CURRENT COMPOSITION, DISTRffiUTION AND RELATIVEABUNDANCE OF THE FISH STOCKS OF LAKE VICTORIA,UGANDA

JOHN OBBO OKARONONFisheries Research InstituteP.O.Box343,Jinja, Uganda

ABSTRACT

An experimental bottom trawl survey was carried out in the Uganda sector of Lake Victoria duringthe period May 1993 through May 1995 with the aim of establishing the current composition,distribution and abundance of the fish stocks. A total of 205 successful one-hour hauls were takenusing the 25.4mm mesh size codend trawl net during the 19 cruises. Fourteen fish taxa (excludingthe haplochromines) were recorded with Lales nilolicus constituting the bulk (97 %) of the fishretained. Haplochromines and L. nilolicus were encountered in all areas sampled while Nile tilapia(Dreochromis nilolicus) and other tilapiines were restricted to waters less than 20 metres deep. Anaverage catch of 154 kg/hr was obtained in waters less than 30 metres deep. Species diversity andrelative abundance varied with depth. Only two of the fifteen fish taxa (haplochromines and L.nilolicus) were recorded in waters deeper than 30 metres and the bulk of the fish by weight (92 %)was obtained in waters less than 30 metres.

INTRODUCTION

Lake Victoria had a multispecies fisherydominated until the 1970s by the tilapiines andhaplochromine cichlids, but with importantsubsidiary fisheries ofmorethan 20 genera ofnon-cichlid fishes suchas thecat fishes (Bagrusdocmac, Synodontis sp. and Schilbe mystus),the mudfishes (CLarias spp.), the lung fish(Protopterus aethiopicus) and Labeovictorianus (KUDHONGANIA andCORDONE, 1974). Stocks of most ofthesespecies declined and others disappearedfollowing the introduction of three tilapiinespecies (Oreochromis niloticus - Nile tiiapia,Oreochromis leucostietus, and Tilapia zilliz)during the 1950s and the Nile perch (Laresniloticus) during 1960. The fishery thenbecame dominated by the Nile perch and the

Nile tilapia and only one ilative Rastrineobolaargentea (OKARONON et aI, 1985;OGUTU-OHWAYO, 1988; OKARONONand WADANYA, 1991). Fig. 1 showschanges in fish production between 1970 and1994. These changes were accompanied byrapid increases in fish yield.

Lake Victoria has alsp become an importantsource offish not only for local consumptionbut also for export and a number of fishprocessingplants have been constructed alongthe shores of the lake. Subsequently, thefishing effort in the Ugandan sector ofthe lakeincreased from 3,200 canoes in 1972 to 4,500in 1988 (41 % increase) and 8,000 by 1990(78%increase). This increase infishing effortand investment was made without clear_knowledge ofthe magnitude and sustainability

90 JOHN OBBO OKARONON

the fishery. Thi:preliminary findparticular referencdistribution and nstocks in the Ugcu

METHODOL04

Data were coHecbottom trawling 1

Uganda sector of1993 through Mavessel IBIS whiet1969/71 survey. Tinto three zones: I:to Bukakata, II) BKiyindi to Ugandaeach zone samplilselected transects. 'those successfuHy1

survey ofKUDHO

o

:r<nlL

80 -J ~e:[ C>Z ­o w- ~

60 ~ >­=- a:J

lL. Zo 0

40 W i=.." U<[ ~t- a% 0l1J a:::

ZO ~ CL.

l1JQ..

x

199"J

x X.1---*-1--_ _ X

1990

><

x)(

)( x

'985

x )( 1(1 x

1980

Elltlmated Catch

Percent0"O. 0' Not1onot

".h p,o due t Ion

xx "

x >< X

IAl

db

x ~

o~~..JiT~.4l'l-!1970

<n~zg 180

ooo

:...... 120:r:ul-e:[u

80owI-<t:::l; 40l-V>w.J

z 100 p~~ 1~I=m~l9§ii2 :"" .--== ~ ::: :;'. - =.---= -l­V>o 800..~

ou:r: 60

~u

w 40'-"<tI­Z

~ 20-V~~~~ ~~~~/f%a.

