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THE A leA JOU L

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(Afr. J Trop. Hydrobiol. )

Yo. 1 1994 ASPECTS OF THE BIOLOGY OF THE SARDINE, LIMNOTHRISSA n illustrated key to the flinji. (Mimeographed MIODON (BOULENGER), IN ~as Development Ad­ ndon). R. HUDDART ision des Synodontis Kariba Research Unit, Private Bag 24 CX, CHOMA, . e Royal de I'Afrique 'en, Belgium. 500p. Present address:- ~). The ecology of the 4, Ingleside Grove, LONDON, S.B.3., U.K. is (Pisces; Siluoidea) Nigeria. (Unpublished ABSTRACT o the University of Juveniles of U11lnothrissa 11liodon (Boulenger) were introduced into the man-made Lake Kariba in ngland). 1967-1968. Thirty months of night-fishing for this from Sinazongwe, near the centre of the Kariba North bank. from 1971 to 1974 are described. Biological studies were carried out on samples of the catch during most of these months. Limnological studies were carried out over a period of four months in 1973. Li11lnothrissa is breeding successfully and its number have greatly increased. [t has reached an equilibrium level of population size at a [ower density than that of Lake Tanganyika sardines, but nevertheless is an important factor in the ecology of Lake Kariba. The growth rate, size at maturity and maximum size are all less than those of Lake Tanganyika Li11lnothrissa. A marked disruption in the orderly progression of length frequency modes occurs in September, for which the present body of evidence cannot supply an explanation.

INTRODUCTION light-attracted sardines in Lake Kariba were The absence of a specialised, plankti­ tested in 1970 and early 1971. Dip nets vorous, pelagic species of fish within the (lusenga nets), as used by Lake Tanganyika middle fIsh fauna occupying the artisanal fisher-men, and lift nets caught new lake became obvious soon after the an average of less than 50 individuals per formation ofLake Kariba in 1963.Juvenile night. Afairly large (50m. by20m.) chiromila clupeid sardines were airlifted from Lake net caught an average of 23 kg. per haul. Tanganyika in 1967 and 1968 in order to Similar-sized catches were obtained fill this vacant niche. Sardines over 25 mm. throughout the lake from the dam wall to in lengthwere unable to survive the catching the Zambezigorge (Devil'sGorge) whether and transporta-tiorf process. Of the two from deep open wateror from shallowbays. species of , Thus Limnothrissa had become fIrmly Limnothrissa miodon (Boulenger) and established throughoutLake Karibaby 1970­ Stolothrissa tanganicae (Regan), the latter 71 and several catches of 0.5 tonnes per is the more pelagic while young. Hence, night1)ave more recently been taken in the only Limnothrissajuveniles could be found Sinazo:ngwe area. The study reported here near the shore and be driven into contain­ describes some investigations into the ers. biology of this species from 1971 to 1974 Unpublished data held by the Research in Lake Kariba. Division of the Fisheries Department of METHODS Zambia! show thatvarious methodsfor fishing Samplesofsardinewere obtainedby fish­ ing at night using lights and a chiromila 1. The Department ofWildlife, Fisheries and National Parks net, as described by COULTER AND of Zambia was split in January, 1974 the fisheries component ZNAMENSKY (1971), slightly modified to becoming the Department of Fisheries of Zambia. 32 RHUDDART

suitlocal conditions. Unless othetwise stated, Samples of66 sardines with a roughly even RESULTS the net used measured 50m. by 20m. The length distribution were taken and their Polpulation lev buntwas of8 mm. mesh nylon. All samples lengths and weights measured. There were The total cat were obtained fromwithin 5 km. ofthe Sina­ 66 spaces on each of the cyclostyled data ably after April zongwe Fisheries Training Centre, itself160 sheets prepared for this study and it was km. from the dam wall and within basin III found that the size of sample dictated by ofthe lake as described by COCHE (1968). the fIlling-in of one of these sheets was These samples, unless otherwise stated,were adequate for the construc-tion ofa length/ soc taken within I km. from the shore in water weight curve. From each length/weight 15-20metres deep. Fishingstartedat2000hrs curve the weights at the arbitrarily-chosen local time and continued until the dawn of lengths of40mm., 55mm. and 70mm. were the following day. The sample for laboratory read off. These are shown plotted study was most often taken from those fish against calendt;r time in Fig. 6. K caughttoward the endofthe fishing period, A large, random sample of 400-1600 due to theirfresher condition, and therefore sardines was measured for length only, the a sample from, for instance, the night of llSualsamplesize beingbetween 700and 1000 IIthand 12thSeptemberis referred to below individuals. A length frequency histogram as a sampIe from September 12th. was prepared for each such large sample so As manyhauls as possiblewere made each that the mode or modes of the distribution night in order to render more accurate the could be distinguished. K calcul"ted catch per haul. A crew of10 men For the purpose ofthis analysis, the stages was originally necessary, using three boats of gonad development are distinguished llo,C and making amaximum ofonly 6-7 hauls per into only two groups, ripe and unripe. Ripe night, so thatmostofthe available research gonads occupied mostofthe body cavity and resources were expended in the routine the eggs were a deepmustard yellowin colour sc runningofthis commercial-scale operation.'TIlls and fully formed. Testes were opaque and situation gradually improved as con­ white. ELLIS (1970) separated female tinuous development of net-handling Limnothrissa into three stages of gonad • hydraulicmachinery made 24hauls pernight developmentand males into two. Samples of possible by early 1974 using only two boats 120-150 fish were taken for gonad examina­ andsix crewmembers. The number oflight­ tion. boatswas increasedinproportion to the number An all-glass displacement sampler was ofhauls per night (unmanned "light-buoys" used to obtain water s

