by
C. Machena
Officer-in-Charge
Lake Kariba Fisheries Research Institute
P.O. Box 75, Kariba, Zimbabwe
| ABSTRACT |
| This review paper looks at research and fisheries development activities between 1984 and the present. Research is now focusing on resolving problems related to fisheries management. The pelagic fishery between Zambia and Zimbabwe lands 30 000 t annually. This is sufficient justification to expend a considerable amount of resources to manage the fishery efficiently. This is being done though the joint Zambia/Zimbabwe SADC Fisheries Project, and a stock assessment group has been set up for this purpose. |
| Management of the inshore areas is also being re-organized through giving resource rights to the fishing communities, to give them a better incentive to exploit the resource sustainably. Socio-economic studies being carried out are supposed to provide the background for this. |
| At the same time, basic research focusing on an increased understanding of the ecosystem has not been ignored. It is becoming increasingly clear that the ecosystem is beginning to stabilize and changes in populations are becoming predictable. |
At the time the dam was built across the Zambezi River in 1958, Kariba was the biggest artificial lake in the world. The lake attracted international attention following the resettling of about 50 000 people who were ecologically dependent on the flood regime of the river, the massive game rescue exercise (Operation Noah) and the flooding of a fragile ecosystem. Sceptics wondered whether the exercise was worthwhile, and therefore followed with keen interest the early development of the lake (Balon, 1978).
The Lake Kariba Fisheries Research Institute was established in 1963 as a joint project between Zambia and Zimbabwe, with funding from UNDP and technical assistance from FAO. The overall mandate given to the Institute covered three areas of responsibility: research, management and extension in the context of fostering fisheries development within the lake and its environs. The Institute initiated a series of studies and these are continuing. Lake Kariba is probably one of the most studied lakes on the African continent. These studies have covered hydrology, limnology, aquatic botany, fish systematics, fish biology and ecology, stock assessment, socio-economic aspects, etc.
Marshall (1984) gives an account of pre-impoundment studies and a review of the development of the lake up to 1984. The present paper continues from where Marshall left off, reviewing studies carried out between 1984 and the present. There has been no attempt to repeat Marshall's work, but, for the sake of continuity in some sections, brief reference has been made to his review.
Earlier studies put emphasis on limnology and fish biology. This trend has now changed. Increasingly it has been realized that the exploitation of the inshore fisheries resource cannot be separated from the complex socio-economic environment of the fishing communities. Governments have also incurred a lot of costs in policing and enforcing management regulations. The focus is now on providing to the fishing communities the resource rights and the appropriate authority to exploit and manage the fishery resource. Besides fishing, the lake and its environs also accommodate a lot of other activities, including lake-based transport, power generation, wildlife utilization, tourism and its hotel infrastructure, urban development, irrigation, etc. Current studies are now focusing on the development of a master plan for the area, to form the basis for integrated development and management. This will incorporate a park plan for the lake that will aim to reduce conflict between fishing and other lake-based activities.
With respect to the pelagic fishery, emphasis is on defining the appropriate level of fishing mortality. It is becoming apparent that an economic break-even point will be reached in fishing effort before biological overfishing takes place. The optimal fishing effort will likely be determined by economic factors (Anon., 1992).
Since the lake is managed separately between the two countries, the Zambia/Zimbabwe SADC Fisheries Project has been set up to coordinate research and development activities on the lake.
From a fisheries point of view, the most important papers on the various aspects of the Kariba ecosystem are listed below.
Physico-chemistry, morphology and hydrology:
Allison (1969); Begg (1969, 1970, 1974a); Bowmaker (1973b, 1976); Caulton (1970); Coche (1968, 1969, 1974); McLachlan and McLachlan (1971); Mitchell (1970, 1973); Petersen et al. (1987); Robarts and Southall (1977); and Ward (1978, 1979).
Plankton and benthos:
Begg (1974b, 1976); Bowmaker (1973b); Kenmuir (1980); Lake Kariba Report No35, 36; Masundire (1991, 1992); Moyo (1991); McLachlan (1969, 1970a, 1970b); McLachlan & McLachlan (1971); Magadza (1980); Mills (1973); Mitchell (1975); and Ramberg (1987).
Macrophytes:
Bowmaker (1973a, 1973b); Machena (1987, 1989); Machena and Kautsky (1988); Marshall and Junor (1981); Mitchell (1969, 1970); Mitchell and Rose (1979); and Thomas (1974).
Fish systematics and distribution:
Balon (1974a, 1974b, 1975); Begg (1974a); Bell-Cross (1972, 1976); Bowmaker, Jackson and Jubb (1978); Jackson (1961); Jubb (1961, 1967, 1976a, 1976b); Kenmuir (1975b); Mitchell (1976, 1978); and Sanyanga (1990).
