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LIMNOLOGY AND FISHERIES OF THE NYUMBA YAMUNGU, A MAN-MADE LAKE IN TANZANIA

T.PETRDepartment ofZoology, Monash University, Clayton, Victoria 3168, Australia

INTRODUCTION

After the closure of the large dam on theZambezi River in 1958, which resulted in theformation of Lake Kariba, much attentionhas been paid to this and to similar largeman-made lakes, with surface areas morethan 1,000 km 2 , which were formed later onvarious other African rivers. Because oftheir large size they have attracted theattention of fishery scientists, who antici­pated large fish catches. Considerably lessattention has been paid to the lesser lakeswhich appeared on smaller African rivers.These lakes, such as Lake Ayame in IvoryCoast, and Lake Nyumba Ya Mungu inTanzania, are less than 200 km' and usuallydo not appear even on more detai led regionalmaps. However, the fish density explosionin some of them soon after their formationwas no less spectacular than that in some ofthe large African man- made lakes.

The present paper deals with the limnologyof Nyumba Ya Mungu, a man-made lakein Tanzania (Kilimanjaro Region), duringthe 7th and 8th years after the elosure of theRuvu dam. The area investigated includedthe River, which is one of the two riversentering the lake.

RESULTS AND DISCUSSION

N}'umba ya MunguLocation: Tanzania, Kilimanjaro Region,

3' 40'S: 37' 25'EAltitude: 663 m a.s.l.

Climate: dry, with one rainy 'season (April-­June).

surrounding vegetation cover: savanna, withsemi-arid scru b.

Dam closed (on the river Pangani) on1.12.1965.

Dam full on 29.4.1968.Surface area when full: 150 km'Maximum length: 32 kmMaximum width: 8 kmMaximum depth: 48 mVolume: 9 X 10' m'.Period of investigations: June, 1972-April,

1973 (3 visits).Physico-chemical Characlerisrics of the Lakeand the Rm'u River

The longitudinal dilferences in watertransparency reflect the amount of suspendedparticles, ineluding plankton, in the water.The upper part of the lake, infl uenced by theRuvu River, had more turbid water duringfloods (June, 1972), than during the dryseasons. This was due not only to theincreased concentration of particles broughtin by the river, but also to the stirring up of

40 T. PETR

sediments by wind action on the ~haHow

water. In the dam area high turbidity ofwater ""as recufiJed ilt the peak of the dryseason in December, 1972, but this can beattributed to dense phytuplankton. Theturbidity of the: Ruyu River is higher thanthat of the dam area hut lower than that ofthe inflow area. Secchi disc readings sho",'that during the rainy and flood seasons wateris about four times Jes~ transparent in theinflow area than in the dam area. 1 awardst he end of the dry 'icason this difference isless than during floods.

The surface water t,mpcratur~ is higherduring the dry season than during the floods,und the difference is morc pronounced inthe shallDw area of the lake where the riverenters. In the dam area thc.:re is only a smalltt-mpcrature gradient, with the differencebe:t\1ieen the surface and 2:l.5 m depth beingonly O.SoC at the beginning of rains (April,

1973). lim' ever, 3t the peak of the dryseason (December, 1972), in the same area,the temperature difTeren(.;e between the

surface and 15 m depth was 2.3°C (Table I).In this month there was little oxygen presentbeluw 7 m depth. Both the temperatureand the oxygen gradients indicate that thelake, at least in the dam area, was s.tratified

at that time, and als.o in April 1973, when in22.5 m depth only 1.5 mg.!] O2 was recorded.

It appears. that even 7 years after thecompletion of the dam, the lake chemicallystratifies. although it is pmsible that this5tratification is limited to the dam area,which is narrow and somewhat protected

from wind and currents by the land con­figllration. As no mea~urementswere carried

out elst:wht::re in the main lake. it is notkno\\'n whethe; the deoxygenated waterforms only pockeb in the deepest part!>such as ubserved tn Lake Kariba (COCHE1963), or whether the wbolc lake is stratifiedduring the dry season, with a continuouslayer of deoxygenated deep water, as forexample in Lake Volta (CZERNlN-CHU­DENITZ 1971). It is noteworthy that thewater is stratified in the dam area only 300 m

Table 1 (Cominued)..

At the intake-3rd 11Depth (m) Ter

Surface1357

101314

Table 2. Plwsico-cher

Table 1. F~ysko-ch~mkal Data for Nyumba Ya Mungu (dam area)

1972 1973Date 2 Jun. :\ Jun. 31 Dec. 4 Apr. 4 Apr. Timetime 16.':;:0 i2.3J II.CO 12.30 1245deoth (m) S 10.0 S S ~2.5

SeecTerr

Secchi disc (cml 167 - - 141 -temperature "C 25 6 25.1 284 27.8 "27.3

OCYI

oxygen mgtl R.9 7.3 9.1 8.5 1.5Fro

conductivity K2u pmhos/cm 020 8CO 960 810 ~Wcon,

pH 8.9 8.8 8.9 8.7 B.4pH

N03-N mgiI 10 7 0 12 ilNCNC

NOz-N mg/} 0 0 0 0.07 0.02 POP0 4.P mg!l 0.02 0.05 0.04 0 0 SiSi mg/l - 11.5 28.0 30.0 N'Na mg/l 122.2 1274 - - - KK mg/l 1>.0 12.7 - - - 0Ca (mg/l CaC03) 35 35 - 0 0 SCS04 mg/l 7 8 12 14 9total alkalinity (mg/I as CaC03) 200 ~CO 340 )05 380

