Fish poplLlation changes in the Sanyati Basin, Lake Kariba, Zimbabwe D.H.S. Kenmuir Lake Kariba Fisheries Research Institute, Department of National Parks and Wildlife Management, Zimbabwe Large-fish population changes in the Sanyati Basin of Lake Kariba This paper considers the major natural large-fish population between 1960 and 1975 are evaluated and discussed. The early changes in the Sanyati Basin of Lake Kariba (Figure 1) between lake population following closure of the dam wall in 1958 was similar to the pre-impoundment riverine population with Labeo 1960 and 1975. With the exception of Jackson (1960) who dealt spp., Distichodus spp., Clarias gariepinus and two characid with the immediate effect of darn closure on the river fish species dominating gill-net catches. Exceptions were mormyrids, population, nothing empirical has yet been published on this scarce in the new lake although abundant in the river, and Oreochromis mortimeri, scarce in the river but expanding rapidly aspect of the biology of Lake Kariba fish, despite the lake be­ in the lake. Productivity in the new lake in terms of ichthyomass ing in existence now for 25 years_ The subject has, however, relative to later years was high. In later years following closure been discussed partially and in general terms by van der Lingen several of the early abundant mostly potamodromous species declined rapidly (C. gariepinus, Labeo spp., Distichodus spp.,) and (1973), Bowmaker, Jackson & Jubb (1978) and Kenmuir (1978, by 1975 they were unimportant. Mormyrids, cichlids and two 1983), while unpublished data on some aspects are those of silurid species increased significantly in catches as did the Donnelly (1971), and the report ofKenmuir (1977), from which characin, Hydrocynus vittatus; the latter as a result of the expan­ sion of the freshwater sardine population, from 1970. While early this paper is derived. populations were essentially herbivorous later populations were . largely insectivorous/molluscivorous. Concomitant with these ) 0 changes new species appeared, while a few disappeared. Possi­ 1 ble sources of origin of the new arrivals are discussed. Salient Zambia 0 2 features of the entire evolutionary process are summarized, while d some of the lessons leamt from Kariba, and a few recommenda­ e t tions arising from this, are listed in the conclusion. a d S. Afr. J. Zool. 1984, 19: 194 - 209 Kilometres ( 100102030 r + e Bevolkingsveranderinge van grootvis tussen 1960 en 1975 in die 10 0 10 20 .... h Sanyati-kom van die Karibameer word g9avalueer en bespreek. Miles ~?, s i "i l Die bevolking net nadat die meer tot stand gekom het na die .. b ." voltooiing van die wal in 1958, het ooreengestem met die Sinazongw u P rivierbevolking voor opdamming, en Labeo spp., Distichodus spp., CJ,. ~ e Clarias gariepinus en twee spesies van die Characidae het die "'", h kiefnetvangste oorheers. Uitsonderings was die Mormyridae wat t "'~ y skaars was in die nuutgevormde meer maar volop in die rivier, en b Oreochromis mortimeri wat skaars was in die rivier maar wat in d die meer vinnig in getalle toegeneem het. Produktiwiteit in die e t nuwe meer, gemeet in vismassa, was hoog relatief tot latere jare. n Met verloop van tyd het van die vro9a volop en meestal riviervis­ a r soorte in die meer vinnig skaars begin raak (c. gariepinus, Labeo g spp., Distichodus spp.,) en teen 1975 was hulle onbelangrik. Mor­ e c myridae, Cichlidae en twee spesies van die Siluridae het beteke­ Zimbabwe n e nisvol in vangste toegeneem asook Hydrocynus vittatus; laasge­ Inset: SANYATI BASIN c a: Township area i noemde as gevolg van die toenemende sardientjiebevolking vanaf l b: lakeside area r 1970. Waar die vroee bevolkings hoofsaaklik plantvretend was, c: Naodza e was lateres hoofsaaklik inseklskulpvretend. Gepaardgaande met d: Sanyatl West d n hierdie veranderings het nuwe spesies bygekom, terwyl 'n paar u Figure 1 Map of Lake Kariba showing the Sanyati 8asin (85) in relation verdwyn het. Moontlike plekke van oorsprong van die nuwe aan­ y komelinge word bespreek. Die hoof kenmerke van die verande­ to the rest of the lake. Basins 1 and 2 are essentially riverine, whereas Basins a rings word opgesom terwyl sommige van die lesse wat by Kariba 3 to 5 are lacustrine and probably showed the same fish population change w e geleer is en 'n paar aanbevelings wat hieruit voortvloei, in die t characteristics. Inset is 85 enlarged to show netting areas. a gevolgtrekkings gegee word. G S.-Afr. Tydskr. Dierlc. 