'dines with a roughly even RESULTS in the number of hauls per night (Fig. 1). on were taken and their Polpulation level The catch per haul remained steady until tIts measured. There were The total catch per night rose consider­ Decem-ber, 1973 when replacement of h of the cyclostyled data ably after April, 1973 due to the increase paraffin pressure lampswith propane lamps for this study and it was ze of sample dictated by one of these sheets was :onstruc-tion ofa length/ SOO-r------;("':":'1) om each length/weight A at the arbitrarily-chosen ,55mm. and 70mm. were e are shown plotted ime in Fig. 6. Kg am sample of 400-1600 ured for length only, the ~ingbetween700 and 1000 gth frequency histogram :ach such large sample so nodes ofthe distribution ihed. :ofthis analysis, the stages ,ment are distinguished ps, ripe and unripe. Ripe lostofthe body cavity and lmustard yellow in colour Testes were opaque and 500 170) separated female three stages of gonad lalesinto two. Samples of • lken forgonad examina- Kg

)lacement sampler was er samples for primary s. Otherwise, all water I 0 I lined using a Nansen (B)\ terminationswere made 5 :1 Winkler method as ~RMAN (1968) and light Kg leriments followed the 0 0' 0 d by VOLLENWEIDER i'J100 filament gill nets with J and of various mesh t overnight for trapping Fig. 1 Limrwthissa catches from night fishing. A: total catch per night B: average catch reI' haul for that ltoryupon Limnothrissa. night C: proportion of full moonlight occurring. dependent on the lunar cycle and cloud. 34 RHUDDART

andmuch heavierweightingofthechiromila The considerable detrimental effect of revealed. Somt net to suit its faster operation resulted in moonlightonsardine catches (Fig. 1) follows sampIingatthirl an increase. Adverse weather conditions thesituation inLake Tanganyikawhere com­ in Fig. 2. caused much late starting and early aban­ mercialfishing is suspendedduringfull moon, According t< donment of fishing trips inJanuary, 1974, buttherearenoseasonal fluctuationsinsardine is nearest to tlH preventing an associat-ed increase in total landed like those in Lake Tanga-nyika: the ofdawnanddm catch. records of the CENTRAL STATISTICAL 4 metres durin OFFICE (1971) show that sardine landings through thewatl increased markedly in Lake Tanganyika of the sardine: duringAugust-Novemberin 1969 and 1970 4 metres ofwat andwere associatedwith minimainJune:July Proportion of in 1969andApril-May in 1970. The catch per Therewas al haul ofintroduced sardine in Lake Kariba 1000l 2.00 in deepoflShon 18 Oct -1 10 Noy (which is, in this-case, a closeapproximation chiromilasamp to catch peruniteffort) has beensteadysince within 1 kIn oft atleast 1971. There have beenno detectable 1. A year-by-yt changessince, apartfrom those induced by jj II steadily falliI j improvedfishing technique. Fishing continued although this .. I over all phases of the moon in the present monthly avera! LJI study, resulting in a lower average catch per The length 0 0 haul thanin 1970,when fishingwas suspend­ lOOO 100 ed duringfull moon. 20 Oct 1 II Noy The greater turbidity of Lake Kariba Table 1 Fish sa percel '- water, especially toward the narrow and ~ n .0 shallow upper end of the lake, compared E z=' with that in Lake Tanganyika means that light-fishing is less effective and the catch per haul lower. COULTER (1968) found Secchi disc readings of 5-18 m. in Lake TanganyikawhileCOCHE (1968) quotesfig­ january ures of 3-9 m. in Lake Kariba. The present February study found Secchi disc readings in the March range 1-6 m. in that limited part of Lake April Kariba under investigation (basin III as de­ May jWle j scribed byCOCHE (1968). Diel Vertical Migration July Samplingwas repeated atshortinten'als August .~~. oftime in the same location, in 15mmwater September October Fig. 2. The number of Limnothrissa caught in depth, using a small chiromila net without chirromila hauls at thirty-minute inteIVais sardine-attractinglights. Thisnetwas 28m x November without attraction by artificial light. There 14min dimensions andits geometrywassuch December was no moonlighton the 18th and20th October Total for the ~ that 80% of its effective fishing volume lay 1971 while there was roughly of maximum Year moonlight on lOth and 11th November 1971. within 4m ofthe surface ofthe water. Time­ All times are expressed in local time, which related changes in the number ofsardines Percentagefor is GMT plus two hours. present in the surface few metres were the year BlOLOGYOFllMNOTHRISSAMIODONINLAKEKAIUBA 35

