Lake Kariba Research Articles

Earthquakes in Zimbabwe

Earthquakes are one of the most destructive natural forces, in both human and economic terms. For example, since 1900, 10 earthquakes have occurred that each killed over 50 000 people. Earthquakes in modern industrialized areas can be also be very costly, even if well designed and constructed buildings save many lives. In Zimbabwe, beginning in February 1959 prompted by fresh seismic (earthquake) activity after the Kariba Dam was sealed in December 1958, seismograph stations began to be installed in the region. Seismographs detect ground vibrations. By 1970, the network consisted of 6 stations (including 1 each in Zambia and Malawi), was operated by the Zimbabwe Meteorological Services from the Goetz Observatory in Bulawayo. Almost all earthquakes in Zimbabwe occur in the Hwange-Zambezi-Kariba or Eastern Highlands areas. Sensitive engineering structures such as the Kariba Dam and power generating facilities around Hwange lie in these areas. However, the central part of Zimbabwe (including the two main cities, Harare and Bulawayo) has almost no earthquakes, only ~1% of the total. Fortunately, most earthquakes in Zimbabwe are quite small: of the ~3 400 earthquakes recorded in a 32 year period, only 30 exceeded a magnitude of 5 on the Richter Scale. The largest earthquake recorded here was in 1963, beneath the newly filled Lake Kariba, measuring 5.8 on the Richter Scale. The Zimbabwe Science News Volume 34 Number 3&4, December 2000, pp. 51-53

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

The fish population of the lower reaches of the Sanyati River was studied using multifilament gillnets and monthly sampling was conducted over 13 consecutive months. A total of 15 fish species belonging to seven families were collected and variations in monthly catches and catch per unit effort and condition were observed. The five most common species exhibited a seasonal breeding pattern, with most breeding occurring in the rainy season. The exotic Nile tilapia Oreochromis niloticus was found to have established itself in the Sanyati River. The results of this study were compared with those of previous workers on Lake Kariba.

Exotic species in large lakes of the world

Many of the large lakes of the world have been exposed to the introduction of exotic species. We have reviewed here the introduction of aquatic species in 18 large lakes on five continents (Laurentian Great Lakes, African Great Lakes, several Canadian lakes, Lake Titicaca, Lake Baikal, Lake Ladoga, Gatun Lake, and Lake Biwa). We found that human activities, social preferences, and policy decisions are often associated with the spread of species in these large lakes. However, the spread and resulting ecological effects of introduced species varied among the case studies reviewed (ranging from the failure of brown trout introduction in Lake Titicaca to successful introduction of Nile Perch in Lake Victoria). Those species that did establish successful populations often had major impacts upon the ecosystems of these lakes via a variety of processes, including predation, disturbance, habitat modification and competition. Although introduction of predators often negatively impacted native species (e.g. Nile perch in Lake Victoria, peacock bass in Lake Gatun), species introduced to lower trophic levels (e.g. sardine in Lakes Kariba and Kivu, rainbow smelt in Canadian Lakes) affected fisheries and altered food web structure as well. Exotic species in large lakes of the world were not limited to fish species: plants (e.g. in Lakes Baikal and Biwa), invertebrates (e.g. in Lake Ladoga), and parasites and pathogens (e.g. in Lake Titicaca) have been introduced, but it was often difficult to discern the food web and ecosystem effects of these organisms. Exotic species also impacted socio-economic systems, having both positive (e.g. Lakes Victoria, Titicaca, Kivu, and Kariba, and the Laurentian Great Lakes) and negative (e.g. Lakes Victoria and Titicaca, and the Laurentian Great Lakes) repercussions for humans who depended upon these lakes for food and income. Unfortunately, our understanding of the impacts and extent of introductions on large lake ecosystems often remains speculative at best. The introduction and spread of exotic species will continue to threaten large lakes of the world into the twenty-first century. Exotic species introductions are a global problem that deserves global attention and understanding.