Fish biology and ecology:
Balon (1971, 1974b); Chifamba (1990); Cochrane (1978); Coke (1968); Donnelly (1969); Hustler and Marshall (1990); Jackson (1960); Joubert (1975); Karenge (1992); Kenmuir (1973b, 1975a); Lake Kariba Report No 17; Marshall (1982, 1988a, 1991); Mitchell (1976, 1978); and Ramberg et al. (1987).
Population dynamics, production, biomass and commercial statistics:
Anon. (1992); Balon (1973, 1974a); Bazigos, Grant and Williams (1975); Chifamba (1991); Cochrane (1978); Junor (1981); Karenge (1992); Kenmuir (1973a, 1984); Kolding, Tirasin and Karenge (1992); Lake Kariba Report No 17, 38, 40; Langerman (1984); Lindem (1988,1992); Machena, Kolding and Sanyanga (in press); Mahon and Balon (1977); Marshall (1979, 1985, 1987, 1988b, 1988c); Marshall and Langerman (1988); Minshull (1973); Mitchell (1976, 1978); Mudenda (1989); Murphree et al. (1989); Thorsteinsson, Sanyanga and Lupikisha (1991); and Walter (1988).
Socio-economic aspects:
Balon (1978); Colson (1960, 1971); Hutton (Pvt) Ltd (1991); Machena (1985, 1986); Magadza (1986); Murphree et al. (1989); Scudder (1962, 1972, 1975, 1980); Skehel (1969); Walter (1988); and Webster (1975).
Recent studies on the phytoplankton communities in Lake Kariba (Cronberg, in prep.) revealed that the recorded mean biomass for the whole lake was relatively low, between 0.5 and 1.5 mg/l (wet weight), although locally high values of 7.8 mg/l have been found. The phytoplankton communities have now developed predictable seasonal and annual variations in species composition and abundance.
In comparison to the early stages of lake development, there have been a number of changes in species composition. Thus the early communities were characterized by large-sized species, and the number of species per group have changed over time. A list of the common phytoplankton species in Lake Kariba in 1986–1989 is given in Table 1.
Table 1 Common phytoplankton species in Lake Kariba, 1986–1989.
| CYANOPHYCEAE | DIATOMOPHYCEAE |
| Anabaena “maxima” | Cyclotella spp. |
| Anabaena “nygaardii” | Melosira granulata |
| Anabaena sp 3 | M. granulata var. angustissima |
| Aphanizomenon recta | M. tenella |
| Aphanothece clathrata | Synedra spp. |
| Cylindorspermopsis spp. | |
| Gomphosphaeria sp. | |
| Microcystis aeruginosa | CRYPTOPHYCEAE |
| Lyngbya limnetica | |
| Oscillatoria spp. | Cryptomonas spp. |
| CHLOROPHYCEAE | DINOPHYCEAE |
| Actinastrum hantzschii | Ceratium furcoides |
| Ankyra judayi | Gymnodinium sp. |
| Chlamydomonas sp. | Peridiniopsis elpatiewskyi |
| Crucigenia tetrapedia | P. pygmaeum |
| Eutretramorus sp. | Wolozynskia sp. |
| Koliella elongata | |
| Monoraphidium contortum | |
| CHRYSOPHYCEAE | TRIBOPHYCEAE |
| Synura australiense | Gonyostomum sp. |
| Merotrichi sp. | |
| HAPTOPHYCEAE | SMALL MONADS |
| Chrysochromulina sp. |
(Source: Cronberg, in prep.)
Dominant algae species in Lake Kariba are Gleotrichi sp., Oscillatoria and Lyngbya spp. The important substrata for the attached algae are aquatic macrophytes and dead trees. Amongst the macrophytes, preferred substrata are Lagarosiphon and Najas, while some species seem to be avoided (Ramberg et al., 1987). The attached algae had a mean biomass (d.w.) of 60 g/m2 in the 0–5 m depth zone, where they were evenly distributed and contributed about 30% of macrophyte biomass (Figure 1) (Ramberg et al., 1987).
| Biomass (dry weight) per bottom surface area at the low water level situation: in the inundation zone, aquatic macrophytes and algae on macrophytes in the littoral zone, and algae on drowned trees |
| (Source: Ramberg et al., 1987). |
 |
Figure 1 Biomass per bottom surface area at low water level
There are seven species of submerged macrophyte in Lake Kariba. Species diversity is low compared to, for example, Lake Memphremogag (Canada), with sixteen species (Chambers, 1987). The hydrolittoral zone is subjected to annual fluctuations in lake level, creating the physically unstable habitat which is the major reason for the low species diversity (Machena, 1989). Successful plant colonizers in unstable habitats need broad tolerance limits to cope with rapidly changing environments and rapid dispersal mechanisms to colonize new bottoms as the environment changes.
