968, resulting in thc formation of a lake of macrophytes are poorly developed, but they I deposits about 150 km' surface area. This equatorial are abundant in the inflow area. especially lake is situated in a savanna region. The during tbe lowest water level in April. deposits present limnological study was carried out In the inflow area the fol1owing plants are hs feeder on this lake in the 7th and 8th years of common: Typha dominguensis, Cyperus spp., re impoundment. The Ruvu, a major inflowing Phragmites mallr;fjanlls, and Ludwigiu river, was also investigated. erecfa-al1 fringing the shores and forming 2. The lake in the dam area was found to islands; Cerarophyllum demersum, Pisfia y ~shing pressure on be stratified at the peak of the dry "ason stratiotes, and A=oila nilotica--submerged d In the vt::ry effective (December) and at the beginning of rains or floating plants. Jers of adult Tilapia. (April). An oxygen discountinuity between 7. Experimental fishing with monofila­ lat also in large man­ 5 and 9 m. depth was recorded at the peak ment nets of various mesh sizes established llta and lCariba, the of the dry season, leaving the deep water the presence of 14 species of fish in the lake "rred in the 5th and practically without oxygen. At that time, and 9 species in the river. Another species nt and then a decline plankton was abundant, dominated by was recorded from commercial landings 'ETR in press). Microcysfis and copepods. from the lake. It is quite probable that more .a particular mesh 3. The conductivity of water increases species inhabit the river, which was fished liche, which is char­ in the sequence River Ruvu-inflow area­ only during the day. . , of a certain fish dam area. At the dam (800-960 ~mhos) 8. Diel experimental catches in the dam for Lake Kainji, it is about twice as high as in the Ruvu area of the lake revealed that Synodontis manage and utilize River (450-550 ~mhos). The Ruvu River puncwalofus and Clurias mu.\'sambicus are Iyumba Ya Mungu water may originate in Lake Jipe, which predominantly nocturnal. The night catches h . giJl net, which is slightly salty_ The pH in the dam area did not give a higher total weight than the wIdely used by the is on the alakaline side (pH 8.4-8.9), wbile day catches. Jes not catch im­ that in the influe area is close to neutral 9. In the dam area of the lake Tilapia ;YS the Tilupia to (pH 6.8.-7.2), being a!feited by the riverine dominated the weight of fish in experimental I possibly several water (pI! 6.7-6.8). The relatively low pH of nets. Tilapia jipe, Tilapia pangani, and Ihe size at which the Ruvll River may be due to tbe presence Sarofherodon esC'ulentus constituted 70 ~~ . other hand, the of humic acids released into the river when and 51 % of the total weight in June, 1972 1 to result in a it passes through a swamp on its way from and April, 197:1 respectively. Commercial the adult Tilapia Lake Jipe to Nyumba Ya Mungu. landings from the open lake consist almost a certain size, 4. The concentrations of sodium and entirely of Tilapia and SarofherodufI, while umber of young potassium are higher in the lake-dam area those from the inflow area also contain thorough survey than in the Ruvu River. It is possible that a large proportion of Clarias mossambicus. gill-nets On fish the high sodium content (122.2 mg/I) goes 10. Most fish spawn at the end of floods, es of the lake together with a high chloride contcnt, but but Labeo cylindricus seems to spawn at the ep waters, i; the latter was not determined. Salt pans beginning of floods. The smallest ripe female preferred habi- were reported from the area before it was of Tliapia pangani was 20.0 em (total length). l and their age covered by the lake. Ripe females of T. jipe and T. pangani 'itation of fish 5. The morphoedaphic index of 6.0 is of 22.5-25.0 em were very common in 80 mm 1 Sarotherdon. higher than those for large African man- nets, which is only slightly less than the 50 l. PETR