Exotic species in large lakes of the world

AbstractMany of the large lakes of the world have been exposed to the introduction of exotic species. We have reviewed here the introduction of aquatic species in 18 large lakes on five continents (Laurentian Great Lakes, African Great Lakes, several Canadian lakes, Lake Titicaca, Lake Baikal, Lake Ladoga, Gatun Lake, and Lake Biwa). We found that human activities, social preferences, and policy decisions are often associated with the spread of species in these large lakes. However, the spread and resulting ecological effects of introduced species varied among the case studies reviewed (ranging from the failure of brown trout introduction in Lake Titicaca to successful introduction of Nile Perch in Lake Victoria). Those species that did establish successful populations often had major impacts upon the ecosystems of these lakes via a variety of processes, including predation, disturbance, habitat modification and competition. Although introduction of predators often negatively impacted native species (e.g. Nile perch in Lake Victoria, peacock bass in Lake Gatun), species introduced to lower trophic levels (e.g. sardine in Lakes Kariba and Kivu, rainbow smelt in Canadian Lakes) affected fisheries and altered food web structure as well. Exotic species in large lakes of the world were not limited to fish species: plants (e.g. in Lakes Baikal and Biwa), invertebrates (e.g. in Lake Ladoga), and parasites and pathogens (e.g. in Lake Titicaca) have been introduced, but it was often difficult to discern the food web and ecosystem effects of these organisms. Exotic species also impacted socio-economic systems, having both positive (e.g. Lakes Victoria, Titicaca, Kivu, and Kariba, and the Laurentian Great Lakes) and negative (e.g. Lakes Victoria and Titicaca, and the Laurentian Great Lakes) repercussions for humans who depended upon these lakes for food and income. Unfortunately, our understanding of the impacts and extent of introductions on large lake ecosystems often remains speculative at best. The introduction and spread of exotic species will continue to threaten large lakes of the world into the twenty-first century. Exotic species introductions are a global problem that deserves global attention and understanding.

The Diet of Five Cichlid Fish Species from Lake Kariba, Zimbabwe

The diet of five cichlid fish species from the littoral areas of Lake Kariba was investigated in 1996-97. Serranochromis macrocephalus, Pseudocrenilabrus philander and Pharyngochromis acuticeps fed mostly on calanoid copepods, chironomid larvae, rotifers, phytoplankton and fish fry. Oreochromis niloticus and 0. mortimeri fed mainly on detritus mixed with diatoms. There was no ontogenetic dietary shift in fish between 20-70 mm in length for any species. A dietary overlap existed between the omnivorous species S. macrocephalus, P. philander and P. acuticeps, and between the microphagous species 0. niloticus and 0. mortimeri. There was no difference in gape width between P. acuticeps and P. philander and competition between them may be reduced by their habitat preferences while S. macrocephalus only competes with them as juveniles since adults switch to a piscivorous diet. Direct competition between species 0. niloticus and 0. mortimeri may result in the exclusion of the latter. The diets of P. acuticeps and P. philander have changed since the 1970s reflecting the influences of Salvinia molesta that was widespread at that time Transactions of the Zimbabwe Scientific Association Vol. 74 2000: 16-21

Periphyton fouling as a marginal energy source in tropical tilapia cage farming

In aquaculture, the benefit of autotrophic production within land-based ponds for fish production has long been recognized. In cage culture, organisms growing on the cage net have so far only been considered as a problem. This study investigated the potential production of periphyton on cage nets used in a tropical mixed tilapia culture of Oreochromis mortimeri (Trewavas), Tilapia rendalli (Boulenger) and Oreochromis niloticus (Linnaeus) in Lake Kariba, Zimbabwe. The production of periphyton was assessed experimentally and compared with the energy demand of the caged fish. The tilapias were found to graze intensively on the net, and the primary production of periphyton on the cage net was ≈ 1% relative to the energy demands of the fish.

The food and feeding behaviour of Limnothrissa miodon (Boulenger, 1906) in Lake Kariba, Zimbabwe

The feeding biology of the Lake Tanganyika sardine, Limnothrissa miodon (Boulenger, 1906) was studied to assess the quality and quantity of food items taken. Fish samples were collected at two hour intervals during a series of twenty-four hour trawling surveys in the Sanyati basin of Lake Kariba. The diurnal and nocturnal consumption trends for each food item were determined. Fish stomach contents were first analysed under a dissecting microscope and subsequently on an inverted regular high power Nikon microscope. Food items were identified and quantified for frequency of occurrence and percentage composition. Limnothrissa miodon feeds mainly on the cladocerans Bosmina longirostris, Ceriodaphnia cornuta and Bosminopsis deitersi and the copepods Mesocyclops sp., Tropodiaptomus sp. and Thermodiaptomus sp. Aquatic insect larvae, rotifers and ostracods also occur infrequently in the diet. Several feeding peaks exist during a twenty-four hour cycle with an overall tendency to ingest food during the daytime. Limnothrissa miodon therefore mostly feeds on crustacean zooplankton and it is mainly a visual feeder that depends on availability of adequate light to locate its prey.