The submerged macrophytes in Lake Kariba have a biomass of about 101 000 t (dry weight) (Machena and Kautsky, 1988; Machena, 1989). The biomass is composed of Lagarosiphon ilicifolius (52%), Najas pectinata (33%), Vallisneria aethiopica (11%), Ceratophyllum demersum (3%) and Potamogeton octandrus (0.3%).
TWINSPAN classification (Hill, 1979) led to 27 groupings (Figure 2) of monospecific and mixed communities of the five recorded species (Machena, 1987). Lagarosiphon formed more mixed groupings than the other species, and in fact mixed with all the other species. Vallisneria formed the most monospecific groups, while Potamogeton mixed with neither Ceratophyllum nor Najas.
 | |
| TWINSPAN dendrogram of 279 samples from 18 diving transects along the Zimbabwe shore of Lake Kariba. Similar samples are joined at a low level in the dendrogram. Dissimilar samples are not joined until higher levels. Each species served as an indicator species and the respective levels are shown | |
| (Source: Machena, 1987). | |
| Key: | Vall = Vallisneria aethiopica; Pot = Potamogeton octandrus; Lag = Lagarosiphon ilicifolius; Cer = Ceratophyllum demersum; Naj = Najas pectinata. 3 = >75% cover; 2 = 25 to 75% cover; 1 = <25% cover. |
Figure 2 TWINSPAN dendrogram of macrophyte associations along the Zimbabwe shore of Lake Kariba
Ordination with Detrended Canonical Correspondence Analysis (DCCA) (ter Braak, 1986; 1987) revealed zonation along a depth gradient, with Potamogeton and Vallisneria occurring in shallow water and Najas and Ceratophyllum occurring in deep water. Lagarosiphon occupied an intermediate position (Machena, 1987).
According to Machena (1989), areas colonized by one species have a wide depth distribution of that particular species, whereas areas with several species have restricted depth distributions of the respective species. This indicates inter-specific competition, which is another important factor in structuring communities.
A number of environmental factors control the biomass, plant growth form and adaptive strategies of the macrophyte species. DCCA ordination showed that depth and transparency explained 66.5% of the variation in macrophyte distribution and slope accounted for 20% (Machena, 1987).
The various types of shores in Lake Kariba have been colonized by plants of different growth forms and different strategies. Exposed and rocky shores are not colonized by plants. The short-stemmed and rhizoid Vallisneria and the rhizoid and erect Potamogeton occupy shore areas with fine to sandy sediments. Lagarosiphon is erect and occupies areas where sediments are rich in nutrients, with high light levels and low disturbance. Lagarosiphon is fast growing, canopy forming and competes effectively. Najas, though also rooted and erect, is out-competed by Lagarosiphon and pushed to greater depths.
Studies on growth and production have only been carried out on Lagarosiphon ilicifolius. These results have been extrapolated to cover the other macrophyte species. The mean net production rate from measurement of individual Lagarosiphon shoots was 7.5 mg/g (d.w.)/day, calculated for the Lagarosiphon community using plexiglass enclosures. Community metabolism is dominated by sediment respiration, which was as high as 42%, indicating fairly intensive decomposition processes. Decomposition is important in the re-mineralization of nutrients locked up in dead organic matter.
Macrophyte communities play an important role in providing habitat complexity, food to aquatic animals, in nutrient recycling, etc. In Lake Kariba, there is evidence that Lagarosiphon translocates phosphorus and ammonium from the sediments to littoral water (Machena, 1989).
Tilapia rendalli, a herbivore in Lake Kariba, has shown a preference for Vallisneria aethiopica, both in the wild and in a controlled environment (Figures 3A and 3B). Vallisneria has a high protein and ash content in comparison to other macrophytes (Table 2) (Chifamba, 1990).
As Tilapia rendalli positively selects for Vallisneria, its biomass may be limited by the available biomass of Vallisneria. Tilapia rendalli had an Index of Relative Importance of only 1.1% compared to Serranochromis codringtoni with 19.1% of the total number of species caught in experimental gill nets in Lake Kariba (Karenge, 1992).
The current distribution of the macrophyte species is not related to the order of their colonization (Machena, 1990), but rather to ecological factors. The macrophyte communities probably reached their current distribution in the early 1970s. The submerged macrophyte vegetation might well change further, especially if new species invade.