o L...L.L, I,' iii Ii J, I ,I ii, J , I r~i I i I I I, I i I. I iii I. I. I. 1.1

( Bl~

~~

illIJI]-Tllapllnes

Haplach.-omls

P,olopl"'US

BOIJ"us

~ Loles

IITI1J Rosldneobolo

c::::::::J 'C I 0 d Q S

~ Orhe, species

-.-tINT

-----Dfll

o....0-41. ~

Fig. 1, Fish production from Lake Victoria

ofthe stocks. There are some indications thatfishery yield is on the decline. Despite theincrease in fishing effort, fishery yield from thelake decreased from 132,OOOtonnes in 1989to120,000 in 1991 and 103,OOOtonnes in 1994.In 1994, Nile perch contributed 66.4%,tilapiines (mainly Niletilapia) 20.6% and therest was composed of R. argentea (11.4%),(0.2 %), Protopterus sp. (0.5%), Clarias sp.

(0.4%), Mormyrids (0.4%) and 0.2% B.docmac, (Fig. 1). The only fish stockassessment exercise was undertaken during1969/71 tKUDHONGANIA and CORDONE,op. cit) before the above changes took place.Against the above background stockassessment exercise was started 1993 in theUganda sector of the lake to generateinformation as a guide in the management of Pig. 2.

MASA)(A

•

Sampling

...

Owing to the draught of the IBIS, bottomtrawling was possible only in waters deeperthan 4 metres and because ofother limitations,it waS not possible to sample waters deeperthan 50 metres.

Rastrineobola and small Barbus spp were notsorted out from the bulk of the juvenile Latesniloticus although specimens ofRastrineobolaargentea were frequently observed stuck onthe trawl net.

(1974) A total of 19cruises were made duringthe period, 4 inzone I, 8 in zone IT and 7 in zoneIII. The 25.4mm mesh size codend trawl netwas used for fishing during the survey andtrawl hauls of approximately 60 minutesduration were taken. Trawling was generallycarried outduring the day between0700-1900hours.

- - - - - DEPTH CONTOURS

_. -.INTERNATIO~AL BOUNDARJES

•~

52·

FISH STOCKS OF LAKE VICTORIA, VGANDA 91

O·

METHODOLOGY

MASAKA

•

o 10 20Krn

~

Fig. 2. Sampling zones on lake Victoria

Data were collected through experimentalbottom trawling surveys carried out in theUganda sector of Lake Victoria from May1993 through May 1995 using the researchvessel IBIS which was also used during the1969/71 survey. The area ofstudy was dividedinto three zones: I) Tanzania/Vganda Borderto Bukakata, II) Bukakata to Kiyindi and III)Kiyindi to Vganda/Kenya Border (Fig.2). Ineach zone sampling was carried out in pre­selected transects. These transects were amongthose successfully trawled during the 1969/71survey ofKUDHONGANIA and CORDONE

the fishery. This paper, therefore, carriespreliminary findings of this exercise inparticular reference to thecurrentcomposition,distribution and relative abundance ofthe fishstocks in the V ganda sector ofthe lake.

:3:CI')

lL:80 to-

-l :3:4: ~:z w0

~

60 to-

"" >-=- CD

lL. ZC> 2

40 I.i.J ~

C> U<C: ::::Jl- e:>2:: C>W cr

zo u a..crw0-

0

(0.4%) and 0.2% B.The only fish stockvas undertaken duringt\NIA and CORDONE,ve changes took place.

background stockras started 1993 in thehe lake to generate: in the management of

92 JOHN OBBO OKARONON

", .

RESULTS

A total of 205 successful hauls of 60 minutesduration each were taken during the period ofinvestigation. Themajor preliminary trends inthe species composition, distribution andrelative abundance ofthe fishes in the Ugandasector of Lake Victoria as reflected from thebottom trawl data are as follows:-

Fish species composition

14 fish taxa (belonging to 12 genera) wererecorded (Table 1). Nile perch contributed96.7% by weight, Nile tilapia 2.8%,haplochromines 0.2 %, P. aethiopicus 0.1 %and Synodontis afrofischeri 0.1 %.Othersencountered in very small quantities includedB. docmact Clarias gariepinus, T. zillii,Mormyrus. kannume, O. variabilis, O.leucostictus, Barbus altianalis, S. victoriae,Xenoclarias sp. and Brycinus (Alestes) sp.(Table 2).