lble detrimental effect of revealed. Some typiG\1 examples of such Alestes were always unimodal, insofar as line catches (Fig. 1) follows samplingatthirty-minute intervalsareshown could be determined with the limited catch of ke Tanganyikawhere com­ in Fig. 2. thisspecies. thegreatm,yoritybeingbe-tween ISpended duringfull moon, According to this evidence, Limnothrissa 37 and 60mm. fork length. There was no sonal fluctuatioTlSin sardine is nearest to the surface during the twilight apparentmode progressionwith the passage in Lake Tanga-nyika: the ofdawn and dusk, is absent from the surface oftime. ENTRAL STATISTICAL 4 metres during daylight and is scattered An exceptional sample was taken on the lOW thatsardine landings through thewater columnatnight,a minority 8th of]anuary, 1973 when a single setting dly in Lake Tanganyika of the sardines being within the surface was made at roughly 04.00 hours, close Ivemberin 1969and 1970 4 metres ofwater. inshore for shelter from a strong wind and dwithminimain]une:July Proportion of Alestes lateralis (Boulenger) large waves remaining from a recent storm. vfay in1970. The catch per Therewas an apparent absence ofAlestes The very inshore position and exceptional d sardine in Lake Kariba in deepoflShore areas. Data from the routine turbidity resulted in a catch ofonly 727 fish .se, a close approximation chiromilasamples. taken in 15-20mofwater ofwhich 619 were Limnothrissaand 108 were fort) has beensteady since within 1 km ofthe shore,are shown inTable Alestes. These Alestes are responsible for ehave beenno detectable 1. A year-by-year comparison indicates a the high (4.7%) monthly average ofA.lestes lrtfrom those induced by steadily falling percentage of Alestes presentin]anuary, 1973 Crable 1) and also :hnique. Fishingcontinued although this is not evident from the for the yearly average o,f Alestes present the moon in the present monthly averages. being higher in 1973 than in 1972. alower average catch per The length frequency distributions of Predators vhen fishingwas suspend­ n. lrbidity of Lake Kariba Table 1 Fish samples From Mixed Limnothrissa miodon and Alestes lateralis Catches, with the calculated toward the narrow and percentage of Alestes present. 1 of the lake, compared Tanganyika means that 1971 1972 1973 1974 i effective and the catch NlJS % Nos % Nos % Nos % OULTER (1968) found A lestes A lestes A lestes A lestes IgS of 5-18 m. in Lake OCHE (1968) quotesfig­ January 2736 1.9 2317 4.7 11301 0.1 ike Kariba. The present February 3161 0 6622 0.3 Ii disc readings in the March 5772 1.8 2252 0.8 2800 0.2 at limited part of Lake April 4258 0.4 3038 0.3 tigation (basin 11I as de­ May 1959 1.0 2922 2.9 :1968). June 1921 0.3 4208 0.8 ltion July 1786 0 1592 1.0 peated atshortinten'als August 2465 0.1 3770 2.8 ocation, in 15mmwater September 441 0.2 7043 0.9 12076 0.2 J chiromila net without October 2049 0 5768 0 8138 0.1 ghts. Thisnetwas28mx November 1729 0.3 477 0 2676 0.1 ndits geometrywassuch December 3157 3.1 1180 0 3413 0.7 :tive fishing volume lay Total for the face ofthe water. Time­ Year 7376 34365 49563 20723 the number ofsardines Percentagefor face few metres were the year 1.4 0.7 0.9 0.2 36 RHUDDART

The routine gillnetting programme Small numbers of fish other than fish containingI secured an extensive range of fish species Limnothrissa and Alestes occur in the chiro­ been examine( (including two individuals ofLimnothrissa). mila catches but, except for Eutropiuscontai Ofthose predators captured, only tiger fish, vittatus and Eutrop'ius depressirostris only the most being , were found to contain the occasional individual is encountered. Eutropius from sardines. Ofthe 1,573 Hydrocynusstomachs Hydrocynus has been present in chiromila 1974,cont.ained examined between October, 1971 andJuly, catchessince theircommencementin1970, is Canibalizatic 1972,212 contained recognisable sardine rarelyabsentandsometimeshundredsofthis was noted only remains, most containing a single sardine species are caught per night. Eutropius when eight of tJ but some containing several individuals. A depressirostris first appeared in chiromila belonging to t further 106stomachs containedfish remains catchesinSeptember,1973andsincejanuary, 70mm. fork 1, whichwere otherwise unrecognisable due to 1974 has beenseldomabsent from a night's sardines presen theiradvanced state ofdigestion, butwhich catch, averaging five individuals pernight. A containoneorn were strongly suspected of comprising higher proportionofthe tiger fish caughtin niles were of20· mostly sardine. Six stomachs were en­ the chiromila net contained sardines than belonging to thl countered which contained one Alestes those obtained-from gillnet catches, but present lateralis each, makingapercentage of0.4% always more than 50% had empty stomachs. Length Frequen of tiger fish which contained this species. Only 38 Eutropius from chiromila catches The lengths Twenty-nine stomachs contained other were examined for gut contents and all of quency determ recognisable species. these containedsardines. Moreover, no tiger againstcalende sardinesefficien bunt of the chi 20mm. fork len mens could be i mm sardines. InLak 90 J a smaller maxin L+S . . . nyika where .L ...... L. to 100 mm. st ..... S· 1968A). A cycli .T. ' .. ..S which a mode ;:

2 ..,---i'---::-_---'---1-"'---=-1;-:-...... ,-1---:--. I is present durir oj_: M 1 J 1 J 1 A 1 5 1 0 1 N 1 D mm N' U 1 J I- MA disappearsin Se 90_ 1971 1972 A smaller m - L+S forklengthisal - '0 '• '. ,,' .... ~ . L· ; "_. "'~",,, ... /.,...• L., ... wi th the large - S Although the c .' ...:' .... "..,'. \...... ', '. ::.... ::~. s',." .1: .' modeseems to t of the histogr, 1 \ I M I J I J 1973 mode, in fact, n for the last f( existence. The finally becomt Fig. 3. The mode lengths offorklength frequency distributions ofLimnothrissasamples. L=The mode pronounced s of the larger fIsh. S=The mode of the smaller fIsh. T=The third mode to appear each year. covering thos BI0LOGYOFUMNOTHRISSAMIODONINLAKEKARJJ3A 37