1984, 19: 194-209 t e n i The period under discussion encompasses the lake's fonn­ b darn-wall a ative years after closure in December 1958 and leads S to the beginning of the period of intensive sardine fishing y b operations in the Sanyati Basin in 1976, when the first thou­ d D.H.S. Kenmuir e sand tons of fish were netted_ Subsequently nearly 30 000 tons c Present address: 7 The Fold, Northwood, Mount Pleasant, Harare, u Zimbabwe were caught between 1976 and 1981, of which some 800/0 were d o taken from the Sanyati Basin. Prior to this, slightly over 1 000 r p Received 13 October 1983; accepted 1 February 1984 tons were netted in exploratory commercial fishing between e R s. Afr. J. Zool. 1984, 19(3) 195 1973 and 1975 following the introduction of sardines in 19671 1968. The annual removal of large quantities of fIsh nutrients from one area of the lake must have an effect, directly or indirectly, on other consumer organisms in the aquatic environment. Con­ sidering that nutrients are already lost to the lake fairly rapidly because of the high outflow to volume ratio an additional and ..J en imposed loss must affect the nutrient budget, and will ultimate­ ly have ecological consequences of one sort or another. Thus prior to the recent high sardine catches it was suggested that a change in nutrient flow caused by the development of the freshwater sardine population could be a contributory cause for the decline of Salvinia and sudd mats in the lake (Ken­ muir 1978, p.44), a subject elaborated on more recently by Marshall & Junor (1981). Submerged aquatics might simiIarly be detrimentally affected if the phytoplankton ~ zooplankton l' 1 2 1 '73 1174 '815 ~ sardine ~ fIshermen chain monopolizes and removes open water nutrients circulating after turnover before these become Figure 2 Lake level fluctuations from 1964 to 1975. The lake filled in available to the littoral zones through wind and current ac­ 1963 and then dropped several metres in 1964 before beginning the annual tion. This in turn could affect other populations, such as those regime of fluctuating lake level. of marginal fIsh and freshwater mussels. Thus cognizance should be taken of Balon's warning (1974a) that the sardines (Kenmuir 1980a,b, 1981). Several of these events are sum­ are liable to attain their high density at the price of other taxa. marized and discussed by van der Lingen (1973), Bowmaker Beadle (1974) also recognized the ecological ramifIcations of (1975) and recently more comprehensively by Kenmuir (1978, intensive commercial fIshing. 1983). The year 1975 can be regarded as the end of the period when Human activities with a bearing on the biology of the lake observed fIsh populations reflected the outcome of mainly have involved cichlid stocking from 1959 to 1961 (parnell 1960, natural evolutionary processes, probably not greatly affected 1961); commercial gill-netting in the littoral areas from 1960 by the widely dispersed marginal multi-species gill-net fIShery, and 1962 on the north and south banks respectively (Harding and 1976 as the beginning of a period of intensive mono-species . 1966; Minshulll973); introduction of freshwater sardines (Lim­ ) 0 commercial fIshing, when further evolutionary and largely nothrissa miodon Boulenger) in 1967/1968 (Balon 1971a; Bell­ 1 0 man-induced trends were probably put into motion. Cross & Bell-Cross 1971; Junor & Begg 1971); introduction 2 This paper deals with the fIrst period and presents the fISh & d of a grasshopper to feed on Satvinia (Mitchell Rose 1979); e t population changes that have been determined from experi­ and the development of a commercial sardine fIshery a d mental and, to a lesser extent, from commercial gill-netting, (Cochrane 1976,1978; Kenmuir 1978,1981, 1982b; Langerman ( r prior to the advent of the sardine fIShing industry. It serves 1979; Marshall & Langerman 1979; Marshall & Junor 1981; e h as it baseline against which further population changes may Marshall et al. 1982). s i l be measured. Data on the composition of subsequent fISh b u populations are being collected by the Kariba Fisheries The river and the fish population before impoundment P Research Institute. e Jackson (1961) described the Zambezi in the area to be flooded h t as a sandbank river, rising to great heights during and shortly y Brief history of the lake b after the rains and then dwindling in size as the dry season d The lake started filling in December 1958 and reached peak advanced to a fraction of its former size between alluvial river e t level in September 1963. This phase was marked by an increase banks. As a result aquatic vegetation of any type was scarce. n a of total dissolved solids from 26 ppm in the old river to a r Fish spawning took place in adjacent flooded areas when the g recorded 65 ppm in the new lake (Harding 1966) and repre­ river was high, but once the river receded the population was e c sented one of extremely high productivity characterized by an forced back into the river proper, to lead a competitive and n e c explosive growth of organisms, including plankton blooms, precarious existence in ecologically unfavourable conditions.