Distribution

Trawl surveys of 1993/95 show that the bulkof the fish was in waters less than 30 metresdeep(Table2). Nileperchandhaplochromineswere encountered in all the areas sampledwhile Nile tilapia and other tilapiines wererestricted to waters less than 20 metres depth(fables 1 and 2). The highest diversity of 11fish taxawas recorded in Napoleon Gulfwhichis within the 10-19 metres depth interval (fable1). Specks diversity decreased with depth(Table 2).

Relative abundance

Interms ofmean catchrates (fables 2, 3; Fig.3), the relative abundance ofthedemersal fishstocks varied with depth for most ofthe fishesin the study area.

The meantrawl catch rates ranged from almostzero inBuvuma Channel (Zone III) and DambaChannel (Zone II) to about one metric tonne/hour in MacDonald Bay (Zone III) and KagegiGulf (Zone I) as sQpwn in Fig.2. The meancatch rates generally decreased with increasingdepth for all the species (TabIe 3). An averageof 154 kg/hr was obtained in waters between4-29 metres deep (Fig.3 (b». The mean catchrates were observed to vary with seasons. Forinstance mean catch rates in Zone III variedfrom 196 kg/hr in June and August 1994 toabout 45 kg/hr in-November 1994.

DISCUSSION

The lakewide stock assessment survey of1969/71 using a bottom trawl (KUDHONGANIAand CORDONE, 1974)revealed:-(l) 24 fishspecies excluding the haplochromine cichlidsand that there appeared awell defined trend inthe number ofspecies by depth with maximumspecies diversification occurring in the shallowwaters of the lake, (2) at least 80% of theichthyomass was composed of thehaplochromines and Lates niloticuscontributed less than 0.1 %, and (3) the meancatch rate of 797 kg/hr for waters between 4metres and 30 metres deep.

Table 1. Fi- I

LOCATION

ZONE I

LujabwaSango BayDimo - BaleDimo - BangaBanga - White Ston~

Kasai Hill - MwezaMweza - LambuKagegi GulfNanvuma - Mbugwe(Bokassa)

ZONE II

Kome - Bulago IsKisima - Nsadzi Is.Nsadzi - Jana IsLutoboka ChannelBomangi BayRumbwa - KalangalGaru - BuvuGaru - BujajuBuvu - SongaBuvu - ZingaGaru - Luyo LHSBujaju - Luyo LHSBunjako BayBunjako - Luyo LHBukakata - Luyo LJSari - KasuriSari - Luyo LHSSonga - BukaNyoba - Kawaga LBeya - Dyavoderu

FISH STOCKS OF LAKE VICTORIA, UGANDA 93

LOCATION DEPTH FISH SPECIES(metres)

BJ Ba Bd Cg Ha Ln Mk 01 On Ov Pa Sa Sv Tz Xc

ZONE I

Lujabwa 30 - 39 x x xSango Bay 4-9 xDimo - Bale 4-9 x x xDimo - Banga 20- 29 xBanga - White Stony 20 - 29 x x xKasai Hill- Mweza 10 -19 x xMweza - Lambu 10 - 19 x xKagegi Gulf 10 - 19 x xNanvuma - Mbugwe(Bokassa) 20 - 29 x x

ZONEll

Kome - Bulago Is 10 - 19 x x x x. xKisima - Nsadzi Is. 20- 29 x xNsadzi - Jana Is 40-49 x x xLutoboka Channel 20- 29 x x xBomangi Bay 10 - 19 x x x x xRumbwa - Kalangala 10 - 19 . x x xGom- Buvu 10 - 19 x x.Gom - Bujaju 4-9 x x xBuvu - Songa 10 - 19 xBuvu - Zinga 10 - 19 x x xGom -Luyo LHS 4-9 x x xBujaju - Luyo LHS 4-9 x x xBunjako Bay 4-9 x x x xBunjako - Luyo LHS 4-9 x xBukakata - Luyo LHS 4-9 x x xSari - Kasun 4-9 x x xSari - Luyo LHS 4-9 xSonga - Buka 10 - 19 xNyoba - Kawaga LHS 30 - 39 x xBeya -Dyavoderu Is. 20 -29 x x x

llce

:atchrates (Tables 2,3; Fig.mdance ofthe demersal fishl depth for most ofthe fishes

tch rates ranged from almostlaI1Ile1 (Zone ill) and DambaI to about one metric tonne/ld Bay (Zone ill) and Kagegi;hown in Fig.2. The meanlydecreased with increasingecies (Table 3). An averageobtained in waters between:Fig.3 (b». The mean catch:d to vary with seasons. For:ch rates in Zone III varied1 June and August 1994 toNovember 1994.