T,

F'Fe mg/l 0.05 0 O.OJ 0.01 0.02

~VJn mg/l 0 0 0 0 0 Ttotal dissoh"ed solids mgJI 508 475 - - - Ttotal solids mgjl 513 485 - - - Ssuspended solids mg/l 4.5 10.0 - - Corganie Ir.atter mg/l 2:0 198

De~ember, 1972-300 ill from the intake-I 1.00 hours

Table J (Comjnued). Vertical Pronles of Temperature and Oxygen (dam area)

NYUMBA YA MUNGU, TANZANIA 41

June 1972300 m from the intake-Jed June, 1972--16.00 hours

Depth (m) Temp. 0C ~~ saL. OzSurface 27.6 117

1 27.0 1212 25.5 1183 25.2 1035 25.1 826 25.0 738 25.0 70

10 25.0 48

Depth (m) Temp. 'C ~~ sat. O2Surface 28.4 139

I 28.5 1073 28.2 835 27.3 497 27.2 409 26.4 16

II 26.2 813 26.2 715 26.1 5

At the intake-3ed June, 1972-12.25 hoursDepth (ill) Temp. cC ~.~ sat. 02

Surface 25.6 112I 25.5 1123 25.1 965 25.1 927 25.1 92

10 ~.1 9213 25.1 8514 25.1 85

Table 2. Physico-chemical Data for Nyumba Ya Mungu (inflow area)(samples of surface water above 1-2 m depth)

Date 1972 19734th June 1st Jan. 3 April

Time 13.30 12.00 14.45Secchi disc (em) 45 70Temperature "'C 22.8 26.8 31. 1ocygen mg/l 6.0 7.0Free C02 mg/l 20 40 40conductivity K20 Jl?nhos/cm 520 550 480pH 7.2 7.2 6.8NO"N mg/I 8 9 13N02-N mg/I 0.01 0 0P0 4.P mg/l 0.5 0.3 0.6Si mg/l 30.0 30.0Na mg/1 41.6Kmg/I 4.8Ca (mg/l CaCO,) 60S04 mg/l 9 13 12Total alkalinity (mg/I as CaC03) 185 150 200Fe mg/I o 05 005 0Mn mg/l o 1 0.2 0.2Total dissolved ~oljds mgjl 285Total solids O1g/1 305Suspended solids O1gJl 2.0Organic matter mg/1 110

27.315

8108.4

130.02o

30.0

Ilas 2.3'C (Table l),~tle oxygen present~ the temperature, indicate that the~ea, was stratifiedpril 1973, when inI O2 Was recorded, years after th~

e Jake chemically"'$SIble that this

the dam areaIcwhat protectedy the land Con­~nts were carriedlake. it is nol

ygenated v.ater, deepest parts,ariba (CaCHElake is stratifiedL a Continuous

water, as forERNl N-CH lJ­orthy that therea only 300 m

o9

3800.02o

19734 Apr.

12.45'7 'L_.)

42 T. PElR

Table J, Physico-chemical data for the Ruvu River1972 1973

Dale 5th June 30th Dec. SLh AprilTime 08.45 '5.30 j 1.30Temperature DC 22.6 24.5 25.3Oxygen mg!l 2.5 5.0 1.5Free C02 rngil 6 10 44conductivity K20 ).lrnhos/cm 470 550 450pll 6.8 6.7 6.8NO,-N mg/l 4.5 - 8.0NO,-N mgll 0 0 0PO,.P mg/l 0.3 0.4 0.3Si mg/I 60 30 60Na mgJ1 42,9K mg/I 4.9Ca (mg!l CaC0 3} 60 60 90S04 mg/1 4 5 2Total alkalinity (mg/l a~ CaC03) 225 220 215Fe mg/! 0.1 0.05 0.1'Mn mgfl o. t 0.1 • 0.1Total dissoht:l1 solids mg/1 305Total solids mgf J 307Sw;pended solids mg/1 2.0Organic matter mg/l J4,5

from the intake. This may indicate that thecontinuou& removal of water for the turbinesdisturbs the stratification only in the im­mcdiate vicinity of the intake.

Most of the physico-chemical analyseswere done· with the Hach model DR-FLportable water engineer's laboratory. Tnconsidering the results for P0 4 - P, N0 3 -N,NOz-N. Fe and Mn limitations inherentin the use of this equipment should be bornein mind. Total dissolved solids, suspendedsolids, total solids, and organic mattercontent of stored samples were carried outby tbe Chief Government's Chemist labora­tory in Kampala. The same samples wt;rt;used for potassium and sodium determina·tions, which were carried out on a flamespectrophotometer in the Botany Departmentof Makerere University, Kampala.

Conductivity increases in the foilowingsequence: river Ruvu < inflow area < damarea (Table 1-3). The Ruvu River and theinftow area have only one third the sodiumcontent (41.& 42.0 mg/I) of the dam area(122.2 mg!I), and this agrees with tbc

conductivity values (450-550) ~mhos in theRuvu River and inflow areas as comparedwith 800-960 ~mhos in tbe dam area).It is probable that the high sodium contentof the water is accompanied by a highchloride content, but the latter was notdetermined. It was suggested by BAILEY(1965) that tbe salt pans on the flood plainwould affect tbe chemistry of the lake.But the fairly high conductivity of waterin the Ruvu River, compared with someother rivers of Tanzania, such as the GreatRuaha and its tributaries (PETR 1973),indicates that the lake receives a considerableamount of ions through this inl1ow. Theseare evidently carried from Lake Jipe, whichis considered to be slightly salty (BROWN1971) with a conductivity K ZIl ~, 834 ~mhosf

cm (KILHAM 1971). It is not known howthe salinity is influenced by passage throughthe swamp. situated between Lake Jipe andNyumba Ya Mungu. and this sbould beinvestigated.