An interactive reservoir management system for Lake Kariba

This paper presents a user-interactive decision support system (DSS) for managing of the Lake Kariba reservoir. Built in the fourth-generation computer language IFPS, the system takes into account relevant reservoir characteristics and parameters, such as the amount of hydropower generated, reservoir storage throughout the year and the amount of water released for down-stream usage. The system blends water release rules determined previously using optimization and simulation-based scenario analyses with expert input from an experienced reservoir manager, yielding an intuitive and realistic DSS with which the reservoir manager may easily identify. The DSS also includes a Box-Jenkins time series model that forecasts future inflows. Each month, the system provides the manager with a proposed release schedule, which the manager then uses to explore and evaluate the consequences in terms of the decision criteria, over an extended period of time. The types of information provided to and sought from the manager correspond closely with actual reservoir management practice. An important characteristic of the system is that the manager can quickly explore various different potential release decisions a priori, for a variety of potential inflow scenarios, including predicted inflows for average hydrological years, as well as inflows reflecting extreme events such as drought and flood periods. The manager can compare the results of the release decisions made in the scenario analysis, both with the release strategy proposed by the system and with historical release decisions, thus aiding the manager in establishing effective reservoir management policies in practice. Therefore, rather than a mechanical value, our DSS offers the manager a flexible problem analysis with suggested courses of action. We illustrate the system using example sessions with an experienced reservoir manager. While the system is designed specifically to support the management of Lake Kariba, its extension to a more general class of reservoir management problems is straightforward.. 1782 1528 V 3

Effect of human immigration on the age distribution of schistosome infections in Siavonga community, Lake Kariba, Zambia

A study on the distribution of schistosomiasis in the community at Siavonga revealed Schistosoma haematobium infection in 35.5% of 338 subjects and a geometric mean egg count (GMEC) and (S.D.) of 13.7 (7.2) eggs/10 ml urine. The prevalence of S. mansoni infection among 323 subjects was 60.1%, with a GMEC of 336.8 (4.8) eggs/g stool. Among the infected, 69.5% carried both schistosome species. Although prevalence of infection with either species was highest in those aged 10-14 years, high prevalences of infection were found in older age groups and egg intensities were uniformly distributed throughout all age-groups. The observed diversion from the typically age-dependent distribution of schistosome infections probably reflects exposure to infection relatively late in life, as the result of immigration from non-endemic areas.

Effect of human immigration on the age distribution of schistosome infections in Siavonga community, Lake Kariba, Zambia.

A study on the distribution of schistosomiasis in the community at Siavonga revealed Schistosoma haematobium infection in 35.5% of 338 subjects and a geometric mean egg count (GMEC) and (S.D.) of 13.7 (7.2) eggs/10 ml urine. The prevalence of S. mansoni infection among 323 subjects was 60.1%, with a GMEC of 336.8 (4.8) eggs/g stool. Among the infected, 69.5% carried both schistosome species. Although prevalence of infection with either species was highest in those aged 10-14 years, high prevalences of infection were found in older age groups and egg intensities were uniformly distributed throughout all age-groups. The observed diversion from the typically age-dependent distribution of schistosome infections probably reflects exposure to infection relatively late in life, as the result of immigration from non-endemic areas.

Introgressive hybridization of tilapias in Zimbabwe

Three species of tilapia native to Zimbabwe, Oreochromis mossambicus, O. mortimeri and O. macrochir are not naturally sympatric, but because of transplantation they have been brought into sympatry. Allozymes of fish examined from Lakes Kariba, Kyle and Chivero showed evidence of introgressive hybridization. The data showed non-random mating populations in Lakes Kariba and Kyle. All three reservoirs displayed individuals with intermediate principal component scores and hybrid index scores indicating hybridization.

Introgressive hybridization of tilapias in Zimbabwe

Three species of tilapia native to Zimbabwe, Oreochromis mossambicus, O. mortimeri and O. macrochir are not naturally sympatric, but because of transplantation they have been brought into sympatry. Allozymes of fish examined from Lakes Kariba, Kyle and Chivero showed evidence of introgressive hybridization. The data showed non‐random mating populations in Lakes Kariba and Kyle. All three reservoirs displayed individuals with intermediate principal component scores and hybrid index scores indicating hybridization.

THE BARRED MINNOWS (TELEOSTEI: CYPRINIDAE) OF ZIMBABWE: IS THERE CAUSE FOR CONCERN?