:assessmentsurvey of1969/trawl (KUDHONGANIA1974) revealed: - (1) 24 fishhe haplochromine cichlidsared awe]] defined trend ines by depth with maximumonoccurring in the shallow, (2) at least 80% of theIS composed of the

and Lates niloticus1110.1 %, and (3) the mean~/hr for waters between 4~sdeep.

Table 1. Fish species distribution/occurrence during IBIS trawl cruises (May 1993- May 1995)

94 JOHN OBBO OKARONON

Table 1 Continued .....

~I

LOCATION DEPTH FISH SPECIES(metres)

Bj Ba Bd Cg Ha Ln Mk 01 On Ov Pa Sa Sv Tz Xc

ZONE illNapoleon Gulf 10 - 19 x x x x x x x x x x xThruston Bay 10 - 19 x x xBuvuma Channel 20 - 29 x x xLingira Bay 4-9 x x x x xHome Bay 10 - 19 x x x x x xBugaia 30 - 39 x xMvuja 20 -29 x xBusuji Pt - Luvia I 30 - 39 xMacDonald Bay 10 - 19 x x x xPringle Bay 10 - 19 x x

Remarlcs: Bj = Brycinussp., Ba = Barbus, Bd = Bagrusdocmac. Cg = Clariasgariepinus, Ha = Haplochromisspp, Ln = Lates niloticus, Mk = Monnyrus kannume, 01 = Oreochromis leucostictus, On = O.niloticus. Ov = O. variabilis, Pa = Protopterusaethiopicus, Sa = Synodontisajrojischen', Sv = s.victoriae, Tilapia zillii, Xc = Xenoclarias sp.

Table 2. Bottom trawl mean catch rates (weighed by depth interval) of the various fishes in the Uganda sector of Lake Victoria

.::. c:i tI:il::~ " II15 ~ ;>~ 0 (I).

§- .'J:::tr;~" .g ~"'''''~::z:: 8 .... ~ ~

!:l-'.:!ii:! .::. ::'S § .s

u>< I<

NE-< I< I<

:>C/l I<

«lC/l K I< I<

«l~ K K

C/l~,,'

Depth interval Area I Area II Area III Combined(metres) ..

4-9 10-19 20-29 30-39 40-49 4-9 10-19 20-29 30-39 40-49 4-9 111-19 20-29 30-39 40-49 4-9 10-19 20-29 30-39 40-49

Fish Species:

Brycinus sp. 0.0 0.0

Rarbus sp. 0.0 0.8 0.0

RagTUS docmac 0.1 0.1 125.4 0.1 0.0

Clarirls gariepinus 0.1 9.3 0.0

Roplochromis spp. 0.1 0.6 0.2 0.0 1.2 0.1 0.1 0.0 0.0 0.2 0.2 0.1 0.0 0.2 0.1 0.1 0.0

lares niloricus 64.9 178.9 134.6 204.0 156.7 105.3 144.8 28.9 46.7 182.8 154.61 65.4 0.0 161.4 145.2 143.3 28.9

Monnyrus kannuT/1£ 0.0 0.0

Oreochromis leucosrictus 0.0 0.0

O. niloricus 1.2 0,2 9.9 6.0 10.2 4.3 4.3

O. voriflvilis 0.0 0.0

Proroprerus oerhiopicus 0.3 0.4 0.3

Synodonris afro{lScheri 0.0 0.4 0.2

s. victoriae 0.0 0.0

Tilapia zillii 0.0

Xenoclarirls sp. 0.0 .0.0 0.0

I

l1'l0\

-<ClZ-<c.:J::J

-<;:oEo-<U>-<>~~

-<...:l~oen~Uof­en:I:en>-<~

96 JOHN OBBO OKARONONI I

Co}

IfTable 3. Bottom Trawl Mean Catch Rates (Weighed by Depth Interval) of the Various Fishes in the Uganda