The total alkalinity of the Ruvu River isclose to that of the lake (Tables 1-3). The

inflow area has its 1end of the dry seas<the Ruvu River t]constant throughou1

area of Nyumba ),alkalinity during the(December. \972) th:(June. 1972). Botto]alkalinity than surspeaking, the alka}Mungu and Ruvuabout three times hGreat Ruaha Rive(PETR \973). The hilYa M ungu may faeplankton production

pH values of 8.4­the dam area duringhigh pH results in aIn the area of tbe Rithe River Ruvu itsewas always presentfrom 6.7 to 7.2, beinside in the river, butin the inflow area,riverine water may l:xhumic acids, releasedswamp through wibefore reaching the 1.

phos-phates shOWriver than for tbe lalseems to be higberthe readings for I

bighly accurate. Blake have similarindicates that thisin the growth of diedifference betweenregards their iron

The total dissosuspended solids, I:i

are higher in the d'and the Ruvu Rivecontent at the dansence of the ricb po

The ratio of to

1973~ April11.302l.31.l

14:06.88.0oU)

.1I

50) ~mhos in thereas as comparedthe darn area).

1 sodium contentnied by a high

latter was not:ed by BAILEY, the flood plainy of the lake.::tivity of water~red with Some;h as the Great(PETR 1973),

; a considerable: inflow. Theseke Jipe, whichalty (BROWN~ 834 ~mhos/

)1 known how.ssage throughLake Jipe andis should be

~uvu River isles 1-3). The

inflow area has its highest alkalinity at theend of the dry season (Apnl 1973), but inthe Ruvu River the alkalinity is fairlyconstant throughout the year. The darnarea of Nyumba Ya Mungu has higheralkalinity during the peak of the dry season(December, 1972) than at the end of floods(June, 1972). Bottom waters have higheralkalinity than surface water. Generallyspeaking, the alkalinity of Nyumba YaMungu and Ruvu River is high, in factabout three times higher than that of theGreat Ruaha River and its tributaries(PETR 1973). The high alkalinity of NyumbaYa Mungu may facilitate the high phyto­plankton production in the lake.

pH values of 8.4-8.9 were measured inthe dam area during the day. This relativelyhigh pH results in an absence of free CO2 •

In the area of the River Ruvu inflow and inthe River Ruvu itself, free carbon dioxidewas always present and pH values rangedfrom 6.7 to 7.2, being always on the acidicside in the river, but usually slightly alkalinein the inflow area. The lower pH of theriverine water may be due to the presence ofhumic acids, released into the water from theswamp through which the river passesbefore reaching the lake.

Phosphates show higher values for theriver than for the lake, while nitrate-nitrogenseems to be higher in the-lake. However,the readings for nitrate-nitrogen are nothighly accurate. Both the river and thelake have similar silica contents, whichindicates that this is not a limiting factorin the growth of diatoms. There is very littledifference between the river and the lake asregards their iron and manganese contents.

The total dissolved solids, total solids,suspended solids, and organic matter contentare higher in the dam area than in the inflowand the Ruvu River. The high organic mattercontent at the dam evic'ently reflects the pre­sence of the rich population in the water there.

The ratio of total dissolved solids to the

NYUMBA YA MUNGU. TANZANIA 43

mean depth gives a high morphoedaphicindex of 6.0. This index is higher than thosecalculated for the large African man-madelakes [Volta ~ 3.2-4.5 (REYNOLDS 1973),Kainji ~ 3.2 and Kariba ~ 1.69 (REGIER1971)].

The Ner Plankton and Aquatic Macrophytes

The net plankton of the dam area. wherethree samples (April, June and December)were collected. was always dominated byAficrocystis. }"felosira was also fairly com­mon, but the diatom Nitzschia and Nal'iculawere found in very small numbers. Thezooplankton in all three samples includednauplii, metanauplii and adult copepods(Cyclopidae). but in April 1973 naupliiwere very scarce. No Diaptomidae werefound. The December. 1972 sample containeda certain number of cyc10pid females witheggs, which were absent from the othertwo samples. Cladocera were also found inall three samples, but they were ver:)' scarcein the December sample. Rotifers wererepresented mainly by Brachionus, whichwere present in all three samples. TheDecember, 1972 sample contained twoadditional species of rotifers.

The plankton was richest in December1972 at the peak of the dry season. andpoorest in April, 1973 at the beginning ofrains, when the water level was at its lowest.But in general, plankton seems' to be ab­undant in the main lake throughout theyear, and it should be able to support alarge plankton feeding fish population.

The area of the Ruvu inflow into the lakeis much poorer in plankton than the damarea. The water there is fairly turbid. whichby itself may prevent the development ofphytoplankton. Although there is someplankton present in the water, its compositionis different from that in the dam area.There is a conspicuous absence of Melosiraand of the blue-green MicrocySlis. Thephytoplankton is represented mainly by

44 T. PhlR

various species of diatoms. Closteriurn wasalso regularly found, evidently washed intothe lake from the swamp through whichthe Ruvu River passes further upstream.Small numbers of Oscillatoria. Dietyo­sphaerium and Chrysophyta were presentin both samples. The zooplankton containedseveral species of rotifers. but none wereabundant. The January sample also containednauplius larvae and Copepoda, suggestingthat shortly before that time the cyclopidpopulation was breeding throughout thelake, as large numbers of nauplii were alsopresent in the dam. area in January. Inthe April sample only rotifers were found.The poorconcentration of plankton in theinflow area evidently kd to l:hange in feedinghabits for some fish, especi" lIy Tilapiapangani and T. jipe. In the darn area theyboth fed on phytoplankton, but in the inflowarea their guts contained bottom deposits.