3'; inch mesh size gill nets most commonty REFERENCES A PREL used in commercial fishing, Ripe ft:maJes of Annual Report (970). The Inland Waters-Nyumba FISHER' Ya Mungu Dam-Summary of Statistics during S. escule"'''s (17.1-18.5 em) were collee 1970 (Appendix VI). In: Min. Nat. Res. & ted from the 70 mm nets (about 2i Tourism, Fish. Div. Statistics-Major Fresh­ inch). waters. Dar es Salaam, Tanzania. Ii. Some fish specie" including Tilapia, Annual Rcport (J 971). Ki!imanjaro Region. Fisheries seem to feed on a great variety of diets. Report. Min. Nat Res. & Tourism, nsh. W. B. MUTA l While the Ruvu Ri"'er fish community Division. Dar es Salaam, Tanzania. Bailey, R. G. (1965). Report to the Chir;:f Fisheries consists of insedivores. and deposit and Officer, Min. Agric., Forests & Wildlife. Tan­ aufwuchs feeders, that of the inflow is made zania. on . (Mimeo.), up largely of depo,it feeders, and that of 7 pp. the dam area is dominated by phytoplankton Bailey, R. G. (l966). The dam fisheries of Tan7ania. and bottom deposit feeders. This is not a E.A. Agrie. Forestry J. 32: 1-15. Brown, L. (J.971). East Africa mountains and lakes­ re~mlt of different species composition in Cast Afrjean Publishing House, Nairobi, Dar different areas, but the result of the same es Salaam, Kampala, 122 pp. fish adapting to different food because of Coche, A. G. (1968). Description ofphy:;ical-chcmical INTRODUl its availability. aspects of Lake Kariba, an impoundment in 12. The commercial fish landings have been zambia-Rhodesia -Fish. Res. Bull. Zambia hshmg 1'0 Special Issue: 200--267. has heen ! declining since 1970, and by 1973 they were Czernin-Chudcnitz:, C. W. (t97l). Physico-chemical only 10.5% of those in 1970. Thi, may be conditions of Lake Volta, Ghana-{]NDP/FAO 1960). Thi' the result of very heavy fishing pressure TIX:hnical Report 1, FI:SF/GHA 10,77 pp. water up t on the Jake fish stock, causing large numbers Ita, E. O. (1973). Approach~ to the evaluation and PracticaH), a of ~dult Tilapia to be removed. The dominant management of the fish sLock in Kainji Lake. by ,kin di' Nigeria~Afr. J. Trop. Hydrobiol. Fish., Special fishing gear are 3~ inch gill nets; and although a few whlcJ issue 1: 35-52. fish. Hall reo they do allow mature Ti/apia and Sarothe~ Kilham, P. (1971). .Biogeochemistry or African lakes rodon to spawn at least once, if applied in and rivers. Ph.D. Thesis Duke University, led to a d' large numbers, they may effeeti,'ely remove Durham, N.C., U.S.A. moving on all or almost above a certain size, con­ Lowe, R. H. (1955). New species of Tilapia (Cichlidllc) nllously do from Lake Jipe and the Pangani River -British either too sequently affecting the number of young Museum (Nat. His/.) (Zoo/., 2: 12: 349-368. being produced by them. Petr, T. (1973). A pre.impoundment study, wirh hatched el special emphasis on fishes, of the Great Ruaha Stocks COl ACKNOWLEDGEMENTS: Many thanks i:l.fe due River (Tanzanja) and of ~ome of its tributaries into the [ to Mr. M. A. Kayuza, the Regional Fisheries Officer (River Yovi. and the Little Ruaha) in and around Manageme in Moshi, who provided \lseful data on Nyumba Ya the proposed impoundment areas-Report for established Mungu Dam and 10 his fisheries team stationed on the Uppsala UniversHy, Sweden. 87 rr. (Typed). Manage this lake, who readily helped with the collection of Petr. T. (in press). On some factors associated witb the scientific material. Dr. R. G. Eailey of Chelsea the initial high fish catches in new Afrk:an the modifi' College (UniYersity of wodon) provided invaluable man-made lakes. of the fisb help with the identification of fish. I am also grateful Regier, H. A. (1971). Evaluation offisheries resources in the cat to SWECO officials in Dar es Salaam and Kidatu jn African freshwatcrs-Afr. J. Trap. Hydrobiol. would be for helping organize this survey. Finally, r wish to Fish. 1: 69--S3. continue ( [hank Dr. E. K. Balon, Dr. l.A.E. Bayly. Dr. G. G. Reynolds, J. D. (973). Report on fish production Ganf and Mr. John Melack for their helpful criticism and nutritiun in the Volta Lake-Prepared for this may of the manuscript. the Smithsonian Institution. 47 pp. (Mimeo). problems ment is to cuntin A stud was star