I1000,~

sector ofLake Victoria (in Kg/Hr) during the period February 1994 to May 1995.Figures for 900

the 1969/71 Survey are in brackets and waters of 50 - 79 metres depth were not surveyedBOO

i" 700

during 1994/95. ~ 1500

~ '00

I I~ 400

Depth (metres) 4-9 10 - 19 20 - 29 30 - 39 40 - 49 50 - 59 60 - 69 70 - 79.,a:: lOOx'-' 200~ fil,"

Species: • I u rool

Haplochromis (493.8) (800.2) (639.5) (507.5) (448.0) (486.3) (196.3) ( 29.6)0.8 0.2 0.1 0.1 0 • I

Oreochromis. esculentus(.) .

( 54.1) ( 28.3) ( 4.4) (0.5) (0.0)Oreochromis variabilis (3.2) (0.2) (0.0) (0.0)

900fj0.0 . 800 797

O. niloticus (10.0) (0.9) (0.0) 700 •

9.3 4.3 ~ . • 00 ~O. niloticus (0.2) 0.0 ~ 500 ~

Tilapia zillii (0.4) (0.0) (0.0)li( 400

0.0 .OO~200

Bagrus docmac (18.0) (39.9) (39.3) (36.8) (34.4) (38.8) (27.6) ( 0.2)'00

0.1 0.0 0.0 0_

Clarias gariepinus (26.0) (37.1) (31.6) (20.7) (15.1) (14.5) (14.8) ( 7.1) flUlj,!l'Q '"

0.0Xenoclarias sp ( 0.0) (0.1) (0.1) ( 0.3) ( 0.4) ( 0.6) ( 0.3) ( 0.3)

0.0Protopterus aethiopicus (133.3) (23.1) (7.3) (5.5) ( 1.6) ( 0.5)

IFig. 3. Mean cal

0.3 ·trawlingsI. Lates niloticus (2.0) (0.6) ( 0.4) (a) lal

125.4 161.4 145.2 143.3 28.9 (b) NlSynodontis victoriae (0.4) (0.7) (7.0) (9.0) (11.2) (26.5) (29.4) (14.9) u~

0.0S. afrojischeri (0.1) (0.1) (0.1) (0.0) (0.0) (0.0) (0.0) (0.0)

0.2Barbus altianalis (0.4) (0.5) (0.3) (0.2) (0.2) During the 198

0.0 trawl survey in .Labeo victorianus (0.1) (0.4) (0.0) Victoria (Ugar

0.0Mormyrus kannume (0.4) (0.4) (0.3) (0.4) (0.9) (0.1) (0.0) (1985) and OK

0.0 (1986) .observtSchilbe mystus (0.9) (1.8) (1.3) (0.6) (0.3) (0.1) (0.0) 24 fish spBrycinus spp. (0.0) KUDHONGAJI

Afro-mastacembelus frenatus(O.O) (0.0) (0.0) Iin the 1969/71

Gnathonemus longibarbis (0.0) (0.0) except two, nam

Total (644.0) I and Brycinus 1(935.3) (731.6) (581.5) (512.1) (567.4) (268.4) ( 52.1) contribution to135.4 166.4 145.3 143.4 28.9

from 91.4% in

Hauls/Hr 50 96 37 16 6 •II

I!

Fig. 3. Mean catch rates from experimental bottom·trawlings.(a) lake Victoria, Uganda.(b) Nothern portion of lake Victoria,

uganda (4-3Om depth)

while the contribution of L. niloticusdrastically increased from 5% to 95.6%during the same period, (c) the contributionof other fish species to available stocksgenerally declined, and the mean catch ratesfor all the fish species combined declinedfrom 595 kg/hr in 1981 to 355 kg/hr in 1983and 155 kg/hr in 1985.

There appears to be a well defined trend in thenumbers of fish species in respect to depth.