Before the closure of the dam, a slightmarginal reed swamp was present in partsof the Pangani and Ruvu ri""ers and papyrusand water lily were recorded from the swampnear the railway bridge in the north-eastof the present lake (BAILEY 1%5). Baileyalso found Pistfa and Ceratophyllum in theRuvu River.

During the present survey Phragmiteswas found fringing the bays of tht: dam area,and it was especially abundam along thenorthern shoreline. In general, however,the dam area was found to be very poorin other species of aquatic and wetlandplants.

In the north of the lake, near the inflowof the Ruvu River, a great variety of mano­phytes was present both in the water andalong the shores. In June 1972, during thehigh water level, Ceratophyllum demersumwas common, and a small number cf Pistiastratiofes and Ludwigia erecta were alsopresent. Typha was the dominant plant ofthe shoreline. In April, 1973 the low waterlevel in the lake resulted in the formation

of nurnerou& plant islands, mainly arounddead trees. At that time the quantity ofaquatic plants was considerably higherthan ill June 1972, with the commonestplants being Pislia, Ceralophyllwn, and Azalianilulica. The islanels consisted predominantlyof Typha dominguensis, Cyperus spp., Phrag­mites maurilianus and Ludwigia ereeta, allof them flourishing. It is apparent that aquaticand wetland macrophytes are well developedduring the lowest water level, and they areespecially numerous on the recently exposedmargins of the lake.

Along the Ruvu River, Pahrgmiles andpapyrus grow, and in the water itself thereare fairly large £reas with dense populationsof Ceratophyllum demersum. In quiet sectionsof the river Pislia stratiotes is present insmall amounts. Tt evidently came into thisarea from a swamp situated a few kilometresabove the site.

Fishes of Ihe Lake ~r Ihe Ruvu River

A set of monofilament gill-nets of meshsizes ranging from 20 mm to 100 mm(stretched) were used to capture as widea range of fish species in the lake and theriver as possible. The dam area was fishedon two occasions for 24 hours, with the aimof obtaining data on diel changes in fishactivity and on relative abundance indifferent seasons. In the upper part of thelake and in the Ruvu River only daylightfishing was carried out, and in the riveronly three different mesh size nets wereset. viz. 20. 40 and 60 mm.

Out of the total 14 species recorded fromexperimental catches and commercial fishlandings (TanIc 4), 7 spedes are of potentialcommercial importance. with 4 of thesebeing exploited \'iz. two Tilapia, Sar01hero­don esculentus and Cfarias mossambicus.

In the dam area the smallest mesh sizenets of 20 mm caught predominantly Hap/o­chromis (June, 1972) or Rhahdalesles (April,1973). These were followed by Barbus

Table 4. List of Fish S

Characi<Rhaba

CyprinicBarbu,Barbu.Barb/,{BarbuBarbuLabeo

ClariidaClarj~

Mocho.;;SY1f()(j

AnguilliAngUI

Cichlid~

TilaplTiiapiSarOlHaph

• Num'tOne,

1972.

Table 5. Total Nwnl

DateTimeNumberWet weight (g)Overall mean biom

DateTimeNumberWet "''eight (g)Wet weight (g{hr)Overall mean bion

INets applied: 2'2Nets applied; 2

NYUMBA YA MUNGU. TANZANIA 45

* Numerous specimens were seen in conunercial fish catehes.t One specimen over 100 cm tolal length was eaptured by commercial fishermen in December,

1972.

Table 5. Total Number and Weight for 24-hoe.r Experimental Fishing in Nyumba Ya Mungu

41,050.0

24 hour weight

605.8 g/hr

1973'4-5 April

17.00-11.00 24 hour weight334

6,885.8 14,538.0382.5

1972 1

2-3 June18.00-10.00 10.00-15.00

278 10126,444.7 9.813.2

1,710.4 g/hr

12.00-17 .0077

7,046.91,409.4

15.00-18.00139

4,792.1

20,30,40, 50, 60, 80, 100 mm (stretched), one of each size; each net 1.5 m deep, 33 m long.20,40, 50, 70, 100 nun (stretched), one of cach size; each net 1.5 m deep, 33 m long.

Ruvu NYM NYMRiver in~ow dam

CharacidaeRhabdalestes tangens;s (Lonnberg, 1907)

Cyprinidae + + "Barbus paludinoslls Peters, 1852 + + +Barbus kerstenii Peters, 1868 + +Barbus oxyrthynchlls Pfeffer, 1889 + +Barbus jacksonii GUnther, 1889 + + .;-Barbus !ineomaculafus Boulenger, 1903 + +Labeo cylindricus Peters, 1852 + -j +

ClariidaeClarias mossambicus Peters, 1852 +

MochocidaeSynodonlis punctulatus Gunther, 1889 +

AnguillidaeAnguilla nebulosa labiata Peters, 1852 ( +)*

CichlidaeTilapiajipe Lowe, 1955 + + +Tilapia pangani Lowe, 1955 + + +Sarorherodon esculemus (Graham, 1929) +Haplochromis gr. "bloyeri" Sauv., 1883 + .;- +

DateTimeNumberWet weight (g)OveraU mean biomass

Table 4. List of Fish Species in Nyumba Ya Mungu and in the Ruvu River

tNets applied :2Nets applied:

DateTimeNumberWet weight (g)Wet weight (g/hr)Overall mean biomass

nly aroundiuantity ofbly highercommonestand Azolla

!ominantly'p., Phrag­erecta, alltat aquaticdevelopedI they arey exposed

fromfish

mtialthese'fero-

'1ifes andself therepulationst sections'esent ininto thislometres

)f mesh00 mm.s widemd the

fishedhe aimIn fishce inof theIylight

riverwere

size,plo­pri!,rhus

s

RhabdaJBarbus j

Barbus,Barbus,Labeo cSynodo.TilapiaTilapiaSarothlHaplol

such as Tilapi<debris from sweed beds, ,from the suphytoplanktopiscivores WI

mossambicus,pro bably inh

Differentiawas mosttangensis arThey are b,river, feediJin the infl(vorous. fe l

filamentoon aquaticdam they

Table 6. The Trophin experim

of the man-macCoast which is 0

In spite of th<used in the RUVl

in the inflow anwith the exceptiB. Iineomacu/atu.

were very common in 80 mm nets (about3-!- inch), which is very close to the 31 inchmesh size gill nets whieh are the mostcommon nets used (ANNUAL REPORT1970). Ripe females of Sarotherodon escu­lentus (TL 17.1-185 em) were collectedin April, 1973 [rom 70 mm nets (:1 inch).Ripe females of Labeo cy/indricus wereround in the river and the lake in April,but at the end of rains they were absent.This may indicate that Labeo breeds at theend of the dry season or the beginning ofrains, and this coincides with the breedingof Labeo in the Great Ruaha River (RPETR1973). MatuLity coeRicients calculated fromthe body and gonad's weight for Barbusline omaculatus, B. pa/udinosus, Rhabdalestestangen.'lis, and Synodontis punetulalu.r werehigher at the end or floods than at the onsetof rains, indicating that these species, andpossibly some others, for which not enoughmaterial was available, spawn predominantlytowards the end of Roods.

Bascd on food analysis of 538 fish be­longing to 10 species and captured duringthe two dieI experimental fishings, the trophicstructure of the fish community in the damarea was calculated. Barbus jacksonii andC/arias mossambicus, on which no or inadequate information is available, are notincluded. Primary consumers, i.e. phyto­plankton feeders and deposit feeders (Tilapia,Sarotherodon and Labeo), form 82.3 ~~ ofthe total biomass of the fish capture. Omni­vores (all Barblls species and Synodontis)form 14.4 o~ and carnivores (Haplochromisand Rhabdalestes) 3.3 '; of the total weight(Table 6). The carnivores and most of theomnivorous fish in the lake are small insize, but appear in large numbers. Theabsence of large predators of commercialimportance in Nyumba Va Mungu is insharp contrast with their abundance in thelarge man-made lakes Volta, Kariba andKainji. The trophic structure of NyumbaYa Mungu seems to be much closer to that

46 T. PETR

pa/udinosus and B. lineomaculatus; Tilapiaformed only a minute proportion of the catchfrom these nets. In 60 to 100 mm nets thecatch was dominated by Tilapia. Tilapiadominated the total catch of all nets together,and they were followed by either Labeoor Synodontis.

In June 1972 there was little differencein the nocturnal and day-light catches inweight and numbers of fish. However,in April 1973, the night catch was con­siderably poorer than the day catch. Theoverall results of the diel fishing, when bothcatches are compared, seem to suggest thatthe lake fish catches decreased from June1972 to April 1973 (d. 41 in June 1972compared with 15 kg in April 1973) (Table5). However, these data are not exactlycomparable for three reasons: the fishingin 1973 was carried out about 1 km distantfrom that in 1972, in a different season,and the set of nets applied in 1973 differedslighily from that of 1972. Nevertheless,it is possible that a decrease in fish abundancemay be at least partly responsible for thedifference between the 1972 and 1973catches as indicated by the continuousdecline in commercial catches since J970.

The females or the majority of speciesrespond to the onset or rains (April) bydeveloping gonads and spawning. Largenumbers of the small Barbus lineomaculatusand Rhabdalestes tangensis (97 to 100 %)were found to be ripe females both in Apriland in June. i.e. at the end of floods. Maturefemales of Synodontis formed 43 to 69 ~~,

Haplochromis gr. 'bloyeti' remales formed24 to 38 % and Tilapia females 25 to 57 %of the total adult specimens at the onsetof rains and at the end or floods. The smallestripe Tilapia pangan; was 20.0 cm (totallength), and it occurred in a 40 mm net,this corresponding to about l~ inch meshsize. This size of net is not used by thecommercial fishermen. Ripe females of Tilapiajipe and T. pangani of 22.5-25.0 cm size

....•

--

NYl1MIlA YA MUNGll, TANZANIA 47

4.62.1

Weight( /'~)

1.21.40.80.8

15.211.462.5

100.0

% Weight67.115.214.43.3

2.5111,131

2.925.8

100.0 54,439

Feeding calegories%Number

18.810.030.840.2

the Ruvu River. The Ruvu River seems tohave two distinct trophic groups: a groupof small species such as Barbus, Rhabda­lesfes and Haplochromis, which all feedmainly on chironomids, which they evidentlyobtain predominantly from aufwuchs, anda group of deposit and aufwllchs feeders,

jipe and T. pangani also have a differentdiet in different areas, being bottom andaufwuchs feeders in the river and in theinflow area of the lake, but plankton feedersat the dam (Table 7).