ers of fish other than fish containing more than four sardines has mode formerly existed. Alestes occur in the chiro­ been examined in this study, but nine The sardine population being fished in t, except for Hydrocynus Eutropius contained four or more sardines, the Sinazongwe area in the post-September rop'ius depressirostris only the most being 14 sardines. In contrast, no months is apparentlya different population ldividual is encountered. Eutropius from gillnets, in either 1973 or from that present in the pre-September een present in chiromila 1974,contained sardines. months. An estimate of 1.5 mm. (1972) or commencementin 1970, is Canibalization ofsmall by large sardines 1.0 mm. (1973) permonthaverage growth rate ,ornetimes hundredsofthis was noted only in early September, 1973, during the mostpartoftheyearfor the longer ~ht per night. Eutropius when eight ofthe 34 inldividuals examined modeandof2-4mm. permonthfor theshorter : appeared in chiromila belonging to the unusually large (over mode can be made (Fig. 3). Two modes at Jer, 1973andsincejanuary, 70mm. fork length) length group of 30 mm. can be seen apparently growing at :lorn absentfrom a night's sardines presentat that time were found to roughly 10 mm. per month for periods of20 ve individuals pernight. A contain oneor twojuvenileseach. Thesejuve­ days in january and February, 1974, when 1 ofthe tiger fish caughtin niles were of20-35 mm. fork length, that is, more frequent determinations were being :contained sardines than belonging to the smallestlengthgroup then undertaken. Evidence is presented below rom gillnet catches, but present that Limnothrissa in Lake Kariba can mature 50% had empty stomachs. Length Frequency at40 mm. fork length so th(lt the two estimates r from chiromila catches The lengths of the modes oflength fre­ ofl-l.5mm. andof2-4mm; growth permonth Ir gut contents and all of quency determinations are shown plotted canbe compared to thegrowthrate ofmature rdines. Moreover, no tiger against calender time in Fig. 3. The smallest Limnothrissa in Lake Tanganyika where the sardines efficiently caughtby the 8mm. mesh mostrecently published information shows bunt of the chiromila net were of roughly 12mm.and 14mm. permonth growth ratefor 20mm. fork length but even smaller speci­ 60 mm. and 40 mm. standard length fish mens could be included in a mass ofnetted respectively (MATTHES 1968a).The nature sardines. In Lake Kariba Limnothrissaattains and causes of the disruption of the slow a smaller maximum size than in Lake Tanga­ progression oflength frequency modes in September, 1972 and again in September, .L... nyika where i';dividuals grow rapidly to 100 mm. standard length (MATTHES ..S 1968A). A cyclical phenomenon occurs in which a mode at about 60 mm. fork length 0 is present during 11 months ofthe year and ND 1Il disappears in September. Ql ....:... A smaller mode at about 40 or 50 mm. Ql fork length is also present, existing together E 22.5 "_/""'L.~ .. ' ~ with the larger mode for eight months. .... 25.5 a. 25 S Although the disappearance ofthe shorter Ql 24.5 ::-;::5 .,.:-,. ""0 modeseems to beabrupt (Fig. 3), examination :... of the histograms shows that the shorter ....Ql 24 J F m 1974 mode, in fact, merges into the longer mode ~ 22 for the last four months of the latter's existence. The bimodal distribution in May 50 finally becomes unimodal in june with a i5sasamples. L=The mode pronounced skew towards the lower'(alues, )de to appear each year. Fig. 4. Isotherms, degrees Centigrade, in deepwater near covering those lengths where the smaller Sinazongwe duringJuly-October 1973. ---