Only 14 fish species (excludinghaplochromines) were recorded from the25.4mm mesh size codend bottom trawl netduring the 1993/95 survey. This reflects aspecies diversity reduction down to about ahalfofthe 1969/71 survey results. The64, 38,and 19 mm mesh size codends were used mostduring the 1969/71 survey (KUDHONGANIAandCORDONE, 1974). Thefish taxa recordedduring the 1969/71 survey and not recordedduring the 1993/95 survey include O.esculentus, Schilbe mystus; Mastacembelusfrenatus, Gnathonemus lon'gibarbis. OKEDI(1967) reported thepresence ofG. longibarbisin the shallow inshore wa,ters over muddybottoms adjacent to'extensive papyrus swampsand also in deep waters (30 - 50 metres) nearrocky islands. GEE and GILBERT (1967)recorded Brycinus jacksoni and A. sadleriwhile surface trawling in the Jinja area ofLakeVictoria. HAMBLYN (1966) reportedoccasionally between January and March 1962Brycinussp. in Napoleon Gulf, afterpoisoTling.The absence of records for some of the fishspecies during the 1993/95 survey could,therefore, be partly attributed to the non­sampling ofshallow (less than 4 metres) inshoreand rocky ~abitats suitable to some of thespecies ..

FISH STOCKS OF LAKE VICTORIA, UGANDA 97

."...'"

mlI1J Ma, flil3 - fl88 [=:J 1168 I 71

to'

1000

900

800

Q'700

~600

.00

~ 400..300'"~2005 100

During the 1981-19§5 experimental bottomtrawl survey in the Northern portion of LakeVictoria (Uganda), OKARONON et al(1985) and OKARONON and KAMANYI(1986) .observed the following:- (a) all the24 fish species encountered byKUDHONGANIA and CORDONE (op. cit)in the 1969/71 survey were again recordedexcept two, namely Gnathonemus longibarbisand Brycinus sp., (b) the haplochrominecontribution to the catch by weight declinedfrom 91.4% in 1981 to almost zero by 1985

Z6.5) (29.4) (14.9)

).0) (0.0) (0.0)

~7.4) (268.4) ( 52.1)

0.5)

'.1) (00)

1.1) (0.0)

38.8) (27.6) ( 0.2)

14.5) (14.8) (7.1)

0.6) ( 0.3) ( 0.3)

486.3) (196.3) (29.6)

50 - 59 60 - 69 70 - 79

Various Fishes in the Uganda~ 1994 toMay 1995.Figures forletres depth were not surveyed

/

98 JOHN OBBO OKARONON

IIIlI

i:iii

The fish species diversity decreased withiI}crease in water depth. Only two species(L. niloticus and haplochromines) wererecorded in waters deeper than 30 metresduring the 1993/95 survey.KUDHONGANIA and CORDONE (op. cit),found Haplochromine spp., B. docmac, C.gariepinus, S. victoriae and Xenoclarias tobe eurybathic. On the other hand, thetilapiines and P. aethiopicus and most of thecommercially' minor categories were foundto be more or less oligobathic(KUDHONGANIA and CORDONE, 1974).The absence during the 1993/95 survey ofB.docmac, C. gariepinus, S. vietoriae andXenoclarias in waters deeper than 30 metersmay partly be due to drastic environmentalchanges that have occurred in Lake Victoriaduringthe lastfew decades (BUGENYI, 1991).With the rising thermocline especially in the40 -80 metres depth zone and increasingeutrophication which is partly attributed tofish introductions (BUGENYI, 1991), there islikely to be some cause for uncertainty in thestocks ofcertain species in the affected zonesespecially the high oxygen-demanding NileperchandBagrusdocmac. During the 1993/95 survey the Echo-Sounder readings indicatedconcentration ofaquatic life, including fish, inareas between the surface and 20 metres depthand almost nothing below 20 metres from thesurface.

There has been a decline in catch rates in the 4­29 metres depth zone for all fish speciescombined from 797 kg/hr in 1969/71 to 595,365,355, 155 and 154 kg/hr in 1981, 1982,1983, 1985 and 1993/95, respectively. Thehaplochromine group which dominated up to1983 declined drastically and in the presentstudy the group contributed only 0.2 %ofthe

catch by weight. On the contrary, L. niloticuswhich was insignificant in 1969/71 (at 1 kg/hr) has been increasing and contributed 5,42,58, 148 and 148 kg/hr in the bottom trawlcatches in the 4-29 metres depth zone during1981, 1982, 1983, 1985 and 1993/95,respectively. Similarly in the commercialfishery the contribution of Nile perch (L.niloticus) increased from 13 % in 1981 toabout 85 % in 1988 before declining to 66 %in 1994 (Fig. 1). Nile perch together withNile tilapia and Rastrineobola argentea havedominated the commercial fish landings since1987.