LOWE (1955) noticed that one speciesof Tilapia in Lake Jipe was feeding on avariety of food, such as plant material andbottom debris. This ability to adapt tovarious kinds of food indicates the high de­gree of adaptability of Tilapia, and possiblyalso other species, to new environments,and seems to contribute to their successfuland rapid colonization of ne'.'! reservoirs.

When the fish statistics were collectedin 1970, the predicted 500-1,000 tons of fishin c:ommercial catches per annum (BAILEY1965) for Nyumba Ya Mungu proved to

32281

1,089

14

I Fhytoplanklon feeders2 Bottom deposit feeders3 Omnivores4 Carnivores

Total

Rhabda/eSfes fange1lSi.~

Barbus paludinosusBarbus lineomanulafUSBarbus oxyrhynchusLabeo eylindricusS.vnodontis plme/ulatusTilapia jipeTilapia panganiSorotherodon esculentusHaploehromis gr. "bloyefi"'

Table 6. The Trophic Struclure of Fishes Taken in the NYL'mta Ya Mungu dam area (based on fish capcuredin experimental nets during 24-hour fishing in June, 1972 and Apd], 1973)

Species Feeding No. % No. Weightcategory (g)

4 157 14.5 6793 135 12.4 6903 80 7.3 4413 9 0.8 4212 109 10.0 8.2063 114 10.4 6.2161 172 159 34.113

such as Tilapia and Labeo, feeding on organicdebris from sediments deposited in aquaticweed beds, and on debris and aufwuchsfrom the surfaces of aquatic plants. Nophytoplankton or tripton feeders and nopiscivores were recorded, although Clariasmossambicus, belonging to the latter group,probably inhabi ts the river.

Differential feeding in different habitatswas most conspicuous in RlwbdalestesTangensis and Haplochromis gr. "bloyefj",They are both strictly insectivorous in theriver, feeding on chironomid larvae; butin the inflow area Haplochrornis is omni­vorous, feeding on chironomids and greenfilamentous algae, while Rhabdalesres feedson aquatic and terrestrial insects: and at thedam they both feed on zooplankton. Tilapia

of the man-made lake Ayame in IvoryCoast which i' of a ,imdar size (186 km').

In spite of the limited number of netsused in the Ruvu River, all species presentin the inflow area of Nyumba Ya Mungu.with the exception of B. o:ryrhynchus andB. lineomaculafus, were also captured from

in 80 mm nets (aboutry close to the 3t inchwhich are the most

(ANNUAL REPORTof Sarofherodon escu-

.5 em) were collected70 mm nets ('1 inch).

..abeo cylindricus were>od the lake in April,ams they were absent.at Lobea breeds at the~n or the beginning ofIdes with the breeding;Ruaha River (RPETRlcients calculated from's weight for BarbuslUdinosus, Rhabdalesfesantis puncfu!afus wereoods than at the onsetlat these species, andfor which not enoughspawn predominantly

Ids.

lysis of 538 fish be­and captured duringal fishings, the trophicImmunity in the damrJarbus jacksonh andon which no or inis available, are notISumers, i,e. phyto­pOsit feeders (Tilapia'0), form 82.3 ~~ offish capture. Omni­

es alld Synodon/is)'OTes (Haplochromisof the total weight" and most or theJake are small in

rge numbers. Theors of commercialYa Mungu is inabundance in the

Tolta, Kariba andcture of Nyumbamch closer to that

48 T. "ETR

Table 7. Feeding Habits of Fish in the Dam and Inflow areas of Nyumba Ya Mungu

resulted in very heavy fishing pressure onthis fish population and in the very effectiveremoval of large numbers of adult Tilapia.It has been observed that also in large man­made lakes such as Volta and Kariba, thelargest fish catches occurred in the 5th and6th years of impoundment and then a declinein catches followed (PETR in press).

The application of a particular meshsized gill net for each niche, which is char­acterized by dominance of a certain fishspecies, was suggested for Lake Kainji,in order to successfully manage and utilizeexperimental fishing of Nyumba Ya Munguproved that the 31 inch gill net, whichis currently the one most widely used by thecommercial fishermen, does not catch im­mature Tilapia and allows the Tilapia to'pawn at least once, and possibly severaltimes, before they reach the size at whichthey are captured. On the other hand, thefishing pressure docs seem to result in avery high fish mortality for the adult Tilapiaand Sarotherodon above a certain size,consequently affecting the number of youngbeing produced by them. A thorough surveyof the effect of various size giU-nets on fishpopulations in various niches of the lake,indicating surface and deep waters, isdesirable for establishing the preferred habi­tats of particular fish species and their agegroups, with the aim of exploitation of fishspecies other than Tilapia and Sara/herdan.

be a considerable underestimate. In 1970,i.e. 5 years after the closure of the dam,the total landings were 28,508.5 metrictons. (ANNUAL REPORT 1970). Thiscorresponds to 312 kg per hectare, whichis higher than the harvest from managed,unfertilized ponds in East and CentralAfrica (BAILEY 1966). This was followedby a decline in landings in the followingyears: in ·1971 catches had decreased to 10370.5 tons (ANNUAL REPORT 1971),in 1972 to 7,287.4 tons, and in 1973 it wasestimated that only 3,000 tons of fish werelanded (KAYUZA, personal communica­tion). The 1973 crop corresponds to 33 kgper hel:tare. This value is much closer to theaverage of 27 kg per hectare per annumcalculated for 8 dams of Tanzania withsurface areas ranging from 1.2 to 1,540hectares (BAILEY 1966). There were 3,161fishermen on the lake in 1970, and 3,386in 1971, indicating that in 1971 the catchper fisherman was only one third of that in1970. This evidently became even worse inthe years which followed, with catches in1973 being only I0.5~;'; of those in 1970.