38 R. HUDDART

1973are notclear, and certain limnological further increased depth to zero at four to studies were undertaken during the months six metres. Primary productionwas 41 0 and I 2 close to September, 1973 in order to obtain 590 mgs C.m- day -l respectively. All later IOO~ more evidence in connection with this photosynthesis/depth curvesshowed maxi­ phenomenon. mum production at between two and six Limnological Investigations metres depth and lesser productionat0.5m. The temperature profile of the nearest and one metre depth separated by very low deepwater to Sinazongwe, offthe Dundum­ productionat 1.5 m depth. 0/0 pongo islands, 15 km. distant. duringJuly­ On 29thAugustprimary production was October, 1973 is shown inFig. 4. Thesituation 570 mg. and on 5th Septemberit had fallen 2 inJulyandAugustis ofcomplete mixingwi th to 270 mg. C.m day -l. A minimumin primary an almost uniform temperature between productionwas recordedon lOthSeptember 22°C and 23°C. Warming ofthe entirewater at 240 mg. after which it rose again to 440 .. mass occurred during Septemberand early mgon 19th Septemberand 990 mg C.m-2day October, afterwhicha thermoclinestarted to -Ion 2nd October.A final determination on o form. On 10thDecember,1973thethermo-dine 5th Novembershtlwed primaryproduction to was present from 25mm. (27°C) to be 600 mgC.m-2day-l. Primaryproductionwas J 35mm. (24°C) and on the 15th February thereforeratherlowingeneraland underwent from 30m. (27°C) to 43m. (24°C). These a reductionduringSeptember. results, though more detailed for this one Reproduction Fig. 5. The propc area and short period to time, are in Duringmost ofthe year thesardineswere merged L am agreementwith those ofCaCHE (1968). found in two distinctsize groups (Fig. 3) and 1972 T fish. The September disruption in the slow fish, both of the percentage ofripe fish in each ofthe two are represen growth of Sinazongwe sardines therefore groupsfromJuly, 1972 to March, 1974is shown samples, whe coincide!iwitha generalwarmingofthewater infig. 5. Ripe fish and therefore, presumably, 4, the thicke mass. spawning occur throughout the year. Up to in Septemb~ Weekly Secchi disc readings were taken 31 % ofthe smallersize groupwere found in only1.5 kIn. offshoreandthe amountofwave ripe condition,this maximumfigure occurring period 4 fish (Fi actionwas found to temporarilyincrease the onSeptember19th, 1972whenthesmallersize o flarger fishwai turbidity of the water in the shallow was regarded as comprising fish ofless than rate ofreproduci Sinazongwebush-clearedarea. Readingsfrom 55mm.forklength.Severalripesardinesbetween tion of large fis July to Octoberwere all between fiveandsix 29mm. and 53mm. forklengthwere recorded which undergo( metres. InJune, due to some strong winds on this date. Thisis amuch lowerfigure than proportionofri altematingwi thcomplete calms, readings of the 61-63mm. fork lengthoffirstmaturityfor its establishmel four, three, seven and four metres, were Lake Tanganyika Limnothrissa recorded by upits reproduct recorded. ELLIS (1970). ingyear untilit, 1 Lightand dark bottle experiments were The greatestreproductive activityin 1973 fish whose sex performedatintervalsduringthe third quarter took place inJuly. Thisstudy commencedin tion ofmalesw of 1973 in order to measure the primary late July, 1972 but possibly the greatest the small numt productivity. On9thAugustand28thAugust, reproductive activity in 1972 took place in less than 50%. I grossphotosynthesis/depth curvesofclassic September. In both years a fall in the pro­ constituted 3~ shape were found with marked inhibitionat portion ofripe fish took place in September­ sardines. depth less than one metre, maximum Octoberwhenthe period 1fish were replaced, phtosynthesis at one or two metres and a from whatever source, by period 2 fish and, gradual lessening of photosynthesis with similarly, period 3 fish were replaced by BIOLOGYOF UMNOTHRISSAMIODONINLAKEKARJJ3A 39

ed depth to zero at four to lary productionwas 41 0 and 2 period 3 4 lay ·1 respectively. All later IOO--I::-::....=:J~ --L-_---l...._---~ ! depth curves showed maxi­ )n at between two and six _large fish dlesser productionat0.5m. --small lepth separated by very low 5mdepth. ust primaryproduction was 0/0 5th September it had fallen lay·l. Aminimumin primary ecorded on 10thSeptember which it rose again to 440 ~mberand990 mg C.m·2day r. Afinal determination on owed primaryproduction to o iy·l. Primaryproductionwas )wingeneraland underwent .gSeptember.

Fig. 5. The proportion of ripe fish in Limrwthrissa samples. In period 1, the thi~ker trace represents fthe year thesardineswere merged Land S fish (see Fig. 3), which first appeared in September, 1971. The thmner trace represents nctsize groups (Fig. 3) and 1972 T fish. In period 2, the thicker trace represents L fis~ and the thinner trace represents S :ripefish ineach ofthe two fish, both of which first appeared in September 1972. In penod 3, the merged 1972 Land S fish 972 to March, 1974isshown are represented by the thicker trace. The thinner trace represents T fish, except for the last ~wo samples, where it represents the forerunners of the S fish which appear in Septemb~r 1973. In penod IOd therefore, presumably, 4, the thicker trace represents L, and the thinner trace represents S fish, both of whIch first appeared lroughout the year. Up to in September 1973. r size groupwere found in maxirnumfigureoccurring period 4 fish (Fig. 5). The new popula~on ., 1972when thesmallersize Condition oflarger fish was ata similar, relatively hIgh >mprising fish ofless than Essentially, the weights ofthe three sepa­ rate ofreproductive activity to the oldpopula­ ;everalripesardinesbetween ratesizes offIshvarysynchronously. Greater tion oflarge fish. It is this new population fluctuations occur in theweightofthe larger fork lengthwere recorded which undergoes the sharp reduction in the iamuchlowerfigure than fish than of the smaller (Fig. 6). proportion ofripe fish within one mo~~ of The type of balance used (see methods length offirstmaturityfor its establishment off Sinazongwe, bmldlOg Limnothrissa recorded by section) was such thatthe synchronous fluc­ upits reproductiveactivity duringthe follow­ tuations couldnot have arisen from errors in ingyearuntilit, too, is replaced.Among those thesetting-up ofthe balance. Heavierweight >roductive activity in 1973 fish whose sex was identified, the propor­ rhis study commencedin per length offish is seen to occur in ~ar.ch­ tion ofmales was variable, probably due to April andJune:Julyin 1973. In I972,asImtlar It possibly the greatest the small numbers involved, but was always ity in 1972 took place in J une:July peak occurred but recording only less than 50%. Over the whole study. males started in May. These tluctuations do not h years a fall in the pro­ constituted 32% of the sex identifIed took place in September­ always coincide with fluctuations in repro­ sardines. enod1fish were replaced, ductive activity (Fig. 5) and so probably rce, by period 2 fish and, retlect changes in food supply inaddition to I fish were replaced by changes in the content of reprOductive 40 RHUDDART