Summary Two species of barred minnow, Opsaridium zambezense and O. peringueyi, occur in Zimbabwe. The former is widely distributed in the Zambezi basin and some other rivers but is now absent from Lake Kariba and possibly some of its tributaries. The latter, which was only recently recognised as a distinct species and admitted to the Zimbabwean list, is much scarcer. It occurs in the Save and Limpopo catchments and there are only four specimens in the Natural History Museum of Zimbabwe collection. These specimens are all more than 40 years old and the lack of more recent ones suggest that O. peringueyi may be extinct, or close to extinction, in Zimbabwe. This situation may have come about through habitat alteration and drought, and emphasises the need for further monitoring of the current occurrence/distribution of fish species in the country.

Ecological footprint for assessment of resource use and development limitations in shrimp and tilapia aquaculture

We review estimates of the spatial ecosystem support required to run a typical semi-intensive shrimp farm in a coastal mangrove area in Carribean Colombia, and to produce food inputs and process wastes for large-scale industrially managed tilapia cage culture and small-scale, semi-intensive tilapia pond farming in Lake Kariba, Zimbabwe. The tilapia farming is discussed in relation to the pelagic kapenta, Limnothrissa miodon (Boulenger), fishery and to inshore fisheries in the Lake. The results show that a semi-intensive shrimp farm needs a spatial ecosystem support—the ecological footprint—35 to 190 times the surface area of the pond, mainly mangrove area. Based on the analysis, we conclude that shrimp farming in Colombia is already utilizing close to the full support capacity of its coastal environment. In intensive tilapia cage farming, the ecological footprint for feed production is 10 000 times larger than the area of the cages. In contrast, a tilapia pond farm maintained on offals from fisheries, agriculture and households depends very little on external ecosystem areas. As long as there is a direct market for human consumption of all kapenta caught in the Lake, fish cage farming based on fish meal from kapenta would be doubtful from ethical, ecological as well as resource management points of view, even if it was economically feasible.

Food composition and selectivity of Synodontis zambezensis (Pisces: Mochokidae) in Lake Kariba, and the ecological implications

The diets of 441 catfish Synodontis zambezensis from three different localities in Lake Kariba were analysed. S. zambezensis was found to be primarily a molluscivore, acitively selecting the pulmonate Lymnaea natalensis and chironomid larvae in 59% and 30% of the cases, respectively. Although highly selective, S. zambezensis changed its prey preference according to availability of prey species. Some changes in diet were associated with the emergences of insects such as the flying form of the termite Aletes sp. The range of food items found in the stomachs suggested that the range of fish can also use other food items effectively. The Percent Similarity Index showed significant temporal differences in diet. Evidence showed that S. zambezensis was a benthic feeder but also fed at the water surface. No dietary shifts from the smallest fish (12 cm) to the largest fish caught (38 cm) were observed. However, specimens under 20 cm total length were few and this could have influenced the results. S. zambezensis is an ‘oral sheller’ that removes gastropod shells prior to swallowing the soft tissue and either discards or swallows the smaller shells (< 10 mm height). The results showed that the S. zambezensis does not have a strict food regime and this gives it a better change of becoming widely distributed. There is a high possibility that the food composition will vary as the food abundances and distributions in the lake shift and S. zambezensis is likely to be little affected. Ecological it is important that the squeaker is feeding on a resource which is hardly utilized in the lake.

Spatial and temporal variations in the composition and density of crustacean plankton in the five basins of Lake Kariba, Zambia-Zimbabwe

Spatial and temporal variations in the composition and density of crustacean plankton in the five basins of Lake Kariba, Zambia-Zimbabwe

Respiration, nutrient excretion and filtration rate of tropical freshwater mussels and their contribution to production and energy flow in Lake Kariba, Zimbabwe

The productivity and ecological role of benthos in man-made Lake Kariba was assessed through the use of P/B-ratios and by measuring the metabolism (respiration, N and P excretion) of the most abundant mussel species (Aspatharia wahlbergi, Corbicula africana and Caelatura mossambicensis) in laboratory experiments. For A. wahlbergi also filtration rate was estimated.