 |  |
Figure 3-A Proportions of four species of macrophytes consumed by Tilapia rendalli in the selection trial | Figure 3-B Percentage occurrence of food items in the stomachs of Tilapia rendalli collected from the wild |
Table 2 Percentage water, organic matter, ash and protein contents of some submerged macrophytes of Lake Kariba
| Species and plant part | Water (%) | Organic Matter (%) | Ash (%) | Protein (% d.w.) |
| Lagarosiphon ilicifolius | ||||
| Leaves | 91.7 | 7.4 | 0.9 | 8.6 |
| Whole | 91.8 | 7.4 | 0.7 | 10.6 |
| Ceratophyllum demersum | ||||
| Leaves | 95.3 | 4.1 | 0.6 | 13.0 |
| Whole | 93.2 | 6.2 | 0.6 | 8.6 |
| Vallisneria aethiopica | ||||
| Leaves | 93.7 | 5.4 | 0.8 | 15.1 |
| Najas pectinata | ||||
| Leaves | 94.8 | 4.5 | 0.8 | 12.5 |
| Whole | 94.5 | 5.2 | 0.4 | 9.8 |
(Source: Chifamba, 1990)
Marshall (1984) cited morphology, and hydrology, temperature and oxygen regimes as well as water chemistry, as the factors that influence fish production in Lake Kariba. Lake Kariba is a slightly alkaline and oligotrophic lake with low potential fish production. Both nitrogen and phosphorus are limiting. Some of the reasons for this are given by Marshall (1984). Temperature and oxygen cycles indirectly influence fish production through their effect on nutrient cycles and also directly affect fish distribution and production. The morphology of the lake also determines the relative availability of both shallow and steep shores, and hence the amount and type of vegetation available.
The effect of morphology has been further analyzed by Sanyanga (1990) and Machena, Sanyanga and Kautsky (1991). They identified the relationship between fish species composition and species abundance, with three major environmental variables - depth, slope and transparency - as well as four nominal variables: presence or absence of fishing, and presence or absence of vegetation.
Analyses with DCCA revealed that depth plays a major role in structuring fish communities in Lake Kariba. The bulk of the species, except Synodontis zambezensis, S. nebulosus, Malapterurus electricus, Oreochromis macrochir, and Limnothrissa miodon, is found in shallow water, as observed by Coke (1968). Figure 4 shows the percentage distribution of fish families by depth. Cichlids comprised 69% of total fish population in the 0–3 m depth zone, but only 6% in the 12–20 m depth zone.
| A. Fish distribution at Fothergill |
 |
| B. Fish distribution at Gache Gache |
 |
Figure 4 Percentage distribution of fish by families within the four depth zones in the Fothergill unfished area (A) and the Gache fished area (B) (Source: Sanyanga, 1990)
Lake Kariba is largely steeply sloping, which limits the abundance and distribution of submerged macrophytes and benthic fauna, and therefore the inshore fish production.
Mean lengths of several fish species varied significantly between fished and unfished areas, with some fish species in fished areas having reduced mean lengths (Sanyanga, 1990). Length frequency distributions of the commercial species Serranochromis codringtoni and Hydrocynus vittatus showed a marked absence of smaller and larger length classes in the fished areas, while S. zambezensis was more abundant in the fished areas (Sanyanga, 1990).
These differences between fished and unfished areas must be attributable to fishing pressure. There is selective cropping of fairly large fish as the commercially allowed minimum gill net mesh is 100 mm.
Berg, Klibus and Kautsky (1992) found evidence of bio-accumulation of organochlorine pesticides - mainly DDT and its metabolites - in the fish of Lake Kariba. The herbivorous Tilapia rendalli had 1 900 μg sDDT/g fat, whilst the predatory tigerfish, Hydrocynus vittatus, at the top of the food chain, had 5 000 μg sDDT/g fat. However, even higher levels were found in bottom-living species. Thus a mussel (Corbicular africana) had 10 100 μg sDDT/g fat.
The incidence of DDT was found to be localized in some territorial fish, but not in the highly mobile tigerfish, indicating point discharges of the insecticides. DDT has been used for the control of tsetse flies within the catchment area of the lake.
The fish eagle, Halieetus vocifer, has been found to have a high content of DDT in its eggs, with reduced egg shell thickness (Matthiessen, Kunene and Hutson, 1984).
Hustler (1991) found that the reed cormorant, Phalacrocorax africanus, and the darter, Anhinga melanogaster, are the most important fish-eating birds on Lake Kariba. Together they take equivalent to 12 to 16% of the commercial inshore fisheries. The reed cormorants and the darters take in 20% and 11% of their body weights respectively, and their prey is to a large extent made up of small-bodied species.
The impact of these piscivorous birds varied with the type of lake shore (as they preferred gently sloping areas) and with the season (the birds were numerous between August and January).
Ericksson and Kautsky (1992) also observed that the African openbilled stork (Anastomus lamelligerus) fed on molluscs in the lake. The stork density was highest during the period of low water level (August to December), when mussels were stranded in shallow water. During this period the storks were distributed in relation to the sites where Mutela dubia, the most preferred mussel, was abundant.
In a study of the ecology of the Nile crocodile (Crocodylus niloticus), Games (1990) concluded that the crocodile population consumed about 225 t of fish per year, amounting to about 10% of the yield of the artisanal inshore fishery.