It is almost certain that the lake had thehighest standing crop of fish in 1970 as aresult of very favourable breeding andfeeding conditions between 1966 and 1969and small fishing pressure. It is probablethat the increase in number of fishermen whohave heen employing more and more gear,

Species

Rhabdaleste,f iaflgensis

Barbus paludinosusB. IineOmaCIJlnllfS

B. axyrhyndmsB. jacksoniiLobeo cylindricusSvnodontis punrllliafus

Tilapia jipeG. panganiS. esculenlusHaplochramis lJ,,'. "bloyetj"

Dam area

zooplankton feederomnivoreomnivoreomnivore

bottom depositsommvort:phytoplankton feederphytoplankton feederphytoplankton feederzooplanklon feeder

Inflow area

insectivoreomnivoreomnivoreDiscivoreomnivorebottom deposits

bottom depositsaufwuchs feeder

omnivore

•

SUMMARYI. The NyUlnba

lake on the PangaJ

Region, Tanzania) w1965, and the filling \196R, resulting in theabout 150 km' surfa!lake is situated inpresent limnologicalon this lake in theimpoundment. The Iriver, was also inve~

2. The lake in thebe stratified at the(December) and at(April). An oxygen5 and 9 m. depth"of the dry season,practically withoutplankton was ablMicrocvs tis and c,3. The conductivin the sequence Ridam area. At theit is about twkeRiver (450-550 11·water may origin:is slightly salty. 1is on the aJakaJin(that in the infiue(pH 6.8.-7.2), beiwater (pH 6.7-6.8the RuYU River nof humic acids rtit passes througlLake Jipe to Ny

4, The concepotassium are hthan in the Ru\the high sodiulItogether with athe latter waswere reported 1covered by the:

5. The morhigher than tb

NrUMBA YA MUNGU, TA'lZANIA 49Mungu

lfiowarea

Yore'Oreore'r.Jr.I deposits

depositshs feeder

re

y ~shing pressure ond In the vt::ry effectiveJers of adult Tilapia.lat also in large man­llta and lCariba, the"rred in the 5th andnt and then a decline'ETR in press)..a particular meshliche, which is char­, of a certain fish

for Lake Kainji,manage and utilizeIyumba Ya Munguh . giJl net, whichwIdely used by theJes not catch im­;YS the Tilupia toI possibly severalIhe size at which. other hand, the1 to result in athe adult Tilapiaa certain size,umber of youngthorough surveygill-nets On fishes of the lakeep waters, i;preferred habi-l and their age'itation of fish1 Sarotherdon.

SUMMARYI. The Nyumba Ya Mungu man-made

lake on the Pangani River (KilimanjaroRegion, Tanzania) was closed in Dccember1965, and the filling was completed in April,1968, resulting in thc formation of a lake ofabout 150 km' surface area. This equatoriallake is situated in a savanna region. Thepresent limnological study was carried outon this lake in the 7th and 8th years ofimpoundment. The Ruvu, a major inflowingriver, was also investigated.

2. The lake in the dam area was found tobe stratified at the peak of the dry "ason(December) and at the beginning of rains(April). An oxygen discountinuity between5 and 9 m. depth was recorded at the peakof the dry season, leaving the deep waterpractically without oxygen. At that time,plankton was abundant, dominated byMicrocysfis and copepods.3. The conductivity of water increasesin the sequence River Ruvu-inflow area­dam area. At the dam (800-960 ~mhos)

it is about twice as high as in the RuvuRiver (450-550 ~mhos). The Ruvu Riverwater may originate in Lake Jipe, whichis slightly salty_ The pH in the dam areais on the alakaline side (pH 8.4-8.9), wbilethat in the influe area is close to neutral(pH 6.8.-7.2), being a!feited by the riverinewater (pI! 6.7-6.8). The relatively low pH ofthe Ruvll River may be due to tbe presenceof humic acids released into the river whenit passes through a swamp on its way fromLake Jipe to Nyumba Ya Mungu.

4. The concentrations of sodium andpotassium are higher in the lake-dam areathan in the Ruvu River. It is possible thatthe high sodium content (122.2 mg/I) goestogether with a high chloride contcnt, butthe latter was not determined. Salt panswere reported from the area before it wascovered by the lake.

5. The morphoedaphic index of 6.0 ishigher than those for large African man-

made lakes. However, it seems to be pre~

mature to apply this index to a not yetfully stabilized lake for tbe prediction ofpolential fish harvest.

6. In the dam area of tbe lake, littoralmacrophytes are poorly developed, but theyare abundant in the inflow area. especiallyduring tbe lowest water level in April.In the inflow area the fol1owing plants arecommon: Typha dominguensis, Cyperus spp.,Phragmites mallr;fjanlls, and Ludwigiuerecfa-al1 fringing the shores and formingislands; Cerarophyllum demersum, Pisfiastratiotes, and A=oila nilotica--submergedor floating plants.

7. Experimental fishing with monofila­ment nets of various mesh sizes establishedthe presence of 14 species of fish in the lakeand 9 species in the river. Another specieswas recorded from commercial landingsfrom the lake. It is quite probable that morespecies inhabit the river, which was fishedonly during the day. .

8. Diel experimental catches in the damarea of the lake revealed that Synodontispuncwalofus and Clurias mu.\'sambicus arepredominantly nocturnal. The night catchesdid not give a higher total weight than theday catches.