allowance f II1II urams occurrence -4J duceslowerf 0 employed 0 0 and KENM 0 0 0 0 0 0 therewas a < 0 8 age of Ales 0 (b0 00 0 0 0 0 0 0 0 0 0 0 0 0 contents.T • 0 0 0 0 0 0 0 stage in tl 3{- 0 increase of t 2 fish gut CO] I 0 o 000 KENMl ."-:" 0 whetherth 0 00 cPo 0 0 0 0 .~ o 0 0 0 6' 0 S' ~ 00 o 8 0 gt 0 0 0 q, 0 Limnothriss, ~#...'j or to a prel fish. That 1 o the popul2 AI­ o 0 Q, 0 00 0 6' 0 0 cP 0 o o o 00 000 0 0 0 0 0 0 o 0 00 the stead] o 0 \'r Fig.l,isco proportior o 1. Length in Octobe sion of let Fig. 6. Weight of 70mm fork length Limnothrissa (upper trace), Weight of 55mm fork length Limnothrissa (central trace), Weight of 40mm fork length Limnothrissa (lower trace), as derived from (Fig. 3). 1 length/weight curves of Limnothrissa samples. a slow ar whetherfr tion or fr' products. tiger fish stomachs contained Alestes lateralis popula-ti DISCUSSION in 1964and1965. Obviously, none contained situation Unpublished data held by the Research Limnothrissa. DONNELLY (1971) found that 1972 wa~ 54.3% of tiger fish food items consisted of Division of the Fisheries Department of creased f( Zambiashowthatin late 1970, samplesfrom Alestes in 1967 and 1968. The proportion of of time, tl more than 1 km. offshore and greater than tiger fish containing A. lateralis at that time large ane 20 m. water depth contained an average of can be estimated at nearer 20% than 55% shore, al 9% Alestesindividuals amongthe Limnothrissa, due to the high proportion of empty tiger spawned while within 1 km. of the shore and less fish stomachs always present. No sardines very quic than 20 m. depth, Alestesaveraged43%. The werefound. KENMUIR (1972) found thatin known th propor-tion ofAlestesin the deeperoffshore April, 1969 to March. 1970,20.6% oftigerfish when th( areas had fallen to less than 0.1 % by mid­ food items consisted of A. lateralis while the surfa 1971. The present study shows a continu­ Limnothrissaconstituted 1.5% offood items. to becor ation ofthis trendwith no Alestes beingfound From April, 1970 to March, 1971, he aprelimi in deep offshore areas and a steadilyfalling (KENMUIR 1972) found that A. lateralis ton bloo representation ofAlestesin inshore catches. formed9.6% offooditemswhile Limnothrissa in tempt MATTHES (l968b) found that 5.3% of formed 41.4% offood items. Even making causes e BIOLOGYOFUMNOTHRISSAMJODONINLAKEKARIBA 41

allowance for the fact that the "percentage tember. occurrence" method ofrepresentation pro­ The main advantage of this hypothesis duceslowerfigures than the"numerical" method ' was the ease with which it could be tested. employed by DONNELLY (1971) Ifthe phenomenonwere caused by changes and KENMUIR (1972), it is evident that in the fertility of the water associated with therewas a continuingdecline inthe percent­ the annual thermal cycle of the lake, it 8 age of Alestes to be found in tiger fish gut could be expected to repeat itself at the o 0 o contents. The presentstudy showsa further same time every year. More frequent stage in the decline of Alestes and the length frequency measurements were increase ofLimnothrissa to be found in tiger therefqre undertaken during the period fish gut contents. August-September-October 1973, in KENMUIR (1972) expressed uncertainty parallel with those limited limnological J> o 0 whether that partofthe decline ofAlestesin studies described above. o tiger fish gutcontentsand its replacementby The phenomenon duly repeated itself Limnothrissawas due to changesin population in September, 1973 showing that it was or to a preference for Limnothrissa by tiger indeed probablyassociated in someway with fish. That there has been a true decline in the annual physico-chemical cycle of the o the population of Alestes is now evident if lake. However, an epilimnion only began o o 00 the steady catch per unit effort, seen in toform in October,and primary production Fig. 1, is comparedwith thedeclining Alestes was higher in August and Ottober than in lolJ IFIM proportion of the catch evident in Table September, so that the hypothesis of an I 1974 1. Length frequency studies commenced explosive growth ofsardines in September in October 1971 and a steady progres­ associated with an increase in algal pro­ n fork length Limrwthrissa sion of length modes soon became clear ductionis weakened. trace), as derived from (Fig. 3). This was interpreted as showing In 1972, aJune-July improvementin con­ a slow and steady growth of sardines, ditionwas largely masked by short-term fluc­ whether from a local, Sinazongwe, popula­ tuationsbuta similarincreasewas much more tion or from a ho~ogenous Lake Kariba obvious in 1973. In addition, a mtained Alestes lateralis popula-tion. The disruption of this recently-published study shows thatat least riously, nonecontained situation which occurred in September, in the Sanyati sub-basin of Lake Kariba a :LLY(1971) found that 1972 was interpreted as a greatly in­ bloom coincides with the lod items consisted of creased rate ofgrowth for a limited period May:Juneperiodoftumoverofthewatermass, 68. The proportion of oftime, the original population becoming ratherthan the refonnation ofthethenno-cline A. lateralis atthat time large and dying or moving further off­ inSeptember-October.AzoopIanktonpeakwas learer 20% than 55% shore, and being replaced by recently­ recorded· in June-July followed lortion of empty tiger spawned sardines which were also growing bya "drasticcrashin (zoo) plankton numbers present. No sardines very quickly. From COCHE (1968) it was overtheperiodSeptember-November.....likely R (1972) found thatin known that September-October is the time to be related to the thermal instability of 970,20.6% oftigerfish when the metalimnion begins to form and the epilimnion" (BEGG 1974). l of A. lateralis while the surface layer ofthe lake basin III begins Although an explosive increasein growth ed 1.5% oHood items. to become stabilised. It was supposed, as rate could still be used to explain the :0 March, 197 I, he a preliminaryhypothesis, thata phytoplank­ "September phenomenon" in the length fre­ lUnd that A. lateralis ton bloom analagous to the "spring bloom" quency mode progressions, broughtabout,for emswhile Limnothrissa in temperate marinewaters takes place and example, by an emergence of 1items. Even making causes exceptional sardine growth in Sep- larvae at that time,. the whole body of 42 RHUDDART