Table 3 Number of fish netted per fleet setting from the Kariba Station in Lake Kariba
| SPECIES | 1961 | 1962 | 1963 | 1964 | 1972 | 1973 | 1974 | 1975 |
| Hippopotamyrus discorhynchus | .. | .. | .. | 0.03 | 8.23* | 4.57 | 8.40 | 5.58 |
| Marcusenius macrolepidotus | .. | .. | .. | .. | 1.12 | 2.23 | 0.95 | 0.58 |
| Mormyrus longirostris | 0.19 | 0.01 | 0.01 | .. | 1.41* | 3.19 | 9.45 | 7.00 |
| Mormyrops deliciosus | 0.01 | .. | .. | .. | .. | 0.15 | 0.68 | .. |
| Alestes imberi | 14.01* | 13.45 | 6.90 | 1.55 | 7.88* | 3.07 | 3.13 | 18.47 |
| Hydrocynus vittatus | 29.35* | 25.76 | 20.45 | 14.84 | 55.53* | 27.38 | 44.59 | 39.73 |
| Distichodus schenga | 3.99* | 1.70 | 0.51 | 0.21 | 0.18 | .. | 0.22 | 0.10 |
| D. mossambicus | 2.80* | 1.15 | 0.36 | 0.09 | .. | .. | .. | .. |
| Labeo altivelis | 12.79* | 9.61 | 3.50 | 0.78 | 0.18 | .. | 0.22 | 1.05 |
| L. congoro | 29.45* | 12.25 | 4.47 | 0.92 | .. | .. | .. | 0.05 |
| Oreochromis mortimeri | 12.08* | 4.89 | 1.98 | 0.48 | 2.35* | 5.61 | 10.22 | 13.98 |
| O. macrochir | 0.31 | 0.14 | 0.02 | .. | .. | .. | .. | 0.63 |
| Tilapia rendalli | 1.15 | 0.31 | 0.10 | .. | 0.47 | 0.38 | 1.22 | .. |
| Serranochromis codringtoni | 0.16 | 0.07 | 0.05 | 0.01 | 6.18* | 4.57 | 11.72 | 7.68 |
| Synodontis zambezensis | .. | 0.12 | .. | .. | 1.59* | 2.03 | 10.27 | 4.05 |
| Eutropius depressirostris | 0.15 | 0.07 | 1.05 | 0.12 | 1.53* | 1.73 | 3.68 | 3.26 |
| Clarias gariepinus | 8.02* | 4.37 | 2.63 | 0.26 | 0.47 | 0.11 | 2.77 | 0.63 |
| Heterobranchus longifilis | .. | .. | .. | .. | 0.06 | 0.11 | 0.36 | 0.21 |
| Number of settings | 80 | 83 | 94 | 89 | 17 | 26 | 22 | 19 |
(Source: Kenmuir, 1984)
Note: * = 8 most common species
Table 4 Number of fish netted per fleet setting from the Lakeside Station in Lake Kariba
| SPECIES | 1960 | 1961 | 1963 | 1964 | 1967/68 | 1972 | 1973 | 1974 | 1975 |
| H. discorhynchus | .. | .. | .. | .. | 0.26 | 18.06* | 18.44 | 10.76 | 3.19 |
| M. macrolepidotus | .. | .. | .. | .. | .. | 6.21 | 5.59 | 5.15 | 1.30 |
| M. longirostris | .. | .. | 0.09 | .. | 0.08 | 1.16 | 1.62 | 4.53 | 8.42 |
| M. deliciosus | .. | .. | .. | .. | 0.63 | 0.05 | 0.33 | 0.34 | 0.11 |
| A. imberi | 63* | 44 | 19.36 | 24.66 | 3.59 | 0.84 | 0.33 | 4.11 | 20.49 |
| H. vittatus | 58* | 40 | 55.45 | 65.16 | 41.16 | 101.42* | 62.29 | 139.73 | 84.30 |
| D. schenga | 3* | 4 | 2.18 | 0.91 | 1.49 | 0.10 | 0.07 | 0.26 | 0.04 |
| D. mossambicus | 5* | 9 | 3.55 | 1.25 | 0.77 | .. | .. | .. | .. |
| L. altivelis | 216* | 74 | 29.64 | 16.41 | 9.54 | 0.10 | .. | 0.03 | 0.42 |
| L. congoro | 17* | 18 | 12.27 | 11.25 | 3.57 | .. | .. | .. | .. |
| O. mortimeri | 38* | 1 | 21.00 | 8.16 | 2.78 | 15.32* | 13.40 | 50.84 | 40.07 |
| O. macrochir | .. | .. | .. | .. | .. | .. | .. | 0.03 | 0.23 |
| T. rendalli | .. | 3 | 0.45 | 0.91 | 0.11 | 1.37* | 1.11 | 4.38 | 4.34 |
| S. (Sa) codringtoni | .. | 1 | 1.00 | 0.41 | 0.18 | 15.37* | 23.03 | 39.15 | 47.73 |
| S. zambezensis | .. | .. | .. | .. | 0.52 | 0.79 | 1.92 | 3.11 | 1.69 |
| E. depressirostris | .. | .. | 0.91 | 2.91 | 0.36 | 11.68* | 6.03 | 12.11 | 3.81 |
| C. gariepinus | 12* | 29 | 16.55 | 13.33 | 0.90 | 0.05 | 1.85 | 10.96 | 6.46 |
| H. longifilis | .. | .. | .. | .. | .. | .. | 0.03 | 0.07 | 0.19 |
| Number of settings | 1 | 1 | 11 | 12 | 61 | 19 | 27 | 26 | 23 |
(Source: Kenmuir, 1984)
Note: * = 8 most common species
Kenmuir (1984) analyzed experimental gill netting data collected by the Lake Kariba Fisheries Research Institute (LKFRI) at two stations in the Sanyati Basin (close to Kariba Town) between 1960 and 1975 (Tables 3 and 4), and Karenge (1992) analysed data collected at one of those stations between 1969 and 1991.