9. In the dam area of the lake Tilapiadominated the weight of fish in experimentalnets. Tilapia jipe, Tilapia pangani, andSarofherodon esC'ulentus constituted 70 ~~

and 51 % of the total weight in June, 1972and April, 197:1 respectively. Commerciallandings from the open lake consist almostentirely of Tilapia and SarofherodufI, whilethose from the inflow area also containa large proportion of Clarias mossambicus.

10. Most fish spawn at the end of floods,but Labeo cylindricus seems to spawn at thebeginning of floods. The smallest ripe femaleof Tliapia pangani was 20.0 em (total length).Ripe females of T. jipe and T. panganiof 22.5-25.0 em were very common in 80 mmnets, which is only slightly less than the

INTRODUl

hshmg 1'0has heen !1960). Thi'water up tPracticaH), aby ,kin di'a few whlcJfish. Hall reoled to a d'moving onnllously doeither toohatched elStocks COl

into the [

Managemeestablished

Managethe modifi'of the fisbin the catwould becontinue (this mayproblemsment isto cuntin

A studwas star

W. B. MUTA

A PRELFISHER'

REFERENCESAnnual Report (970). The Inland Waters-Nyumba

Ya Mungu Dam-Summary of Statistics during1970 (Appendix VI). In: Min. Nat. Res. &Tourism, Fish. Div. Statistics-Major Fresh­waters. Dar es Salaam, Tanzania.

Annual Rcport (J 971). Ki!imanjaro Region. FisheriesReport. Min. Nat Res. & Tourism, nsh.Division. Dar es Salaam, Tanzania.

Bailey, R. G. (1965). Report to the Chir;:f FisheriesOfficer, Min. Agric., Forests & Wildlife. Tan­zania. on Nyumba Ya Mungu Dam. (Mimeo.),7 pp.

Bailey, R. G. (l966). The dam fisheries of Tan7ania.E.A. Agrie. Forestry J. 32: 1-15.

Brown, L. (J.971). East Africa mountains and lakes­Cast Afrjean Publishing House, Nairobi, Dares Salaam, Kampala, 122 pp.

Coche, A. G. (1968). Description ofphy:;ical-chcmicalaspects of Lake Kariba, an impoundment inzambia-Rhodesia -Fish. Res. Bull. ZambiaSpecial Issue: 200--267.

Czernin-Chudcnitz:, C. W. (t97l). Physico-chemicalconditions of Lake Volta, Ghana-{]NDP/FAOTIX:hnical Report 1, FI:SF/GHA 10,77 pp.

Ita, E. O. (1973). Approach~ to the evaluation andmanagement of the fish sLock in Kainji Lake.Nigeria~Afr. J. Trop. Hydrobiol. Fish., Specialissue 1: 35-52.

Kilham, P. (1971). .Biogeochemistry or African lakesand rivers. Ph.D. Thesis Duke University,Durham, N.C., U.S.A.

Lowe, R. H. (1955). New species of Tilapia (Cichlidllc)from Lake Jipe and the Pangani River -BritishMuseum (Nat. His/.) (Zoo/., 2: 12: 349-368.

Petr, T. (1973). A pre.impoundment study, wirhspecial emphasis on fishes, of the Great RuahaRiver (Tanzanja) and of ~ome of its tributaries(River Yovi. and the Little Ruaha) in and aroundthe proposed impoundment areas-Report forthe Uppsala UniversHy, Sweden. 87 rr. (Typed).

Petr. T. (in press). On some factors associated witbthe initial high fish catches in new Afrk:anman-made lakes.

Regier, H. A. (1971). Evaluation of fisheries resourcesjn African freshwatcrs-Afr. J. Trap. Hydrobiol.Fish. 1: 69--S3.

Reynolds, J. D. (973). Report on fish productionand nutritiun in the Volta Lake-Prepared forthe Smithsonian Institution. 47 pp. (Mimeo).

ACKNOWLEDGEMENTS: Many thanks i:l.fe dueto Mr. M. A. Kayuza, the Regional Fisheries Officerin Moshi, who provided \lseful data on Nyumba YaMungu Dam and 10 his fisheries team stationed onthis lake, who readily helped with the collection ofthe scientific material. Dr. R. G. Eailey of ChelseaCollege (UniYersity of wodon) provided invaluablehelp with the identification of fish. I am also gratefulto SWECO officials in Dar es Salaam and Kidatufor helping organize this survey. Finally, r wish to[hank Dr. E. K. Balon, Dr. l.A.E. Bayly. Dr. G. G.Ganf and Mr. John Melack for their helpful criticismof the manuscript.

3'; inch mesh size gill nets most commontyused in commercial fishing, Ripe ft:maJes ofS. escule"'''s (17.1-18.5 em) were colleeted from the 70 mm nets (about 2iinch).

Ii. Some fish specie" including Tilapia,seem to feed on a great variety of diets.While the Ruvu Ri"'er fish communityconsists of insedivores. and deposit andaufwuchs feeders, that of the inflow is madeup largely of depo,it feeders, and that ofthe dam area is dominated by phytoplanktonand bottom deposit feeders. This is not are~mlt of different species composition indifferent areas, but the result of the samefish adapting to different food because ofits availability.12. The commercial fish landings have beendeclining since 1970, and by 1973 they wereonly 10.5% of those in 1970. Thi, may bethe result of very heavy fishing pressureon the Jake fish stock, causing large numbersof ~dult Tilapia to be removed. The dominantfishing gear are 3~ inch gill nets; and althoughthey do allow mature Ti/apia and Sarothe~

rodon to spawn at least once, if applied inlarge numbers, they may effeeti,'ely removeall or almost above a certain size, con­sequently affecting the number of youngbeing produced by them.

50 l. PETR

l