The relatively high yields of L. niloticus inLake Victoria during the 1980s is most likelydue to the abundance ofthe suitable prey, thehaplochromines. In the absence of thehaplochromines L. niloticus has switched toother prey like R. argentea, Caridina sp. andits own juveniles (OGARI, 1985; OGUTU­OHWAYO, 1985 and 1990; MWEBAZA­NDAWULA, 1990; WANDERA, 1993). Thedynamics ofthe current prey ofL. niloticus arenot yet clear and, therefore, the future of thisspcecies in Lake Victoria will very muchdepend on the stocks of its current prey. Thesituation is further compHcated by the drasticenvironmental changes leading to reduction inoxygen levels in deeper waters, affecting thehigh oxygen-demanding Nile perch and otherfish species. The invasion ofthe lake by waterhyacinth (Eichhornia crassipes) is a negativedevelopment. This weed and other floatingweeds in dense mats have been reported tocreatedeoxygenated conditions intropical lakes(LITTLE, 1966) and, consequently, to renderwaters covered by it unsuitable for breedingand nursery grounds for most fish species.The continued presence ofthis weed in the lake

will obviously ;consequently, the

ACKNOWLEDG

This study was madCommunity who nand the Uganda GOIsurveys. I am greatof FIRI (Dr. F.Wordination ofthe WI

and various FIRI s(assistanceduringthcMr. J.Kamanyianduseful suggestions.Mrs Ruth Byekwaslmanuscript and Mproduction ofthe fii

REFERENCES:

Bugehyi, F.W.B.,LimnologChangingFisheries 0

In: SsentonMeza (edsSeminar 0:

Manageme:Lake Vi(RegionalPJPlanning (31/91 (EN)

Gee, I.M. and I

ExperimelLakeVietoAnn. Rep.Res.Org.,

I

FISH STOCKS OF LAKE VICTORIA, UGANDA 99

e contrary, L. niloticusat in 1969/71 (at 1 kg/Ilg and contributed 5,Ihr in the bottom trawltres depth zone during1985 and 1993/95,

rly in the commercialon of Nile perch (L.rom 13% in 1981 tofore declining to 66 %e perch together with~eobolaargentea havecial fish landings since

elds of L. niloticus inlIe 1980s is most likelyfthe suitable prey, thethe absence of the

oticus has switched to'Itea, Caridinasp. andARl, 1985; OGUTU-1990; MWEBAZA­

\NDERA,1993). Theprey ofL. niloticus arefore, the future ofthisoria will very much.its current prey. Theplicated by the drasticleading to reduction in.waters, affecting the~ Nile perch and otherm ofthe lake by water'assipes) is a negative~d and other floating'lve been reported toIitions in trq>icallakesnsequently, to rendermitable for breedingIr most fish species.)fthis weed in the lake

will obviously affect the stocks and,consequently, the catches.

ACKNOWLEDGEMENTS

This study was made possibIe by the EuropeanCommunity who rehabilitated the R.V. IBISand the Uganda Government who financed thesurveys. lam greatly indebted to the Directorof FIRI (Dr. F.W.B. Bugenyi) for the co­ordination of the work. I thank the IBIS crewand various FIRI scientists for their valuableassistanceduringthesurveys. Dr. T. Twongo,Mr. J.Kamanyi and other ARI scientists madeuseful suggestions. I would also like to thankMrs Ruth Byekwaso for the preparation ofthemanuscript and Mr. S.N. Sowobi for theproduction ofthe figures.

REFERENCES:

Bugehyi, F.W.B., 1992. The Effect ofLimnological Factors and theChanging Environment on theFisheries of Northern Lake Victoria.In: Ssentongo, G.W. andF. L. Orach­Meza (eds.). Report of NationalSeminar on the Development andManagemept of Uganda Fisheries ofLake Victoria. UNDP/FAORegional Project for Inland FisheriesPlanning (IF/P), RAF/87/099ffD/31/91 (EN): 36-58.