evidence points toan increased food supply There is much evidence for a reduced into the newly-fon in June and July and to the "September rate ofgrowth of Lake Kariba Limnothrissa an airliftfrom Lake phenomenon" occurringina time ofdearth. relative to those in Lake Tanganyika. Ifthe 1968. Perhaps a marked drop in the abundance aberrantmonthofSeptemberis disregarded, The catch of s. of food organisms precipitates large-scale the growth rate inferred from length fre­ remained unchan! sardine migrations in September,leading to quency mode progressions is less than any periodatamuchlm disruptions in local length frequency published estimate for Lake Tanganyika Tanganyika: there patterns. No explanation is put forward to Limnothrissagrowth (ELLIS 1970). The lesser sardine populatior accountfor the post-September population size at maturity is probably due to a slow dense than in Lake invariably stabilising into two length growth rate, while the adoption of a can­ i\lestes laterali~ . frequencymodesoverthreeyears ofsampling. nibalistic feeding potential seems to be de­ newopen-waterpel The fact that these cyclical changes in termined, not surprisingly, by size rather formation of the length frequ~ncymode progression repeat than by age. The much smaller maximum replaced by Limn themselves so exactly suggests that the size recorded fro~LakeKaribaindicates that samplingprogrammc population level of introduced sardine in the life span incompleted before maximum of i\lestes from d, the lake is already in equilibrium. A steady growth has occurred, although it could and itsreduction fr population level is also indicated by the possibly be related to the short history of (1974) ofthe catch unchangingcatch perunit effort. Limnothrissa in Lake Kariba. Sinazongwe shore It is probable that the population of The importantrole now being played by declined, and Limn Limnothrvssa in Lake Kariba, at least in the Limnothrissa in the Lake Kariba ecosystem importance as a foo Sinazongwe area, is less dense than in Lake promises a significant commercial sardine the mostimportanl Tanganyika. Shoalsare notoftenseenin day­ fishery, made more valuable by the proximity Lengthfrequem light although sardines are often seen ofthis lake to the consumer centres and by estimate of the gro nosing the surface ofthe water atdusk. The the apparent absence ofa regular seasonal to bemade. Thisral catch per haul is only a fraction of that drop in catch as found in Lake Tanganyika. publishedestimate f achievedin Lake Tanganyikaifa chiromila net However, the apparentlesserpopulation Tanganyika Limnc is usedand onlyaminute fraction ifa 'lusenga' density and growth rate of Lake Kariba annual, cyclical phe: (dip) netis used. Shoals ofsardine are readily Limnothrissa combined with the relatively theadultsardine pc observedin daylightin Lake Tanganyikaand, small size ofLake Kariba itself, prevent the lengthof70-75mm duringnight-fishing, industrial purse-seiners prospective Lake Karibasardinefishery from sardines during do notconsider itworth theirwhile to make approachingthe size ofthe Lake Tanganyika population quickl a setting unless large numbers are visible fishery. The advent ofa commercial fishery tinct size groups w under their artificial lights. In Lake Kariba, in Lake Karibawill provide regular samples and 55-60mm.Atfi sardines are not readily seen in the illu­ ofLimnothrissa from different regions ofthe grows much more minated zone ofwater under a light-fishing lake andenablemore concentrated biological size groupandfrna lamp, although larger catches per setting studies to be undertaken. The presentuncer­ ata median length are associated with visible, albeit very few, tainty concerning the annual cycle in The reason fo sardines. Itispossible, however, thatgreater the biology off Sinazongwe Limnothrissa orderly progressil turbidity ofthe water mightaccount, at least might then be resolved. modes in Septen partially, for these effects. Lake Kariba SUMMARY annual physio-cht sardine catchesare monospecific, consisting Artificial lights and a chiromila netwere butwithoutconspi only ofLimnothrissa, while Lake Tanganyika used to obtainsamplesof'Sardine, Limnothrissa mass as a whole be catches include both Limnothrissa and miodon (Boulenger) ,0ffSinazongweon Lake ber. priorto the fOJ Stolothrissa, of which the latter at present Kariba, from September, 1971 toFebruary, 1974. while primarypro( (1974) forms the greater portion. Limnothrissajuveniles had been introduced roughly half of 1 BIOLOGYOFUMNOTHRISSAMIODONfNLAKEKARIBA 43

:h evidence for a reduced into the newly-formed, man-made lake in preceeding and succeeding months. fLake Kariba Limnothrissa anairliftfrom Lake Tanganyikain 1967and The minimum size of spawning adults in Lake Tanganyika. Ifthe 1968. was much less than in Lake Tanganyikaand IfSeptemberis disregarded, The catch of sardines per unit effort nevermore than 50% ofany size groupwere inferred from length fre­ remained unchanged over the sampling found in ripe condition. A significant ogressions is less than any period ata muchlower rate than thatin Lake n umberofripe sardineswas found through­ ate for Lake Tanganyika Tanganyika: there is little doubt that the outthe yearalthough there was, perhaps, an 1:h (ELLIS 1970). The lesser sardine population in Lake Kariba is less increase ofspawningactivi ty inbetweenJune is probably due to a slow dense than in Lake Tanganyika. and September. The sardines were also in lIe the adoption of a can­ Alestes lateralis, which had invaded the betterconditionbetweenJuneandSeptember ~ potential seems to be de­ new open-waterpelagic niche soonafter the but better condition was not exactly lrprisingly, by size rather formation of the lake, had been largely synchronous with a greater proportion of ~ much smaller maximum replaced by Limnothrissa by 1971. The ripe fish. nLake Karibaindicates that samplingprogr.ur.unerecordedtheelirrllna-tion ACKNOWLEDGEMENfS mpleted before maximum of Alestes from deep, open-water areas Special thanks are due to FisheriesAssi tant, lrred, although it could and its reduction from 1.4% (1971) to 0.2% A. Situmbeko and Research Assistant, S. ed to the short history of (1974) ofthe catch from within 1 km. ofthe Velemufor theirinterestand helpatall stages ake Kariba. Sinazongwe shore. Similarly, Alestes has of this study. trole now beingplayed by declined, and Limnothrissa hasincreased, in REFERENCES Ie Lake Kariba ecosystem importance as a food speciesfor Hydrocynus, Begg, G.W. (1974). The influence ofthermal icant commercial sardine the most importantpredatorin the lake. and oxygenstratifica60nonthevertical evaluable by the proximity Length frequencystudieshave enabledan distributionofzooplanktonatthemouth consumer centres and by estimate of the growth rate of Limnothrissa ofthe SanyatiGorge, I,..ake Kariba. Kariba ~nce ofa regular seasonal to be made. Thisrate ismuchlowerthan any Studies, 4, 60-67. mnd in Lake Tanganyika. publishedestimateforthegrowth rate ofLake CentralStatisticalOffice, Lusaka (1971): Fish­ pparentlesserpopulation Tanganyika Limnothrissa; A very regular, eries statistics (natural waters) 1970. vth rate of Lake Kariba annual, cyclical phenomenonoccursinwhich Official statistics November, 1971. bined with the relatively the adultsardine population, with a median Coche,A.G. (1968). Description ofphysico­ Kariba itself, prevent the length of70-75mlQ, is replacedwith smaller chemical aspects of Lake Kariba, an ~ribasardine fishery from sardines during September. The new impoundmentinZambia-Rhodesia. Fish. ze ofthe Lake Tanganyika population quickly stabilises into two dis­ Res. Bull. Zambia, 5: 200-267. It ofa commercial fishery tinct size groups with medians at 30-40mm Coulter,G.W. (1968). Hydrologicalprocesses Iprovide regularsamples and 55-60mm. Atfirst, the smallersize group and primary production in Lake m differentregions ofthe grows much more quickly than the larger Tanganyika. Proc. 11th conf. Great re concentrated biological size groupand finally merges into the latter, Lakes Res. 609-626. Internat. Assoc. taken. The present uncer­ atamedian length of45-50mm, duringMay. Great Lakes Res. g the annual cycle in The reason for the disruption in the Coulter,G.W. andYA Znamensky (1971). The ;inazongwe Limnothrissa orderly progression of length frequency chiromila net. A method for catching lIved. modes in September was sought in the light-attracted fish inLake Tanganyika. annual physio-chemical cycle of the lake, Fish. Res. Bull. Zambia, 5: 215-224. and a chiromila netwere butwithoutconspicuous success. Thewater Donnelly, B.G. (1971). The fish population les ofsardine, Limnothrissa mass as a whole became warmer in Septem­ changesonLake Karibabetween 1960 ), offSinazongwe on Lake ber, prior to theformation ofa thermocline, and 1968. Part II, Characidae and ber,1971 toFebruary,1974. while primaryproductivityinSeptemberwas Citharinidae. Internal Report, lIes had been introduced roughly half of that in the immediately L.K.F.R.I. Kariba, Rhodesia. 44 NOT}

Ellis, C.M.A. (1970). The size at maturity Matthes, H. (1968a). Preliminary investiga­ and breeding seasons of sardines in tions into the biology of the Lake PERI( southern Lake Tanganyika. African]. Tanganyika clupeids. Fish. Res. Bull. ponto) ~ Trop. Hydrobiol. Fish., 1:59-66 Zambia, 4: 39-45. Golterman, H.L. (1968). Methodsforchemical Matthes, H. (1968b). The food and feeding analysis offresh waters. IBP Handbook habits of the tiger-fish, Hydrocyon B, Blackwell, Oxford. 172p. vittatus (Cast., 1861) in Lake Kariba. Kenmuir, D.H.S. (1972). Report on a study Beaufortia, 15: 143-153. of the ecology of the tigerfish, Vollenweider,R.A. (Ed.) (1969).AManualon Hydrocynus vittatusCaste1nau,in Lake Methods for Measuring Primary Pro­ Kariba. Internal Report, L.K.F.R.I. duction in Aquatic Environments. IBP Duringe< Kariba, Rhodesia. Handbook12. Blackwell,Oxford. 213p. reefcarideat , a small speC! occasions fn These shrim] to Periclimen, tent differen pereiopods, colourpattern a differentho that the speci separatespecit Periclimen

•

Fig. 1. Perdi' female, :