The fishery for Rastrineobola argentea in Lake Victoria: estimation of potential yields using a new approximate model based on primary production

Sustainable fishery yields of Lake Victoria dagaa, Rastrineobola argentea, a silvery zooplanktivorous cyprinid, are estimated in the absence of catch/effort and survey data. The starting point is an estimate of sustainable yield of a fish in another large lake in the region, the baseline lake. This is used to estimate the potential yield of a fish occupying an analogous niche in a second lake, the target lake. Several baseline lakes may be used to provide results for the target lake. A new approximate model scales fish yield in baseline and target lakes according to the ratio of the logarithm of primary productivity. We modify this model to account for differences in productive capacity in the two taxa by scaling the result according the ratio of Φ′ values, the auximetric growth parameters. Here we employ sustainable fishery yields of freshwater sardines, Limnothrissa and Stolothrissa, from Lakes Tanganyika and Kariba as the baselines for the model. For Lake Victoria dagaa, the validity of linking primary production and zooplanktivore production one trophic level removed in the food web is considered a reasonable approximation. The principal uncertainties in applying the model are evaluated and estimated quantitatively using a bootstrap procedure derived from error distributions of the model input parameters. Rastrineobola population biomass estimated from the model's fishery yield results is very similar to previously published values. The potential sustainable yield of 5.18–8.14 t km −2 in relation to recorded 1990 landings of 1.24 t km −2 (84 000 t over the whole lake, and the latest data available to us) suggests that catches may be safely doubled with minimal risk. Although this finding is sensitive to the accuracy of the catch records, it is in accord with the small number of other estimates.

Managing aquaculture for sustainability in tropical Lake Kariba, Zimbabwe

In Lake Kariba, Zimbabwe, small-scale pond farming and experimental cage-culture of Tilapia fishes have been running for some years and there are now plans for large-scale aquaculture. As a basis for deciding on how aquaculture could be developed to improve the chances for sustainable resource use and long-term maximised fish production in the lake, we compare the potential ecological life-support demand of two alternative aquaculture methods. First, the economic and ecological resource demand, expressed in industrial and solar energy units, respectively, were estimated for semi-intensive pond farming and intensive cage farming. Next, the ecosystem areas appropriated by the two farms for production of feed, oxygen, and phosphorus assimilation were estimated. Our estimates indicate that intensive cage farming would require about 17 800 MJ solar energy (Gross Primary Production) to produce 1 kg of fish. The industrial energy input would be more than 1.5-times higher (about 85 MJ/kg) compared to semi-intensive pond farming (about 50 MJ/kg). Intensive cage farming must be supported by ecosystem areas that are all substantially larger than the area of the farm itself. The ecosystem area for feed production is the largest (21 000 m 2 m −2), but the areas required for oxygen production (160 m 2 m −2) and nutrient assimilation (115 m 2 m −2) are of special importance since they must be located close to the farm. For semi-intensive pond farming, oxygen production and nutrient assimilation could probably be provided within the pond system, and no external life support from Lake Kariba would be needed. At least from an ecological point of view, semi-intensive pond farming is more sustainable than intensive cage farming because it needs a smaller input of external resources to survive. However, a moderate level of intensive cage farming should not be ruled out in Lake Kariba. Aquaculture has potential to become successful in Lake Kariba, but only if it is developed within a linked economic, social, and ecological framework.

Aquatic snails of the Bulinus africanus group in Zambia identified according to morphometry and enzymes

The Bulinus africanus species group (Planorbidae) of freshwater snails has been reported to be represented in Zambia by two species, B. africanus (Krauss) and B. globosus (Morelet), both named as intermediate hosts for Schistosoma haematobium. Uncertainty in identification of these snails from morphology led to the present investigation, combining morphometry (shell and copulatory organ) with enzyme analysis. Observations of both kinds were made usually on the same individual snails, from collecting sites mostly in the Lusaka area or at Lake Kariba. Particular attention was given to the proportional relationship between the penis sheath and the preputium of the copulatory organ, a character used previously to distinguish B. africanus from B. globosus in south-eastern Africa. The enzyme profile MDH-1, AcP-2, PGD-1 and PGM-2 was common to all snails examined from 25 populations; GPI and HBDH were polymorphic. The enzyme data indicate that the samples represent a single species. Shell characters varied continuously. The copulatory organ was generally of the form known for B. globosus. Although the copulatory organ of a few individuals had proportions overlapping the range reported for B. africanus, the present variation was continuous and was not bimodal. It is concluded that all these specimens are conspecific and may be identified as B. globosus. Previous identifications of B. africanus from Zambia appear to need substantiation and it seems that if this species is present at all in the sampled areas, it must be uncommon. It is relevant in regard to possible strain differences within S. haematobium in Zambia, that our observations indicate that only a single species of intermediate host is involved in transmission.