Gee, LM. and M.P. Gilbert, 1967.Experimental trawling operations onLake Victoria.Ann. Rep. E. Afr. Freshwat. Fish.Res. Org., 1966: 33-46.

Hamblyn, E. L., 1966. A note on the inshorefish population of Napoleon Gulf(Lake Victoria). Ann. Rep. E. Afr.Fresh. Fish. Res. Org., 1965: 23­35.

Kudhongania, A.W. and 1. Cordone, 1974.Batho-spatialdistribution patterns andbiomass estimate of the majordemersal fishes in Lake Victoria. Afr.J. Trop. Hydrobiol. Fish., 3: 15-31.

Little,E.C.S., 1966. The invasionofman­made lakes by plants. In (R. H.McConnell, ed.) Man-made Lakes:The London Symposium pp.75-86.

Mwebaza-Ndawula, L. 1990 .. The role ofinvertebrate organism~ in the fisherypotential of Lake Victoria.Proceedings of a workshop on"Prevailing activities on the LakeVictoria basin with particularreference to thefisheries oftheLake".Mwanza, Tanzania8-9March 1990:56 - 60.

Ogari,J., 1985. Distribution,.FoodandFeedingHabits ofLates niloticus in the NyanzaGulfof Lake Victoria (Kenya). FAOFishRep. 335: 68-80.

Ogutu-Ohwayo, R., 1985. The effects ofpredation by Nile perch, Latesniloticus (Linne), introduced into LakeKyoga (Uganda) in relation to thefisheries of Lake Kyoga and LakeVictoria. FAO Fish. Rep., 336: 18­41.

100 JOHN OBBO OKARONON

: i

Ogutu-Ohwayo, R., 1988. Reproductivepotential of the Nile perch, Latesniloticus (L.) and establishment ofspecies in Lakes Kyoga and Victoria(East Africa). Hydrobiologia. 162:193-200.

Ogutu-Ohwayo, R., 1990. The decline ofthe native fish species of LakesVictoria and Kyoga (East Africa)and the impact of introduced species,especially the Nile perch, Latesniloticus and Nile tilapia,Oreochromis niloticus. Environ. Bioi.Fish., 27: 81-96.

Okaronon J.O., T.O. Acere and D.L.Ocenodongo, 1985. The current stateofthe fisheries in the Northern portionofLake Vietoria(Uganda). FAOFish.Rep.~ 335: 89-98.

Okaronon, J.O. and J.R. Kamanyi, 1986.Recent trends in the fisheries of thenorthern portion of Lake Victoria,Uganda. UFFRO SeminarNovember 1986.

Okaronon, J.O. and J. Wadanya, 1991.Fishery resource base for the Ugandasector of Lake Victoria. In:Ssentongo, F.W. and F.L. Orach­Meza(eds.) 1992. ReportofNationaJSeminar on the Development andManagement of Uganda Fisheries ofLake Victoria. UNDP/FAORegional Project for Inland FisheriesPlanning (IFIP), RAF/87/099!ID/31/91 (EN.): 21-35.

Okedi J., 1967. The biology ofsome mormyridfishes in Lake Victoria basin. Thesissubmitted for the Degree ofDoctor ofPhilosophy at Makerere UniversityCollege, University ofEast Africa.

Wandera, S.B. 1993. Seasonal abundance,vertical migration and the fishery ofDagaa Rastrineobola argentea inthe Uganda waters of Lake Victoria.In Marshal, B.E. R. Mubamba(eds.). CIFA Occasional paperNo. 19.

GROWTH INILOTICUS]

Abstract

The growth rates ofin lakes Victoria andgrew faster in Lake]Victoria (18.9 ± 1.4There were significalGrowth rates in Lake(p < 0.001) but thosother (p = > 0.05).supply due to increasignificantly higher,reported in several n

INTRODUCTION

Growth amongenvironmental (temperature, salin(BROWN 1957). Ethe most importantrates in the tropics ware high and vary 1increase ordecrease;and is affected by thland size of prey (M.KEMPINGER & Chigher the quality ojgrowth rate. ForagiJwith type and abundcfish can survive for Ieno food but this will

and affect reprodu~can also changeChanges ingrowthp

: