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An assessment of the fish population of the lower reaches of the Sanyati River, Zimbabwe

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An assessment of the fish population of the lower reaches of the Sanyati River, Zimbabwe
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   African Journal of Aquatic Science 2000, 25: 84–88 Printed in South Africa — All rights reserved Copyright © NISC Pty Ltd  AFRICAN JOURNAL OFAQUATIC SCIENCE EISSN 1727–9364 An assessment of the fish population of the lower reaches of the SanyatiRiver, Zimbabwe W Mhlanga Lake Kariba Fisheries Research Institute, Private Bag 2075 Kariba, Zimbabwee-mail: lkfri@africaonline.co.zw  Received 1 July 1999, revised 14 November 2000 The fish population of the lower reaches of the Sanyati River was studied using multifilament gillnets and monthly sampling wasconducted over 13 consecutive months. Atotal of 15 fish species belonging to seven families were collected and variations inmonthly catches and catch per unit effort and condition were observed. The five most common species exhibited a seasonal breed-ing pattern, with most breeding occurring in the rainy season. The exotic Nile tilapia Oreochromis niloticus was found to haveestablished itself in the Sanyati River. The results of this study were compared with those of previous workers on Lake Kariba. The Sanyati River is an important river, contributing about10% of Lake Kariba's total annual inflow. Productivity in theEastern Basin of the lake is strongly influenced by the flowof the Sanyati where Masundire (1989) observed that theSanyati River had a significant effect on zooplankton abun-dance, increases being attributed to localised floods thatbring in nutrients. Several fish species migrate from Lake Kariba up theSanyati River to breed. Kenmuir (1973) observed thatTigerfish, Hydrocynus vittatus , Castelnau, migrates up theSanyati to spawn, although the exact locality of the spawn-ing areas was not determined. In a study of the MwendaRiver, Bowmaker (1973a) noted that, besides H. vittatus ,several other species also migrated upstream to breed.These included Momyrus longirostris , Peters,  Alestes(Brycinus) imberi  , Peters, Labeo cylindricus , Peters, Labeocongoro , Peters, Labeo altivelis , Peters, Clarias gariepinus ,Burchell, Labeo lunatus , Jubb. Although Bowmaker reportedthe occurrence of L. lunatus in Lake Kariba, it is generallybelieved that this species occurs in the Upper Zambezi andOkavango systems. This identification may have been dueto an error and further studies will have to be carried out toverify its occurrence in the lake. Several workers have stud-ied the fish populations in the Sanyati River. Whilst the focusof these was on H. vittatus (Begg 1974, Kenmuir 1973,1975, Langerman 1981, 1984), there is limited informationon the other fish species occurring in the Sanyati.Estimates of the annual fish catches from the artisanalgill-net fishery in the fishing villages adjacent to the SanyatiRiver from 1980 to 1998, have varied from 32.76 tonnes in1998 to 417 tonnes in 1984 (Songore et al  . 1999). Duringthe same period the catch per unit of effort (cpue) in thesevillages varied from 2.8kg/100m in 1981 to 15.8kg/100m in1990, and over the last 7 years (1992–1998 inclusive) thelowest cpue (3.7kg/100m) was in 1996 while the highestcpue was in 1993 (7.1kg/100m).The aim of this study was to assess the current status of the fish populations in the Sanyati River (Figure 1).Comparisons were made with previous studies in theSanyati and Mwenda Rivers. Materials and methods Multi-filament gillnets were set monthly, perpendicular to theshoreline, from June 1997 to June 1998 in the Sanyati River.Two sets of 38, 63, 89, 114 and 140mm stretched mesh netswere used, one bottom-set, the other top-set. Condition factor (c.f.) was calculated using the equation: c=W * 100/L 3 (Pauly 1984)whereW=Weight in grams (ungutted weight)L=Length in centimetres (standard length) Results and discussion  Atotal of fifteen fish species belonging to seven familieswere caught (Table 1). The Redbreast tilapia ( Tilapia rendal-li  Boulenger), chessa ( Distichodus schenga Peters), Electriccatfish ( Malapterurus electricus Gmelin) and African mottledeel (  Anguilla bengalensis labiata Peters) were recorded inthe angling censuses of 1973 (Begg 1974) and 1980(Langerman 1981). These four species were not recorded inthe current study. Nkupe ( Distichodus mossambicus Peters)was recorded only in the 1973 study, while Labeo congoro and Mormyrops anguilloides were only recorded in the 1980 Introduction Keywords : Fish population, Sanyati river, Zimbabwe, abundance, breeding, condition factor   study. The absence of these species requires further inves-tigation to determine whether this is due to differences insampling gear or is a result of a decline in their populations. Hydrocynus vittatus was the most abundant species inthe Sanyati River (Table 1). Previous studies in the SanyatiRiver also noted the predominance of H. vittatus (Begg1974, Langerman 1981). The second most common speciesin Sanyati River was B. imberi  . The five commonest species,which constituted 93.6% of the total, were H. vittatus , S.zambezensis , B. imberi  , S. intermedius and L. altivelis .Notably, the tenth most abundant species in the SanyatiRiver was the exotic cichlid O. niloticus .In a study of the status of O. niloticus in Lake Kariba dur-ing 1993/1994, Chifamba (1998) observed that it occurredonly in the area near a fish farm that had been culturing thisspecies. Acage-culture project to rear O. niloticus wasestablished in the Eastern Basin of Lake Kariba. It is likelythat this project contributed to the spread of O. niloticus tothe Sanyati River. These results show that O. niloticus established itself in the Sanyati River. Ratios of males to females varied among the species(Tables 2 and 3). Of the ten species compared, six, namely H. discorhynchus , M. longirostris , B. imberi  , H. vittatus , L.altivelis and O. mortimeri  , had more males than femaleswhereas in S. intermedius , C. gariepinus , S. zambezensis and S. codringtonii  ,the females predominated. Severalspecies were excluded from the comparisons as they hadsample sizes of less than 10. Fish population of the lower reachers of the Sanyati River, Zimbabwe85 FamilySpeciesNumber of fishPercentageCharacidae Brycinus imberi  50025.04 Hydrocynus vittatus 60930.5Cyprinidae Labeo altivelis 1276.36 Labeo cylindricus 40.2 Schilbe intermedius 1587.91 Clarias gariepinus 281.4 Heterobranchus longifilis 10.05 Synodontis nebulosus 10.05 Synodontis zambezensis 47023.54 Oreochromis mortimeri  472.35 Oreochromis niloticus 80.4 Sargochromis codringtonii  30.15 Hippopotamyrus discorhynchus 211.05 Mormyrus longirostris 160.8 Marcusenius macrolepidotus 40.2 Figure 1: Lake Kariba with sampling sites on the Sanyati River indicated Table 1: Species composition and abundance of fish caught in gillnets in the Sanyati River in 1997–1998  Mhlanga86 Kenmuir (1973) noted that, for H. vittatus , the 35–38cmlength-class had the largest percentage of females, while inthe study by Bowmaker (1973b) females became dominantonly in the larger length classes (40–70cm). In the currentstudy a similar trend to that reported by Bowmaker wasobserved where females become more dominant withincreasing size.In Bowmaker’s (1973b) study, males of Mormyrus and Clarias became more dominant with increasing size (lengthclass), whereas in the current study females became moredominant. The trends in sex ratios for Hydrocynus , Brycinus(Alestes) , Synodontis and Schilbe (Eutropius) in the currentstudy were similar to those observed by Bowmaker, wherefemales become dominant in the larger length classes.Monthly catch (by numbers) of the five most abundantspecies showed varying peaks (Table 4). Catches of L.altivelis , S. zambezensis and B. imberi  were highest inSeptember. Catches of H. vittatus were highest in January,while those of S. intermedius were highest in March. Themonthly catch per unit of effort for all the species combinedshowed several peaks (Table 5). On a weight basis, cpuewas highest in January. This is the month when there wasthe highest number of the most abundant species ( H. vitta-tus ). Further sampling will need to be carried out in order toassess whether the cpue patterns are directly linked tobreeding activity and recruitment. In terms of numbers, the highest cpue occurred inSeptember, the month when the highest numbers of S. zam-bezensis , B. imberi  , and L. altivelis were caught (Table 5). Inan angling census of  H. vittatus in the Sanyati in 1973, Begg(1974) observed that the highest cpue (catch per boat-hour)was in October. Bowmaker (1973b) also noted that the high-est catches were in January/February (rainy season) whilethe lowest catches were in June/July (winter). The catchesof S. zambezensis in the current study, reached a peak inSeptember, while in her study of the same species,Sanyanga (1996a) reported that peak catches occurred in April. The differences may have been due partly to differ-ences in sampling locality, since the current study was in ariver, while Sanyanga's study was in the open waters of thelake. These studies show that the variation in catches is sea-sonal.Breeding patterns of the five most important species areshown in Table 7. Most breeding activity occurred fromNovember to March. This period coincided with the rainyseason, which Coche (1974) noted was from November to April. Bowmaker (1973b) observed that most of the fish bredduring the summer rains. In Lake Chivero (formerly LakeMcIlwaine) near Harare, the peak-breeding season for B. Table 2: Sex ratios (%) of different size-classes of C. gariepinus , H. vittatus , L. altivelis , M. longirostris and O. mortimeri from the SanyatiRiver [values given are for females] Table 3: Sex ratios (%) of different size classes of B. imberi  , S. intermedius and S. zambezensis from the Sanyati River [values given are for females] Table 4: Monthly catch (in numbers) of B. imberi  , H. vittatus , L. altivelis , S. intermedius and S. zambezensis from the Sanyati River (June1997–June 1998) Table 5: Monthly catch per unit of effort (cpue) of all fish caught in the Sanyati River (June 1997–June 1998)SpeciesLength Class (cm)10.0–19.920.0–29.930.0–39.940.0–49.950.0–59.960.0–69.9n C. gariepinus 100547510010020 H. vittatus 32281866100100468 L. altivelis 59445716 M. longirostris 0456736 O. mortimeri  22465050SpeciesLength Class (cm)5.0–9.910.0–14.915.0–19.920.0–24.925.0–29.930.0–34.9n B. imberi  80427569 S. intermedius 5030667110087 S. zambezensis  –333472100190JunJulAugSepOctNovDecJanFebMarAprMayJun B. imberi  6831479561414474226385217 H. vittatus 351079013982121511957855642832 L. altivelis 597515922486980 S. intermedius 1516125101710366353133 S. zambezensis 467073122374714353574493424JunJulAugSepOctNovDecJanFebMarAprMayJuncpue (No./100m)38303971324121903029352213cpue (kg/100m)6.846.476.5713.575.9615.934.455.518.615.466.994.331.38  Fish population of the lower reachers of the Sanyati River, Zimbabwe87 imberi  was from October to November for females andDecember to January for males (Marshall and Van der Heiden 1977). In the Sanyati River, their breeding seasonreached a peak in December.The highest percentage of mature L. altivelis occurred inNovember. B. imberi  and S. intermedius had the highest per-centage of mature fish in December and March. H. vittatus and S. zambezensis had two peaks of sexually mature fish.The two peaks for H. vittatus occurred in November andJanuary, with the latter peak being the major one. In a studyof H. vittatus in Sanyati Gorge, Kenmuir (1973) noted thatbreeding peaks occurred in November and Jan-uary/February. For  S. zambezensis , the major peak occur-red in December while the minor peak was in March.Most of the breeding patterns observed in this studywere similar to those observed by Bowmaker (1973b) withthe exception of S. zambezensis . In Bowmaker’s study,peak breeding activity occurred in April, whilst in the currentstudy the major peak occurred in December. In the study bySanyanga (1996b), breeding activity reached a peak in Aprilfor females and in April, June and December for males. From Table 6 and Table 7 it is seen that the lowest con-dition factors of the individual species occurred during theperiod of breeding. The poor condition is likely to be linkedto reproductive activity since the fish lose condition as theymobilise energy reserves for reproduction. Kenmuir (1973)also observed that the monthly condition of H. vittatus declined from November. He concluded that there were def-inite seasonal differences in condition. Aseasonal variationin condition was also observed in the current study.In their study of B. imberi in Lake Chivero, Marshall andvan der Heiden (1977) noted that for fish with a standardlength greater than 10cm, the condition factor reached amaximum in October for males, and in November for females. In the current study, the condition factor for