After closure of the dam in 1958 the fish population was similar to the pre-impoundment riverine population, with Labeo spp., Distichodus spp., Clarias gariepinus and two characid species dominating gill net catches while the fish biomass increased.
In the early 1960s some of the early abundant species declined rapidly, e.g., C. gariepinus, Labeo spp. and Distichodus spp. At the same time, mormyrids (e.g., Mormyrus longirostris), cichlids (e.g., Serranochromis codringtoni, S. macrocephalus), and silurids (e.g., Synodontis zambezensis) increased significantly in the catches. Hydrocynus vittatus increased significantly following the introduction and establishment of Limnothrissa miodon.
Using the Shannon Index, Karenge (1992) observed an increased diversity at the Lakeside station between 1969 and 1991 (Figure 5). Fish diversity and abundance followed increased abundance of macrophytes and benthic fauna, which was, for example, the case with Serranochromis macrocephalus (Karenge, 1992).
Changes in fish population are still taking place (Sanyanga, 1990). The catfish Synodontis zambezensis now seems to be the most abundant fish in the lake in terms of catch per unit effort. Synodontis is abundant in fished areas and occurs deeper than the other species. Increases in the population of benthic species is a common trend in the biological development of reservoirs. Studies have been undertaken to develop appropriate gear to commercially exploit this species (Songore, 1992).
Figure 5 Increase in fish diversity (Shannon Index) at the Lakeside station, Lake Kariba, over the period 1969 to 1991
(Source: Karenge, 1992).
Marshall (1984) reported on the estimations of the biomass of inshore species by Balon (1973, 1974) and Mitchell (1976) which sampled selected areas by poisoning blocked-off coves (Table 5). Langerman (1984) sampled on different types of shores with an explosive sampling grid and results are reported in Marshall and Langerman (1988) (Tables 6 and 7). The biomass estimations in Marshall and Langerman (1988) are similar to those obtained from poisoning.
Estimates of the biomass of the pelagic Limnothrissa have been carried out by Lindem (1988, 1992) and Marshall (1985, 1988b). Marshall estimated biomass by fish capture, using a conical lift net without the aid of light. He obtained estimates ranging from 1 to 723 kg/ha, with an overall mean of 59.23 kg/ha. These estimates were made in the eastern part of the lake and extrapolated for the whole lake.
The density of sardines in the lake has also been assessed using hydro-acoustics. Lindem (1988, 1992) carried out survey transects over the whole lake using a portable Simrad EY-M echo-sounder operating at 70 kHz. The survey gave biomass estimates ranging between 16 and 120 kg/ha, with a mean of 37 kg/ha. It is generally assumed that Limnothrissa has two major spawning periods per year in Kariba and develops from egg to adult in 5 to 6 months. The production potential of this fish is therefore high.
Hydro-acoustic studies are now being conducted on a regular basis under the auspices of the joint Zambia/Zimbabwe SADC Fisheries Project.
Table 5 Standing stock of major fish species (i.e., >1% of total sampling stock) in Lake Kariba
| SPECIES | Balon (1973) | Mitchell (1976) | Langerman (unpubl) | |||
| kg/ha | % | kg/ha | % | kg/ha | % | |
| Mormyrops deliciosus | 92 | 15.1 | 126 | 30.0 | ||
| Hippopotamyrus discorhyncus | 96 | 15.9 | 70 | 16.7 | ||
| Marcusenius macrolepidotus | 8 | 1.8 | ||||
| Mormyrus longirostris | 18 | 3.0 | 45 | 10.6 | ||
| Hydrocynus vittatus | 31 | 5.1 | 10 | 2.5 | 6.4 | 15.3 |
| Alestes lateralis | 29 | 4.8 | 6 | 1.4 | 13 | 3.1 |
| A. imberi | 13 | 3.0 | ||||
| Labeo altivelis | 6 | 1.0 | 53 | 12.6 | ||
| Eutropius depressirostris | 9 | 1.5 | ||||
| Clarias gariepinus | 52 | 8.6 | 13 | 3.2 | ||
| Heterobranchus longifilis | 14 | 2.3 | 7 | 1.7 | ||
| Malapterurus electricus | 48 | 8.0 | 10 | 2.3 | 5 | 1.3 |
| Serranochromis codringtoni | 13 | 2.2 | 12 | 2.9 | 33 | 8.0 |
| S. macrocephalus | 12 | 2.9 | ||||
| Pharyngochromis darlingi | 1.1 | |||||
| Oreochromis mortimeri | 97 | 16.0 | 22 | 5.2 | 25 | 6.1 |
| Tilapia rendalli | 56 | 9.3 | 54 | 12.7 | 190 | 45.3 |
| TOTAL | 580 | 96.0 | 409 | 97.2 | 408 | 97.6 |
| Sampling period | 1968–1971 | 1972–1974 | 1981–1982 | |||
(Sources: Marshall, 1984; Langerman, unpubl.)
Table 6 The inshore fish families (% by weight) in Lake Kariba Studies
| FAMILY | Balon (1974) | Mitchell (1976) | Marshall and Langerman (1988) |
| Anguillidae | - | - | - |
| Mormyridae | 34.30 | 59.15 | 0.95 |
| Characidae | 10.01 | 0.93 | 26.00 |
| Distichodontidae | 0.44 | 0.11 | - |
| Cyprinidae | 1.72 | 1.68 | 11.60 |
| Schilbeidae | 1.59 | 0.30 | - |
| Clariidae | 10.90 | 4.90 | 0.03 |
| Malapteruridae | 8.00 | 2.28 | 3.80 |
| Mochokidae | 3.50 | 5.12 | 4.77 |
| Cyprinodontidae | - | 0.01 | - |
| Cichlidae | 29.86 | 22.42 | 52.85 |
(Source: Marshall and Langerman, 1988)
Table 7 Inshore fish biomass (kg/ha) in relation to environmental factors
| Station | Biomass(1) | Substrate | Vegetation | Slope | Exposure |
| 1 | 650 | Sand/organic | Dense | Gentle | Sheltered |
| 2 | 415 | Sand/silt | Dense | Gentle | Sheltered |
| 3 | 340 | Sand/silt | Dense | Gentle | Sheltered |
| 4 | 320 | Sand | Dense | Gentle | Sheltered |
| 5 | 211 | Sand | Dense(2) | Gentle | Exposed |
| 6 | 145 | Sand/silt | Dense | Gentle | Exposed |
| 7 | 144 | Rocks | Some(2) | Gentle | Exposed |
| 8 | 112 | Rocks | None | Steep | Sheltered |
| 9 | 64 | Rocks | None | Steep | Sheltered |
| 10 | 33 | Rocks | Some | Steep | Sheltered |
| 11 | 31 | Rocks | None | Steep | Exposed |
| 12 | 272 | Rocks | Some | Steep | Exposed |
| 13 | 280 | Sand | None | Steep | Exposed |
(Source: Marshall and Langerman, 1988)
(1) Mean biomass for stations 1 to 6 = 346.8 kg/ha; mean biomass for stations 7 to 11 = 76.8 kg/ha.
(2) There was no vegetation from water's edge to 1 m depth because of wave action, but vegetation was present in water >1 m deep.
Table 8 Growth, maturity and mortality related parameters of 22 species of fish from Lake Kariba, with 95% confidence limits (in brackets) calculated when possible
| FAMILY and SPECIES | Lengt h type(a) | Lmax (cm) | L∞ (cm) | K (yr-1) | to (yr) | φ' | Lmat(50%) (cm)(b) | a(b) | b(b) | M (yr-1) | Z(c) (yr-1) | |
| Characidae | ||||||||||||
| Brycinus imberi | T | 17(b) | 14.8 | 1.404 | 0.589 | 2.488 | 12♂/12♀ | 0.036 | 2.79 | 2.32 | 1.91 | |
| Brycinus lateralis | T | 14 | 14.7(d) | - | - | - | - | - | - | - | 3.87 | |
| Hydrocynus forskahlii | S | 58(b) | 56.7(e) (±5.04) | 0.323 (±0.396) | -0.338 (±0.396) | 3.016 | 27♂/30♀ | 0.020 | 2.98 | 0.61 | 0.68 | |
| Micralestes acutidens | S | - | 7.2 | 0.627 | -0.489 | 1.512 | - | - | - | 1.67 | 2.48 | |
| Cichlidae | ||||||||||||
| Oreochromis mortimeri | T | 48 | 54.3(f) (±2.58) | 0.256 (±0.026) | 0.351 (±0.055) | 2.878 | 29♂/22♀ | 0.023 | 2.98 | 0.53 | 0.83 | |
| Pharyngochromis darlingi (g) | T | 10 | 15.7 | 0.660 | -0.293 | 2.211 | - | 0.008(h) | 3.01 | - | 5.28 | |
| Pseudocrenilabrus philander (g) | T | 8 | 8.4 | 1.620 | -0.137 | 2.058 | - | 0.008(h) | 3.03 | - | 8.28 | |
| Serranochromis codringtoni(b) | T | 39 | 37.3 (± 11.8) | 0.799 (±0.439) | -0.189 | 3.046 | 24♂/21♀ | 0.019 | 3.05 | - | 1.00 | |
| Tilapia rendalli | T | 38(b) | 48.5(f) (± 1.20) | 0.145 (±0.005) | -0.419 (±0.014) | 2.533 | 23♂/21♀ | 0.034 | 2.89 | 0.38 | 1.16 | |
| Clariidae | ||||||||||||
| Clarias gariepinus | T | 82 | 86.3(d) | - | - | - | 37♂/34♀ | 0.015 | 2.83 | - | 0.44 | |
| Heterobranchus longifilis | T | 115(b) | 121.1(d) | - | - | - | - | 0.003 | 3.22 | - | 0.59 | |
| Clupeidae | ||||||||||||
| Limnothrissa miodon (i) | T | 10 | 13.5 (±7.00) | 0.950 (±0.900) | -0.020 (±0.130) | 2.238 | - | 0.00001(h) | 2.86 | - | 4.6 | |
| Cyprinidae | ||||||||||||
| Labeo altivelis | T | 49 | 51.6(d) | - | - | - | 25♀/27♂ | 0.023 | 2.96 | - | 0.60 | |
| Malapteruridae | ||||||||||||
| Malapterurus electricus | S | 85 | 89.5(d) | - | - | - | - | - | - | - | 1.08 | |
| Mochokidae | ||||||||||||
| Synodontis nebulosus | S | 14 | 14.7(d) | - | - | - | - | - | - | - | 0.94 | |
| Synodontis zambezensis | T | 31 | 32.6(d) | - | - | - | 18♀/15♂ | 0.009 | 3.18 | - | 0.90 | |
| Mormyridae | ||||||||||||
| Hippopotamyrus discorhynchus | T | 32(b) | 34.9 (±0.15) | 0.158 (±0.001) | -0.634 (±0.003) | 2.284 | 13♀/11♂ | 0.057 | 2.53 | 0.44 | 1.87 | |
| Marcusenius macrolepidotus | T | 30(b) | 44.3(f) (±3.44) | 0.121 (±0.015) | -1.319 (±0.092) | 2.376 | 14♀/11♂ | 0.025 | 2.76 | 0.34 | 1.30 | |
| Mormyrops deliciosus | S | 100 | 136.8(f) (±14.35) | 0.078 (±0.012) | -1.116 (±0.097) | 3.164 | 46♀/44♂ | 0.015 | 2.87 | 0.19 | 0.53 | |
| Mormyrus longirostris | T | 80(b) | 70.8 (± 1.04) | 0.224 (±0.009) | -1.207 (±0.051) | 3.050 | 47♀/34♂ | 0.018 | 2.86 | 0.45 | 0.53 | |
| Schibeidae | ||||||||||||
| Schilbe depressirostris | T | 36(b) | 42.4(f) (±2.00) | 0.095 (±0.006) | -0.762 (±0.026) | 2.232 | 15♀/16♂ | 0.015 | 2.89 | 0.30 | 0.97 | |
| Schilbe mystus | S | 34 | 38.4(f) (±7.17) | 0.081 (±0.009) | -0.801 (±0.048) | 2.077 | - | - | - | 0.28 | 0.68 | |
Notes: (a) T = total length; S = Standard length;
(b) Data from Karenge (1992);
(c) Data from Mahon and Balon (1977);
(d) Programme did not converge; preliminary estimation of asymptotic length (Pauly, 1984);
(e) Unweighed data;
(f) Probably overestimated;
(g) Data from Hustler and Marshall (1990);
(h) Total length in mm;
(i) Data from Anon. (1992).
(Table in general based on: Kolding, Tirasin and Karenge, 1992)
(Source: Anon., 1992)
Figure 6 Mean length of all samples 1970–1991 (excluding experimental sampling of juveniles
(Source: Anon., 1992)
Figure 7 Growth curve of kapenta from otolith ring counts on juvenile and commercial size kapenta