B.imberi reached a peak in September. Conclusions The current results show that seasonal variations in the totalcatch, condition factor as well as the sex ratios of somespecies in the Sanyati River were similar to those observedby previous workers on Lake Kariba and Lake Chivero. Thesex ratios of several species were, however, different. Although breeding patterns were generally similar, anexception was observed for S. zambezensis . It is clear thatdifferent population studies sometimes differ in their resultsfor certain species. References B EGG GW (1974) Report on a four-month angling census in theSanyati Gorge, Lake Kariba in 1973 . Lake Kariba FisheriesResearch Institute unpublished project report No. 19, 22pp.B OWMAKER  AP(1973a) Potamodromesis in the Mwenda River, LakeKariba . In Ackermann WC, White GF and Worthington EB (eds)Man-Made Lakes: Their problems and environmental effects.Geophysical Monograph 17, American Geophysical Union.Washington DC, pp 159–164.B OWMAKER  AP(1973b)  An hydrobiological study of the MwendaRiver and its mouth, Lake Kariba . PhD thesis, University of theWitwatersrand, South Africa, 576pp.C HIFAMBA PC (1998) Status of Oreochromis niloticus in Lake Kariba,Zimbabwe, following its escape from fish farms . In Cowx IG (ed.)Stocking and Introduction of fish. Fishing News Books, Oxford, pp267–273.C OCHE  AG (1974) Limnological study of a tropical reservoir. InBalon EK and Coche AG (eds) Lake Kariba: a man-made tropicalecosystem in Central Africa. Monographiae Biologicae, Junk, TheHague 24 : 1–242.K ENMUIR DHS (1973) The ecology of the Tigerfish, Hydrocynus vit-tatus Castelnau, in Lake Kariba. Occasional Papers of theNational Museums and Monuments of Rhodesia B5(3): 115–170.K ENMUIR DHS (1975) The diet of fingerling tigerfish, Hydrocynus vit-tatus Castelnau, in Lake Kariba, Rhodesia.  Arnoldia Rhodesia 7 :1–8.L  ANGERMAN JD (1981) Report on the 1980/81 angling census in theSanyati Gorge . Lake Kariba Fisheries Research Institute unpub-lished project report No. 41, 16pp.L  ANGERMAN JD (1984) Optimum harvest strategies for Tigerfish inLake Kariba, Zimbabwe . MSc Thesis. University of Witwatersrand, Johannesburg, 186pp.Marshall BE (1982). The influence of river flow on pelagic sardine Table 6: Monthly condition factor of B. imberi  , H. vittatus , L. altivelis , S. intermedius and S . zambezensis from the Sanyati River (June1997–June 1998)JunJulAugSepOctNovDecJanFebMarAprMayJun B. imberi  2.592.782.753.002.532.621.362.512.592.302.532.432.68 H. vittatus 1.611.811.753.211.671.850.832.332.511.511.661.811.52 L. altivelis 3.522.582.632.602.703.801.132.652.782.442.572.60 S. intermedius 1.431.571.281.421.251.550.871.291.411.371.351.331.20 S. zambezensis 1.911.902.002.022.002.051.102.042.012.061.971.891.85 Table 7: Monthly percentage of B. imberi  , H. vittatus , L. altivelis , S. intermedius and S. zambezensis in breeding state in the Sanyati River (June 1997–April 1998)JunJulAugSepOctNovDecJanFebMarApr  B. imberi  000012.744.461.114.36.905.6 H. vittatus 2.23.813.332.020.960.335.380.755.6010.5 L. altivelis 0000066.7000011.1 S. intermedius 0000002016.742.146.818.5 S. zambezensis 10.804.50.925.018.4100.050.033.363.651.9  Mhlanga88 catches in Lake Kariba. Journal of Fish Biology  20 : 465–469.M  ARSHALL BE, V  AN D ER H EIDEN JT(1977) The biology of  Alestesimberi  Peters (Pisces: Characidae) in Lake McIlwaine, Rhodesia. Zoologica Africana 12 : 329–346.M  ASUNDIRE H (1989) Zooplankton composition and abundance inrelation to water transparency and predation in Lake Kariba,Zimbabwe.  Archiv für Hydrobiologie 33 : 513–520.P  AULY D (1984) Fish population dynamics in tropical waters: Aman-ual for use with programmable calculators . ICLARM Studies andReviews 8, 325pp.S  ANYANGA RA(1996a) Variations in abundance of Synodontis zam-bezensis (Pisces: Mochokidae) Peters 1852, in the inshore fish-ery of Lake Kariba. Fisheries Research 26 : 171–186.S  ANYANGA RA(1996b)  Age, growth estimates and reproductive biol-ogy of Synodontis zambezensis (Mochokidae) Peters 1852 inLake Kariba, Zimbabwe . PhD Thesis, Stockholm University,Sweden, Part 3, 20pp.S ONGORE N, M OYO  A, M UGWAGWA M(1999) 1998 Fisheries Statistics .Lake Kariba, Zimbabwe Shore. Lake Kariba Fisheries ResearchInstitute, Unpublished project report No 95, 38pp.
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