Garry M. Bernacsek
Via Vascellari 34
00153 Rome, Italy
Songo, Tete, Mozambique
|No time series data are available for fish catch and fishing effort for Cahora Bassa reservoir. Fisheries development inputs have been very few. The Cahora Bassa ecosystem suffers from two major constraints: unpredictable large fluctuations in water level and high clay load reducing light penetration significantly. Downstream effects are poorly documented but available information suggests substantial ecosystem disruption. Fisheries development has been impaired by warfare, lack of government infrastructure inputs, insufficiencies of fishing gears and crafts, inappropriate processing practices, difficulty in marketing catch and conflicts between fishermen and some official bodies. The sardine Limnothrissa miodon is well established in the reservoir but no fishery has as yet developed for it. Pre-impoundment environmental impact research was inadequate and was carried out much too late to influence dam design. A fisheries development plan was formulated but not implemented. Predictions of potential fish yield, species catch composition and nuisance macrophyte risk are thought (or known) to be inaccurate. It is suggested that such inaccurate predictions cause wastage of scarce development funds. There is at present no special fisheries management policy for Cahora Bassa. It is suggested that better planning capability for reservoir fisheries can be achieved if: (i) biologists are allowed to have an input into dam planning, particularly water level management; (ii) the type of government development inputs required are carefully assessed and compatible with the inherent development capacity of the fishery; and (iii) there is an improvement in accuracy of prediction techniques for biological parameters important for fishery development.|
|Il n'existe aucune série de données chronologiques sur les captures ni sur l'effort de pêche dans le réservoir de Cahora Bassa. Les activités en faveur du développement des pêches ont été rares. L'écosystème de Cahora Bassa se heurte à deux obstacles importants: les fortes fluctuations imprévisibles du niveau des eaux et l'épaisse couche d'argile qui réduit beaucoup la pénétration de la lumière. On connaît assez mal les effets en aval mais d'après les renseignements disponibles il semble que l'écosystème ait été fortement désorganisé. Le développement des pêches a été freiné par les hostilités, le manque d'infrastructures gouvernementales, la médiocrité des engins et des bateaux de pêche, la mauvaise adaptation des pratiques de traitement, les difficultés de commercialisation des captures et les conflits entre les pêcheurs et certains organismes officiels. La sardine Limnothrissa miodon s'est bien adaptée dans le réservoir mais elle n'est pas encore exploitée. Les recherches sur les incidences de l'endiguement sur le milieu ont été insuffisantes et effectuées beaucoup trop tard pour avoir une influence sur la conception du barrage. Un plan de mise en valeur des pêches a été élaboré mais n'a pas été exécuté. Les prévisions sur le rendement potentiel de poisson, la composition des captures par espèces et les dangers que présentent les macrophytes sont d'après ce qu'on pense (ou sait) peu précises. On estime que ce type de prévisions inexactes entraîne un gaspillage inutile des fonds de développement déjà insuffisants. Il n'existe actuellement aucune politique spéciale d'aménagement des pêcheries pour Cahora Bassa. On pourrait obtenir une meilleure planification des pêcheries en réservoir si: i) les limnologistes étaient autorisés à participer à la planification des barrages, particulièrement en ce qui concerne l'aménagement du niveau des eaux; ii) le type des apports de développement requis du gouvernement étaient soigneusement évalués et compatibles avec la capacité inhérente de développement de la pêcherie; iii) on améliorait l'exactitude des techniques de prévision en ce qui concerne les paramètres biologiques qui présentent de l'importance pour le développement des pêches.|
The general aim of individual reports in this volume is to compile documentation of factors which have affected fishery performance in each particular reservoir. These can be grouped into two broad categories:
Biological productivity (and its alterations as a function of time) inherent in the ecosystems;
Development inputs such as resource evaluation, supply of fishing gear and craft, establishment of transport and marketing networks and training programme.
Cahora Bassa (Fig. 1), the fourth largest reservoir in Africa after Volta, Nasser/Nubia and Kariba, has the least developed fishery of any major African reservoir. Since closure of the dam (5 December 1974) there has been little fishery infrastructure development due to warfare instability in the area and other national government priorities. The fisheries of Cahora Bassa such as exist at present have had to ‘go it alone’ as it were. There are virtually no historical data on fish catches or fishing effort, and the limnological/environmental data base, skewed and incomplete as it is, is far more comprehensive. There is therefore little scope at present for performing a meaningful factor analysis of Cahora Bassa for either of the two categories listed above. A relatively comprehensive compilation and interpretation of the available fisheries data (collected primarily by the FAO project team working currently at Cahora Bassa) and limnological data ware carried out recently by Bernacsek and Lopes (in press). This project technical report would appear to meet the requirements of the CIFA reservoir synthesis as far as technical documentation is concerned and there is little point in a repetition here of the considerable technical data presented.
The overall objective of the CIFA Reservoir Synthesis activity as set out in the guidelines for authors is “to improve our planning capability so that in future we will be able to more fully realize the fishery potential of new reservoirs”. Cahora Bassa certainly provides several poignant lessons on fisheries planning and implementation. Accordingly, it was decided by the present authors to shift the emphasis of this Cahora Bassa synthesis away from data compilation (which has in any case already been completed in Bernacsek and Lopez (in press)) to an interpretative problem identification/solving approach more directed towards the overall stated objective of the synthesis activity. Only limited data pertinent to particular discussions will be presented below and the reader is referred to Bernacsek and Lopes (in press) for more comprehensive documentation. Table 1 gives the more important technical data for the dam, power station and reservoir.
The essential and primary characteristic of Cahora Bassa, as for any other reservoir, is that it is a mass of water stored for some predetermined use or uses. In this case the primary use is for electricity generation, and secondarily for downstream flood control. Fisheries, if they develop, are considered a spin-off benefit. Accordingly, no special provisions are made in the management of the impounded water mass for fisheries. In practice this means that the initial requirement of any ecosystem for ‘stability’ (i.e., predictability of annual cycles of variation of main ecosystem medium) is not usually met in Cahora Bassa reservoir.
The observed water level fluctuation for the seven hydrological years of the life span of Cahora Bassa (Fig. 2) is the result of several factors. The fifth and last turbine of the power station was only installed in 1977, and accordingly the first three years of life are not representative of the ‘normal’ operational mode. The following year (1978) was the year of the ‘Kariba flood emergency’ during which a massive flood was released from the Kariba reservoir upstream without sufficient advance warning being given to the Cahora Bassa power plant/dam manager. 1979 and 1980 are the only two years which can be considered hydrologically and operationally ‘normal’. During 1981 and 1982 there have been extended periods during which only one turbine was operational - the result of electricity transmission capability having been lost due to sabotage of the transmission lines.
Observed changes in water level and surface area for the years 1977 to 1981 are given in Table 2. Annual drawdowns for flood water storage range between 6.98 m and 14.06 m. The lowest figures are for the ‘normal’ years 1979 and 1980. Drawdown rates are also the slowest for these years (0.78 and 1.05 m/month). However, even during ‘normal’ years there is a relatively large loss in reservoir surface area of about 20 percent which disturbs fishery practices and may disrupt reproduction of littoral-nesting fish species such as tilapiines.
The design water level fluctuation curve is compared to the observed mean for the two ‘normal years’ in Fig. 3. While broadly comparable, some important differences exist. The design curve is ‘peakier’, with more rapid filling and drawdown rates than the observed curve. On the other hand, the observed curve lacks the four month stable period (March to June) of the design curve. While it is possible from a fisheries perspective to formulate arguments in favour of one or the other curve, it is clear that there is leeway for curve tuning that would not undermine the primary and secondary functions of the dam/reservoir and yet still provide a better (though unlikely to be optimal) curve behaviour for fisheries - both bioproduction and fishing activity.
Several factors complicate the ‘guestimate’ of the hydrological ‘stability’-predictability of Cahora Bassa. The reservoir has a relatively huge ratio of live water volume to dead storage volume (68.00 km3:5.00 km3). At the same time the maximum annual flood storage capability of 280 km3 is exceeded by the design flood volume of 327 km3. The catchment area of Cahora Bassa is large (847 272 km2) and complex. The two principal sub-basins, Kariba and Kafue, are impounded while a third, the Luanguwa, is not. Flood predictability (timing, magnitude and duration) is accordingly poor. Hydroelectrica Cahora Bassa (HCB) power company dam operators are therefore inclined to take advantage of the large live storage volume and to manage the Cahora Bassa water mass in a manner which gives safety margins to prevent either dam crest overtapping or exposure of the turbine intakes (at 295.00 m a.s.l.).
An improvement in flood prediction capability coupled with increased dam discharge capacity1 would alleviate this situation. As long as there is no disruption in electric power generation a higher degree of environmental medium ‘stability’ and predictability would appear possible.
1 The present total dam discharge capacity of 16 250 m3/sec would be raised to 21 450 m3/sec if a second planned power station (known as the Phase II project) is built on the north shore.
Physical parameters (such as temperature) and chemical parameters (such as nutrients and oxygen) are particularly critical variables controlling biological production in aquatic ecosystems. Typically lentic systems undergo two distinct phases within an annual cycle:
A ‘mixing’ phase during which there is relative physical and chemical homogeneity throughout the system; and
A ‘stagnation’ phase during which heterogeneity is established as a gradient along depth.
Cahora Bassa reservoir exhibits both of these phases. However, due to the short replacement time of the water mass (i.e., 0.61 year) of Cahora Bassa a third factor is superimposed on these two-that of a longitudinal heterogeneity gradient. Mean monthly affluent inflows at Zumbo and discharges through the dam for 1979–1980 are shown in Fig. 4. The main ‘block’ of affluent water enters the reservoir during the March to May period, while May to July and December–January are the main periods when ‘blocks’ of resident water are discharged from the reservoir. The work of Bond et al. (1978) during the first year of reservoir life shows ‘blocks’ of water (‘labelled’ by temperature) moving from west to east along the main axis of the reservoir (Fig. 5), with depth stratification superimposed on this pattern during the latter months of the year. The effect of this longitudinal heterogeneity on biological production in Cahora Bassa is completely unknown.
Various chemical parameters of the reservoir are listed in Table 3.
Cahora Bassa displays some physical and chemical features of interest:
Thermal stratification is only well developed in the deeper eastern part of the reservoir. Dominant winds from the southeast prevent stratification from developing in the shallower central and western basins.
Secchi disc transparency is low (0.60–0.85 m in June–September 1982 period) due to a high concentration of clay particles.
Oxyclines develop in the eastern basin coinciding with thermal stratification (occasional oxygen depletion probably occurs) but not in the central and western basins.
There is only a small increase in dissolved solids (1.24 times) and conductivity (1.03 times) as the affluent water mass passes along Cahora Bassa reservoir. This is less than for other reservoirs and is probably due to the short residence time of Cahora Bassa.
There appears to have been a doubling in sodium chloride content of Cahora Bassa water since late 1980 (Figs. 6 and 7). The origin of this extra salt load is unknown but domestic sewage from Zimbabwe is suspected.
There are no chemical data available for the ‘bloom’ period (initial high productivity) of the reservoir, but it is suspected that this took place mainly in 1976 because the operational water level was not reached in 1975.
Cahora Bassa water has a high clay load (108–109 particles per litre). This reduces light penetration to only 0.60 m to 0.85 m (Secchi disc transparency) in the June–September period. The poor light penetration results in a pelagic phytoplankton community depauperate in both species and density1. In turn, the zooplankton community is also depressed. As a result the food resources of the pelagic zone may be extremely limited for planktivorous fish such as the sardine Limnothrissa miodon. Since there would seem to be little possibility for controlling the entry of clay into the reservoir this situation must be accepted as an unalterable inherent limitation. It has been shown that filter-feeding zooplankton are capable of removing clay particles from the water mass and this ‘mechanism’ could cause partial clearing1.
The submerged trees of many reservoirs provide a huge surface area for aufwuchs growth. Cahora Bassa is no exception and periphyton is an important component in the diet of several commercial fish species. There is evidence that when pelagic plankton concentrations are low, even sardines transfer to aufwuchs for feeding. A notable feature of Cahora Bassa is that the reduced light penetration results in periphyton growth on only the first few meters of canopy below the surface. Furthermore, the relatively rapid and large changes in water level of Cahora Bassa result in the aufwuchs standing crop being either exposed to desiccation during drawdown or submerged into the aphotic zone during filling. Aufwuchs growth therefore continually shifts up and down the long axis of the submerged trees.
The evident heavy utilization of aufwuchs by commercially important fish species for food presents a serious long-term problem to the fisheries of Cahora Bassa. Canopy break-up will eventually take place and much of the large substrate surface area presently available for aufwuchs growth will be lost. In view of the poor light penetration of the water, it seems unlikely that benthic algal growth (which will be restricted to within a few metres of the shoreline) can significantly compensate for the expected large-scale, long-term loss in aufwuchs production. As even sardines utilize aufwuchs, all sectors of the fishing industry would be affected. It is therefore important to begin monitoring the break-up rate of the canopy and at the same time some indices of fish production to establish possible correlations. Options to compensate for canopy break-up should be investigated. If no natural mechanism(s) for rechannelling nutrients into usable bioproduction in the absence of extensive aufwuchs substrate area can be detected, the widespread use of artificial substrates should be considered.
1 Based mainly on as yet unpublished results of investigations of the project limnologist, Z.M. Gliwicz.
Although the emphasis of the reservoir synthesis activities is on reservoir fisheries, some consideration must also be given to the downstream environment and its fisheries. The effect that Cahora Bassa dam, in ‘collaboration’ with the three other major upstream impoundments (Kariba, Kafue and Itzhitezhi), has had on the downstream ecosystems appears to be quite severe. During initial filling from 5 December 1974 to early March 1975, river flow was chopped from the normal 2 000–3 000 m3/sec to less than 60m3 day. The recommendation of the ecological impact study team to reduce discharge to no less than 400–500 m3/sec was ignored (Davis, 1975a, 1975b). The immediate results of this virtual desiccation of the Lower Zambezi which are on historical record were:
Wide-scale stranding of reproductively active fish entering floodplains to spawn and consequent massive mortality of stocks by fishing and ‘natural’ causes;
Cutting off of the water supply to Tete town.
It is probable that several other disruptive effects (both to natural and human ecology) took place but have not been recorded.
Cahora Bassa has now completely disrupted the normal downstream hydrological regime. Flood releases occur two or even three times a year (as opposed to one normally) and therefore out-of-season floods occur annually.
No comprehensive impact studies of the downstream effects of Cahora Bassa have as yet been carried out. Some observations during 1975 are probably the result of Kariba rather than solely of Cahora Bassa, but these should give an indication of the kinds of downstream effects Cahora Bassa would be likely to engender:
Dying of mangroves on the seaward side of the Zambezi delta;
Reduction of the number of agricultural harvests realizable per annum;
Reversion of the marginal grass-lands to savanna;
Salination of agricultural soil;
Dropping of water table level and desiccation of top soils;
Reduction in estuarine prawn population density;
Decrease in soil fertility due to cessation of annual silt load input.
It is quite clear that there is at present little integration between Cahora Bassa and the natural and human ecosystems downstream. There is an urgent need for a detailed ecological impact study, and, based on its results, effective governmental action to re-establish some degree of compatibility between the dam and its downstream receiver environments.
Fish production (exclusively table-fish) for Cahora Bassa for 1982 was estimated by Bernacsek and Lopes (in press) at about 4 300 t. The total sustainable annual yield of table-fish has been calculated at 6 700t. This suggests that only 64.1 percent of the table-fish potential is being tapped and that a considerable expansion of this fishery is still possible. If the estimated sustainable sardine yield of 8 000 t is added to the table-fish potential, only about 29 percent of the total Cahora Bassa fish potential is currently being harvested.
Several technical factors have been identified as partly responsible for the present under-developed state of Cahora Bassa's fisheries, and these are discussed in separate sections below. The positive motivating force for overcoming these technical problems (with or without governmental assistance) ultimately resides in the fishermen themselves. Economic forces are, of course, profound stimulants (or depressants) to motivation, but the role of other forces, especially those of a cultural and sociological nature, have not been investigated and therefore their importance cannot be assessed. In a human environment (i.e., Cahora Bassa in the overall context of Mozambique) which has been subjected to several years of warfare and which is presently the subject of an attempt to introduce a new social system, cultural and sociological motivating factors can be expected to take an increased importance compared to other types of human environments where simple economic forces and laws are allowed to take unconstrained effect. It is suggested here that a sound development and management policy (as yet to be developed for Cahora Bassa) should include a component which recognizes and provides for special motivational needs of the fishermen by using incentives for increased production. This would be in addition to providing for fishermen's basic economic necessities.
Bernacsek and Lopes (in press) suggested that at least 1 200 fishermen were operating on Cahora Bassa in 1982. This is equivalent to an effort density of 0.46 fishermen/km2. African lakes and reservoir fisheries however only approach full exploitation at a critical fisherman density of about 1.5 per km2. Accordingly, at least a three-fold increase in fisherman density would theoretically be necessary before maximum Cahora Bassa production can be expected to be realized.
Several other reservoirs have achieved or surpassed the critical fisherman density required for full exploitation in a relatively short space of time after dam closure. Therefore, it is pertinent to determine for Cahora Bassa:
What factors in the past have prevented the density from increasing (as they may have a lingering residual effect), and
What factors in the present may be preventing the density from increasing.
Unquestionably, two prime factors operating in the past have been (in order of importance): (i) war with Rhodesia, and (ii) lack of government input (the latter to a substantial degree the result of the former). From 1975 to 1978 the area around Cahora Bassa was a highly unstable security zone. Rhodesian gunboats penetrated Cahora Bassa reservoir as far downstream as Chicoa, destroying all vessels and docks and harassing villages. Large areas of the south shore of the reservoir, as well as the prime access road (the tarmac Tete-Songo road), were burned with napalm. In such a situation, the Mozambican Government could hardly begin constructing the needed common infrastructure to be used by the fishing and other industries, let alone provide specific inputs to the fishing industry.
The warfare caused a large exodus of people from the Cahora Bassa area. Since the establishment of Zimbabwe as a sovereign state, fishermen have been returning to the reservoir. Their self-identity as Mozambicans is however not strong. Discussion with fishermen at Nyamtimbsa near Chicoa revealed that the memory of warfare is still very strong and there remains fear of a renewal of conflict. This manifests itself, for example, in a reluctance to build permanent housing. This fear is fuelled currently by the operation of an internal anti-government resistance movement in Tete and some other Mozambican provinces.
Government inputs since Zimbabwean independence have been small. Fishermen experience difficulties in marketing their catches. There are no roads running along either shoreline, and direct access to the reservoir is limited to a few towns and villages. A water transport company (Empressa Fluvial) operates small cargo service but dry fish are not a priority. Other than the PESCOM contracting system (a statal company which purchases and markets artisanal and industrial fish catches internally), there is no government-sponsored fisheries support programme to attract new manpower to the industry, and even the PESCOM system is not particularly attractive due to its restrictive and monopolistic terms.
Demographic data for the Cahora Bassa area are not available at the present time. Hence, it is not known if there is an actual limitation in available human resources, or if human resources are adequate, but require voluntary redeployment into the fishing industry.
It is not known if fisherman density will increase further if unassisted by government programmes. Certainly the establishment of a mixed sardine fishery will be almost impossible to achieve without a substantial government investment to make the necessary specialized fishing materials widely available at affordable prices and thus create a positive climate for fishing which would be capable of attracting manpower. Some training programmes in new fishing methods will be necessary to build up the necessary expertise within the fishing communities.
Perhaps the most critical technical problem experienced by Cahora Bassa fishermen is the limited availability of fishing materials. Locally, gillnets are both in extremely short supply and very expensive. The netting material reaching retail outlets in Tete province is often of low quality (i.e., cotton material which quickly rots in water) or of incorrect specifications (i.e., small mesh, thick twine-netting more suitable for bottom trawling). To overcome high prices some fishermen weave their own nets from nylon twine, but the twine is of poor quality and has a low breaking strain. Some fishermen import nets from neighbouring countries, but this is illegal, and the contraband traffic of nets, dried fish and other products causes a conflict between the fishermen and Mozambican security forces. PESCOM operates a contracting system which provides the fishermen with gillnets in exchange for exclusive rights to purchase the catch at fixed prices. Fishermen who may have no option but to enter into contract with PESCOM still prefer to sell their catch to private traders with whom they can negotiate prices (invariably higher than the PESCOM prices) and this causes further conflicts between the two purchasing parties and between PESCOM and the fishermen.
Government priority at present is development of the marine fisheries. Accordingly, few suitable fishing gears are sent to Cahora Bassa reservoir, rather supplies include mainly excess or unsuitable material. Until this situation changes and a steady supply of low-cost, good-quality fishing gear is made available to the fishermen of Cahora Bassa, conflicts arising out of the gear ‘crunch’ and loss of production can be expected to continue, or even intensify, if an attempt is made to ‘seal’ off the border.
No boat building yards as such exist on or near Cahora Bassa reservoir. Closest to a ship building yard is a general metals fabrication plant at Tete which is able to accept contracts to build steel plate boats of various dimensions (provided regional government approval of the contract is received beforehand). All artisanal craft are dug-out canoes built by knowledgeable individuals at various places on the reservoir, but the number of experienced dug-out constructors appears to be small as it is not uncommon for several fishermen to share canoes.
Cahora Bassa is not a particularly expansive reservoir (mean breadth is 10.83 km) but wind action can sometimes generate high-amplitude waves. Offshore navigation in dug-out canoes becomes extremely hazardous under these conditions and plank boats of a more secure shape and design are required to safely operate under such unfavourable weather conditions. Since there exists a good supply of various types of suitable lumber in the area, as well as sufficient electricity for power tools, there appears to be no good reason why a boat yard could not be established in the Songo area.
Smoking of freshwater fish is probably the most common and widespread processing method in Africa. Curiously, it is almost completely unknown in Mozambique. This situation is undoubtedly a residual from the colonial period (i.e., the Portuguese preference for salt-dried fish). The method of salt/sun drying used in Garganta and Chicoa basins was described by Bernacsek and Lopes (in press). It has the following major drawbacks:
A steady and large supply of salt must be transported from the coast. This is expensive both in local and foreign currencies, and any disruption in the salt supply network interferes directly with fish production and consumption.
The method is slow and it is not possible to carry it out during the rainy season.
Dermestid beetle infestation begins immediately during the drying process.
The main positive feature of properly salt-dried fish is the good shelf life, but Bernacsek and Lopes have argued that, since the product is usually consumed within a week, a long shelf life is unnecessary. In any case, the high incidence of dermestid infestation evident in the Cahora Bassa product will reduce the shelf life.
In contrast, smoke processing can overcome the three major drawbacks of salt/sun drying. Wood is free and available in plenty around Cahora Bassa. Smoking is faster and can be carried out during the rainy season. Dermestid infestation can occur only after processing is completed and appropriate sanitary practices can considerably reduce the incidence of infestation. The shorter shelf life of smoked fish is not incompatible with the existing marketing/consumption pattern.
An extension input to introduce high-efficiency smoking technology (such as the Ivory Coast kiln) to the main fishing communities is badly needed on Cahora Bassa - as elsewhere in Mozambique. The introduced techniques should make extensive use of locally-available materials.
Species composition data for commercial catches is meagre, and stock assessment research has only recently been initiated, but the following information is available:
Labeo spp. form an important part of the gillnet catch at Zumbo town in December. These are probably stocks migrating out of the reservoir and up the Zambezi River to spawn.
Hydrocynus vittatus is widespread in Chicoa and Garganta basins and forms a major component of many gillnet catches. It is probably the most highly exploited species at present.
Oreochromis mortimeri is rather restricted in distribution and may form only a major component of gillnet catches during restricted time periods each year at particular localities.
Distichodus schenga, at least in the Chicoa basin, is a major component of the gillnet catch and appears to outcompete tialpiines as the dominant littoral phytivorous table-fish.
Clarias gariepinus appears to occur in good number but is only caught consistently on baited longlines (which are in little general use).
Limnothrissa miodon is well established in Cahora Bassa and is present in a commercially exploitable density. No fisheries exist for it at present due to lack of gear, expertise, and the unstable security situation (i.e., military personnel at Chicoa will not permit light fishing at night in their area).
The general impression is that the available stocks of Cahora Bassa are being rather unevenly exploited at present. Longlining for Clarias gariepinus (and secondary catch elements such as Hydrocynus vittatus, Heterobranchus longifilis and Anguilla sp.) is much more time-consuming than gillnetting. Without the development of a special market for Clarias (or drastic depletion of gillnet-exploitable stocks), there appears to be at present no incentive or advantage for longlining.
A sardine fishery has not yet developed for reasons given above. The relatively intensive exploitation of Hydrocynus vittatus (the chief predator of Limnothrissa miodon) should result in an even greater proportion of total sardine production being made available for fishery exploitation. The development of a sardine fishery (which may have a greater potential yield than the tablefish fisheries: 8 000 versus 6 700 t per annum, respectively - estimated by Bernacsek and Lopes (in press) should be viewed as a fishery development priority on Cahora Bassa.
The primary economic concern of the fisherman is to convert his catch into usable income in the form of currency or directly into material goods. This concern presents several problems to the fishermen of Cahora Bassa. The availability of consumer goods locally is poor. Accordingly, the incentive to accept Mozambican currency in exchange for fish is not strong. At best the fisherman must maximize his Mozambican currency intake to the greatest extent possible to be able to cover the high local cost of replacing his fishing gear, and thus he is more interested in dealing with private traders than in entering into contract with PESCOM (see above).
As Cahora Bassa reservoir extends to the Zambian and Zimbabwean frontier in the west (Zumbo) and to the Zimbabwean frontier in the south (Rio Messenguezi), fishermen operting in these areas have (in the absence of tight border security) easy access to the more lucrative markets in these countries, both for sale of their catch and purchase of consumer goods. It is well known that there is a steady flow of dried fish and consumer goods across the borders, and Mozambican authorities are of course concerned about this loss of food to the Mozambican consumer as well as the undermining of national economic legislation (i.e., illegal export of Mozambican natural resources).
There are several options open for ameliorating the income problems faced by the fishermen. One approach would be for PESCOM to adopt a more realistic and competitive price structure, as well as extending its purchasing network to more fishing communities. This must be coupled with a better retail supply of consumer goods, thus making Mozambican currency a more attractive exchange medium. The export of fish to neighbouring countries could be normalized by imposing tariffs. Fishermen operating on Cahora Bassa who are not Mozambican could be incorporated into a type of licensed ‘gastarbeiter’ scheme.
Two types of ecological studies were carried out in the Cahora Bassa dam impact area prior to impoundment:
Baseline soil, vegetation, hydrology and climatology analysis. These studies were carried out between 1958 and 1961 and had a direct economic thrust (i.e., to assess agricultural and other economic potentials). By themselves they were entirely inadequate for making any comprehensive assessment of the ecological impact of the dam.
Specific studies in order to assess the ecological impact of the dam and generate scientific information for development of the Zambezi valley. This work was carried out under the direction of a government agency (Missao de Ecologia Aplicada do Zambeze). Limited work on the aquatic ecosystems was begun only in 1973 and came too late to influence the design of the dam (construction had already begun in 1969).
It is clear that in the case of Cahora Bassa there was no serious attempt to ecologically optimize the dam design prior to construction, let alone adequately assess the probable impact of the dam design finally selected (mainly on economic and engineering criteria) for construction. Furthermore, after dam closure, proposals put forward by the ecological impact assessment team (for example, the recommendation on minimum downstream river flow during initial filling, see Section 2.4 above) were not implemented and there has been no regular monitoring of the dam's downstream effects during its seven-year life span. The biological and limnological studies of the reservoir carried out during its first year are more of theoretical than applied scientific interest as the reservoir did not reach a normal operational water level and hardly had an opportunity to fully develop its lentic ecological niches and characteristics. As a result, Cahora Bassa has the dubious distinction from an ecological perspective of being the least studied and possibly the least environmentally acceptable major dam project in Africa. It is not clear why so little emphasis was placed on ecological considerations during the planning and post-planning phases of the dam project, but the ‘hidden’ long-term costs of this regrettable omission may be high if no attempts are made in the near future to assess the dam's detrimental effects and implement some forms of remedial action.
As for environmental impact, planning studies for fishery development only began at a relatively late stage (1973). They were therefore not timely enough to exert any influence on dam design/operation schedule. Fisheries were seen as a side benefit of the dam project rather than as a major aspect of it, thus ignoring the potential economic importance of the catch.
A detailed plan for fisheries development on Cahora Bassa reservoir was formulated by P.B.N. Jackson (working for R.F. Loxton, Hunting and Assoc. of Johannesburg) which included comprehensive schedules for funding and infrastructure inputs. A bush clearing plan was also presented. However, primarily due to the unstable security situation almost none of the ‘Jackson Plan’ proposals were implemented. The plan suffers from some technical shortcomings which are the result of the poor predictive capability which existed at the time it was formulated1. Jackson calculated the potential yield (to have been realized five years after dam closure if all development inputs went according to plan) as 5 000 t/yr by direct extrapolation from the Kariba reservoir catch at the time. This was before the sardine fishery of Kariba had properly developed. The potential annual yield of Cahora Bassa is currently calculated as between two and three times this figure (Bernacsek and Lopes, in press).
The predicted species composition of the catch in the Jackson Plan is also now thought to be inaccurate:
|Predicted Catch Composition|
|Jackson Plan||Bernacsek and Lopes (in press)|
|Table fish||3 600 t (72%)||6 700 t (46%)|
|mainly tilapiines and Labeo spp.||mainly Hydrocynus, Distichodus and Labeo|
|Sardines (Limnothrissa)||1 400 t (28%)||8 000 t (54%)|
Despite removal of tilapiines from the shortlist of major commercial species, the catch predictions of Bernacsek and Lopes present a decidedly more attractive development potential panorama than the Jackson Plan.
Absolutely no planning was done for development and/or protection of the downstream fisheries of the Lower Zambezi floodplain.
1 This is not to imply that the present capability is near perfect, but some substantial improvements have been made particularly in the use of the morpho-edaphic index - see Bernacsek and Lopes (in press) for a new equation for reservoirs formulated by Welcomme and Bazigos
In sharp contrast to fisheries, substantial research and extension inputs went into nuisance aquatic macrophytes control. Massive infestations (up to 25 percent to 45 percent reservoir surface area coverage) of Eichhornia crassipes (water hyacinth) were predicted which would interfere both with hydroelectric installations and navigation. Booms were erected near the dam, spraying programmes formulated and even a control project base camp constructed at Tchipalapala near Chicoa. However, after dam closure, macrophytes never covered more than 0.5 percent of the surface area. Wind/wave action and drawdown were found to be highly effective control agents for Eichhornia, and the three other potential nuisance species present in the reservoir were unable to compete successfully and have not become established. Again the predictive capability of the scientists was inadequate (although the predictions that Eichhornia would become the dominant species was correct).
Other than extremely limited bush clearing near the Rio Messenguezi and near Chicoa (as well as near the dam to protect the turbines from debris) and the PESCOM contract purchasing scheme, no special inputs into the fishing industry have been expended on Cahora Bassa. The current MONAP project marks the beginning of comprehensive stock assessment, environmental and fishing technology research on the stabilized reservoir.
The exact position of the areas cleared of bush for fishing purposes is unknown and therefore their effect on catches cannot be assessed.
Several aspects and effects of PESCOM activities were noted above. Overall tonnage purchased per annum (290 t in 1980 and 121 t in 1981) are minor compared to the total estimated reservoir yield (4 300 t in 1982).
No special legislation has been formulated for Cahora Bassa reservoir fisheries management. No licences are presently required to operate fishing gear or vessels and there are no legal size limits for net meshes or fish. The overall state of the fish stocks is good due to low effort and a preference for large mesh sizes (102 and 127 mm stretched) and the fishery appears to be ‘regulated’ by negative economic (difficulty in marketing catch) and technical (poor availability of gear) factors.
The illegal traffic of fish to neighbouring countries has as yet not precipitated any strong executive countermeasures.
Although unable to address extensively the two main documentation categories of the synthesis activity (biological production over time and development inputs) due to lack of data and absence of significant inputs, the above synthesis for Cahora Bassa reveals three factors which are of direct relevance to the overall synthesis objective of improving planning capability for reservoir fisheries. These are as follows:
A close liaison must exist between the fishery biologist/limnologist and the engineering hydrologist involved with dam planning or dam operation. It may be possible to alter the water level design curve of a reservoir to give behaviour patterns more acceptable to fisheries and still meet specifications for other dam functions. Existing dam/reservoirs would have to be assessed to determine the individual operational constraints and hydrological budget, and recommendations made to dam operators to incorporate features into the design curve desirable from a fisheries perspective (i.e., slower drawdown rate or an extended stable level period). For reservoirs in the planning stage preferred water level design curves would have to be produced for each of the various dam design/operation options considered;
In the absence of significant development inputs and in spite of other major constraints (for example, and unstable security situation) reservoir fisheries have a high capacity for self-development up to a certain point. Government development inputs and interventions must therefore be carefully planned: i) not to disrupt or undermine the inherent development capacity; and ii) provide exactly those ‘critical’ inputs (be it gear, vessels, processing technology, and/or marketing networks) required for further production increases;
The predictive capability of biologists for important parameters such as potential maximum sustainable yield, catch species composition and nuisance macrophytes risk may still not be adequate for reservoir planning purposes. Incorrect predictions can result in diversion of scarce development funds into unnecessary and unproductive activities. There is, therefore, a need for better predictive capabilities in the pre-impoundment planning stages of reservoirs.
Extensive hydrobiological bibliographies for Cahora Bassa Reserva are presented in Bernacsek and Lopes (in press) and Ita and Petr (1983).
Bernacsek, G.M. and S. Lopes, (in press) Investigations into the fisheries of Cahora Bassa reservoir seven years after dam closure
Bond, W.J., 1978 The limnology of Cahora Bassa, Mozambique, during its first years. Freshwat.Biol., 8(5):433–47
Davies, B.R., 1975 Cahora Bassa hazards. Nature, Lond., 254:447–8
Davies, B.R., 1975a They pulled the plug out of the Lower Zambezi. Afr.Wild., 29:26–7
Table 1. Technical data for Cahora Bassa
|Date of closure||5 December 1974|
|Crest height above river bed||161||m|
|No. of flood gates||8|
|Total discharge capacity of flood gates||13 600||m3/sec|
|Spill gate discharge capacity||350||m3/sec|
|No. of turbines||5 (4 operational + 1 standby)|
|Maximum generating capacity (5 turbines)||2 075||mW|
|Maximum total turbinated discharge (5 turbines)||2 300||m3/sec|
|Geographical position||15°29'S to 16°00'S;||30°25'E to 32°44'E|
|Extreme length (at 326.00 m a.s.l.)||246||km|
|Extreme width "||39.8||km|
|Surface area "||2 665||km2|
|Maximum depth "||156.00||m|
Table 2. Annual variation in water level and surface area parameters for Cahora Bassa (1977 to 1981 period). These pertain to the first and 16th day of each month and although close are not necessarily the exact peak and minimum of a particular 'hydrological year. Peak levels always occurred during the calendar year but drawdown minimum usually occurred early in the following year. The years 1975 and 1976 have been eliminated as the observed water level fluctuations are entirely unrelated to the ‘normal’ operational fluctuations. The year 1982 has been eliminated since the data are incomplete
|HYDROLOGICAL PARAMETERS||CALENDAR YEARS|
|RESERVOIR WATER LEVEL|
|- Peak (m a.s.l.)||322.34 (Jul.77)||327.64 (Apr.78)||326.96 (Jun.79)||326.06 (May 80)||327.03 (Jun.81)|
|- Drawdown minimum (m a.s.l.)||313.84 (Jan.78)||314.03 (Dec.78)||319.58 (Jan.80)||319.08 (Feb.81)||312.97 (Jan.82)|
|- Annual drawdown (peak-drawdown minimum) (m)||8.50||13.61||7.38||6.98||14.06|
|- Drawdown rate (m/month)||1.42||1.70||1.05||0.78||2.01|
|RESERVOIR SURFACE AREA|
|- At peak water level (km2)||2 398||2 790||2 737||2 670||2 743|
|- At drawdown minimum (km2)||1 829||1 840||2 202||2 171||1 775|
|- Annual fluctuation (peak-drawdown minimum) (km2)||569||950||535||499||968|
Table 3. Chemical data for Cahora Bassa reservoir
(Period March 1977 to April 1982)
|Organic matter||"||3.39||2.60– 5.63|
|Turbidity||Jackson turbidity units||9.50||5–21|
1 Sum of cations, anions and organic matter means
Figure 1. Cahora Bassa reservoir at 326.00 m a.s.l. water level
Figure 2. Historical variation in water level of Cahora Bassa reservoir
Figure 3. Comparison of observed water level fluctuation (mean of two hydrologically and operationally ‘normal’ years) of Cahora Bassa reservoir with the design curve
Figure 4. Mean monthly water level, water gains and water losses from Cahora Bassa reservoir for two hydrologically and operationally ‘normal’ years (1979 and 1980)
Figure 5. Isotherms (°C) along the east-west axis of Cahora Bassa reservoir. (after Bond et al., 1978)
Figure 6. Historical variation in concentrations of three main cations in Cahora Bassa reservoir (surface samples from near dam wall)
Figure 7. Historical variation in concentration of three main anions in Cahora Bassa reservoir (surface samples from near dam wall)
Kainji Lake Research Institute
New Bussa, Kwara State, Nigeria
|The pre-impoundment ecology of the Niger River was relatively well known from studies not connected with the reservoir, and a comprehensive pre-impoundment investigation also was carried out. In comparison with other large African reservoirs Kainji stands out as having filled most quickly, the least mean depth, large annual water level fluctuations, and a rapid flushing rate. Also, it is relatively nutrient poor.|
|Among the nine fish families in the reservoir cichlids dominate the standing stock with 105 kg/ha followed by bagrids with 36 kg/ha. Average total biomass for the shore area was 240 kg/ha.|
|Fishery yields have been varying from 4 500 t/yr to 6 000 t/yr in recent years. Some species are very heavily fished, but lack of adequate fishing methods has inhibited the development of a pelagic fishery on two small, but abundant clupeid fishes. Management and development activities have received attention so far as research and planning, but actual execution of activities on more than a pilot basis still has to be accomplished.|
|Les conditions écologiques du Niger avant son endiguement étaient assez bien connues grâce à des études qui avaient été effectuées auparavant. Par rapport à d'autres grands réservoirs africains on considère que celui de Kainji s'est rempli très rapidement, qu'il est le moins profond en moyenne, que le niveau de l'eau varie considareblement pendant l'année et que son évacuation est rapide. Il est aussi assez pauvre en éléments nutritifs.|
|Parmi les neuf espèces de poisson qui vivent dans le réservoir, les cichlidés constituent le principal stock avec 105 kilos par hectare, suivis des bagridés avec 36 kilos par hectare. La biomasse totale moyenne est de 240 kilos par hectare dans la zone côtière.|
|Ces dernières années, les rendements de poisson ont varié de 4 500 tonnes à 6 000 tonnes par an. Certaines espèces sont fortement exploitées mais le manque de méthodes appropriées a freiné l'expansion de la pêche pélagique de ces deux espèces de clupéidés, poissons petits mais abondants. Les activités d'aménagement et de développement - notamment en matière de recherche et de planification - ont fait jusqu'ici l'objet d'une grande attention, mais il reste encore à entreprendre les activités autrement que sur une base pilote.|
There are numerous published and unpublished data on various aspects of the morphology and physico-chemistry of the reservoir and the Niger River, the biology of the various fish species and ecology of some fish communities, population dynamics, management and development aspects and socio-economics of the fishing communities. The more important published literature is listed here:
Physico-chemistry and morphology:
White, Ed. (1965), Imevbore (1970a and 1970b), Imevbore and Bakare (1974), Imevbore and Visser (1969), Visser (1973), Adenaji (1975, 1976, and 1978), Henderson (1970, 1973a, 1973b, 1975), Imevbore and Adeniji (1977), Sagua and Fregene (1979) and Imevbore (1975). A comprehensive bibliography on Kainji Lake Basin between 1957 and 1978 has been compiled (Ibeun, 1979) and other useful references could be obtained in this publication and in the updated bibliography on Kainji Lake for this report.
Ecology and biology of the fish communities:
These include: Lelek and El-Zarka (1973), White, Ed. (1965), Bakare (1968), Banks, Holden and McConnell (1965), Motwani (1967, 1970), Reed, et al. (1967), Ajayi (1972), Akitunde (1976), Arawomo (1976), Blake (1977a, 1977b, 1977c), Imevbore (1971), Imevbore and Bakare (1974), Olatunde (1977, 1978, 1979); Otobo (1976 and 1977), Willoughby (1974 and 1979) and others to be found in Ibeun (1979).
Fish population dynamics and commercial catch statistics:
Bazigos (1971, 1972 and 1974), Blake (1977c), Ekwemalor (1975, 1977 and 1978), Henderson (1971), Ita (1978, 1980a, 1981a, 1981b), Lelek (1972, 1973, 1975), Lewis (1974), Turner (1971), Blake (1977c).
Fisheries Management and development aspects:
Ita (1973, 1976, 1979, 1981a and 1981b), Nordlund (1973), Otobo and Imevbore (1979), Reed (1970), Sagua and Babaloa (n.d.) and others to be found in Ibeun (1979).
Socio-economics of the fishing communities:
Anthonio (1970 and 1973), Babalola (1976a and 1976b), Jenness (1970, 1973) and Oyedipe (1973).
A comprehensive pre-impoundment scientific investigation was carried out by White, Ed. (1965). The group of experts was stationed at a research station at Shagunu, a small village on the mid-western shore of the lake. Table 1 shows some of the physico-chemical and biological data across a transect of the River Niger adjacent to Shagunu Research Station in July 1965. The average chemical composition of the waters at different stations near Shagunu is given in Table 2.
Earlier attempts have been made to tackle the problems of man-made lakes' ecosystem on a comparative basis, for example, by:
Balon and Coche (1974), Ita (1975), Ita and Morcos (1981), and Petr (1975 and 1978). Table 3 (Balon and Coche, 1974) shows one of the most comprehensive geographical, meteorological, hydrological, physico-chemical and morpho-edaphic classifications of Lake Kariba, Zambia; Lake Nasser/Nubia, Egypt/Sudan; Lake Kainji, Nigeria; Lake Volta, Ghana; and Lake Kossou in Ivory Coast.
From the comparison, Lake Kainji stands out distinctly among others as the smallest in surface area and has the least mean water depth and shore-line length but has about the highest water level fluctuations and water exchange ratio (flushing rate). With the exception of Lake Kossou, the figures for which are not available in the table, Kainji Lake has also the least total dissolved solid contents compared with the other three major lakes.
Table 4 (from Petr, 1975) shows some geographical and morphological characteristics of four of the lakes included in Table 3 but with some information about two other lakes (Lake Ayame in Ivory Coast and Lake Nyumba Ya Mungu in Tanzania) not included in the classification in Table 3. These two lakes have much smaller surface areas than those earlier classified. Table 5 (from Petr, 1978) also shows some geographical and morphological characteristics of six major lakes in Africa including Kainji Lake, together with Lake Brokopondo in tropical South America and Lake Argyle in tropical Australia.
The table includes descriptions for Lake Cabora Bassa in Mozambique not treated in the previous tables.
One interesting aspect of the comparisons in Tables 4 and 5 is the period of filling which shows that Lake Kainji, of all the lakes classified, had the shortest period of filling (3 months) followed by Cabora Bassa which filled up within one year after the closure of the dam.
Table 6 shows a checklist of the fish species caught in the River Niger in 1962 and 1965 and those recorded in coffer dammed arm of the lake in 1966 during the construction period, compared with those captured in the lake between 1969 and 1978 using various sampling gears. Of a total of 119 species recorded in the river, about 101 species were recorded in the lake. One species recorded in the river as Microthrissa miri Daget was a later re-identified as two species of the family Clupeidae namely: Pellonula afzeliusi and Sierrathrissa leonensis.
Table 7 reveals that although 119 species were recorded in the river using assorted fishing methods, only 66 species were actually recorded in the fishermen's catches on the River Niger. Similarly, although about 101 species were recorded in the lake using assorted sampling gears, only 58 species were recorded in the lake in 1976 (Table 8) using only gillnetes in a graded fleet with mesh sizes ranging from 5 to 18 cm. However, gillnets are only one of the gears used by local fishermen in Kainji Lake, hence, more species could have been registered in their catches.
Table 9 shows the relative percentage composition of fish in the River Niger by numbers and weights of the families and species (after Motwani and Kanwai, 1970). The figures in this table could be compared with those of Table 8 (Ita, 1978) to show the variations in the relative importance of the species and families of fish in the river and the lake as recorded in 1976. Imevbore and Okpo (1975) attempted a comparison of the percentage composition of fish in the river between 1965 and 1966 and those captured in the lake in 1969 (Table 10). Their comparison shows a drastic decline in the population of the mormyrids which were first in importance in the river in terms of weight and second in terms of number (Table 10).
Table 11 shows a comparative checklist of fish species in the four major man-made lakes, Kainji, Kariba, Nasser and Volta (Ita and Morcos, 1981). A summary of Table 11 is shown in Table 12. Lakes Kainji, Nasser and Volta show some similarities in the diversity of their fish families with about 22, 17 and 22 fish families respectively. Kariba records over 12 families. Kainji, Nasser and Volta also record the highest number of species (102, 55 and 58 respectively). About nine fish families are observed to occur in all the four lakes.
A subjective classification of fish species in Lake Kainji into groups of relative economic importance with respect to market preference is shown in Table 13. A subjective index of abundance is also given indicating the present state of abundance of the various species. Also the maximum observed lengths and weights in the lake are indicated. Although the clupeids in Lake Kainji are of secondary economic importance, the clupeid population in the lake has not been tapped because of inadequate fishing methods (Atalla lift net attached to dug-out canoes) used by the local fishermen. However, Otobo (1977) estimates a standing stock of over 3 000 t of clupeids in the lake. This fish is not big enough for gillnet capture as is the case in Lake Volta.
Table 14 shows the standing stock of fish in Kainji Lake as estimated by rotenone sampling of inshore waters between 1975 and 1976 arranged in order of relative family importance. The cichlids dominated the catch with a mean standing crop of about 105 kg/ha followed by the bagrids with about 36 kg/ha and the characids, cyprinids, Mochokidae, Malapteruridae, Clariidae, Mormyridae, Centropomidae and Schilbeidae with about 25, 13, 6, 6, 5, 3, 3 and 0.4 kg/ha respectively; other species made up about 1.7 kg/ha. The relative percentage ichthyomass compares favourably with the percentage composition by weight in gillnet catches along the littoral margin of the lake in 1976 (Table 15). The gillnet data on Table 15 is the same as those used in estimating relative composition by number in Table 8. The distribution of fish families in the major habitats of Lake Kainji (shore, surface and bottom of deep waters) is shown in Table 16. Fish are distributed in the ratio of 1:1.7:1.9 at the bottom, surface and shore respectively, an indication of the fact that the shore or littoral margin is the most productive area of the lake. The mean ichthyomass (240 kg/ha) for the shore could be used for extrapolating the concentration of fish in other habitats using the distribution ratios.
Table 17 shows the ichthyomass and density of all species sampled with rotenone in order of decreasing ichthyomass with the cichlids, Citharinus, and bagrids topping the list. The numbering for both ichthyomass and density show order of decreasing importance. Again the data in this table could be converted to percentages for comparison with those in Table 8 from gillnet catches.
A comprehensive study of the food habits of fish species in Kainji Lake has been carried out with the objective of classifying the fish into their respective trophic levels. Table 18 shows a classification of the fish in the four major man-made lakes (Kainji, Kariba, Nasser and Volta) into their respective trophic levels based on the Kainji experience and a summary of food habit studies of other workers in the major lakes. The grouping of fish in Kainji Lake was based on the food items that formed over 50 percent of food of the fish in order to avoid classifying the same species within more than one trophic level.
The classification is intended to stimulate a unified classification of the African man-made lakes' fish species into their respective trophic levels for management purposes and for observing trends in trophic level fluctuations and forage to carnivorous ratios with time.
Tables 19 and 20 show the fluctuations in percentage composition by numbers and weights (respectively) for the fish families between 1969 and 1977. The families have been grouped in their respective trophic levels to permit the observation of the trends in the mean percentage composition of the primary, secondary and tertiary trophic levels and for the estimation of trends in the forage to carnivorous (F/C) ratio. Tables 21 and 22 summarize the figures in Tables 19 and 20, and Table 23 shows the trend in the forage to carnivorous ratio. The decreases in the F/C ratios between 1972 and 1975 are attributed to the effects and after-effects of the Sahelian drought within this period (Ita and Otabusin, 1981).
Table 24 shows a summary of the mean weights of fish species caught with a graded fleet of gillnets (mesh sizes ranging from 5 to 18 cm) in Kainji Lake between 1969 and 1977. The significance of the mean size at production and recruitment of species in the lake has been illustrated in Ita (1980a). The pattern of other species could be studied in the course of the analysis of the synthesis study.
Table 25 shows the estimated total annual fish catch and the number of boats in Kainji Lake after Bazigos (1972) and Ekwemalor (1978). The comparison of the mean catch per boat with the experimental catch per unit effort shows that the lake has attained a natural equilibrium and catch is fluctuating between 6 000 and 4 500 t/yr. Factors responsible for this natural equilibrium could be deduced in the course of the synthesis study.
There has been no systematic approach toward the management and development of the lake fisheries; hence, any observed trend in production is purely a natural control. Proposals have, however, been made (Ita, 1981a) toward a systematic management approach aimed at increasing the yield from the current 5 000 t to the pre-impoundment prediction of 10 000 t. Biological indices of overfishing in the lake have been highlighted and measures for control advanced. A minimum mesh size of 3 inches has been proposed as the most ideal for the lake. Methods adopted in arriving at this conclusion are contained in Ita (1981a). Table 26 shows vital statistics for the commercially important species in Kainji Lake which could be useful in the course of the synthesis if the same information could be obtained for other reservoirs. Tables 27 and 28 show the growth and fecundity rates of the major commercially important species. Growth rates of other major species and their fecundity rates are still under investigation. This information, it is still hoped, will facilitate management recommendations for the optimum production of desirable species in the lake within a state of balance between the prey species and the predators.
Processing, marketing and distribution of fish is still at the artisanal level. There has been no organized processing of fish for commercial purposes except that recently introduced by the Niger River Basin Development Authority (NRBDA) on a small scale, depending on supplies from about twenty local fishermen. Kainji Lake Research Institute is also processing fish (supplied by a small group of fishermen) on an experimental basis. Marketing and distribution are also carried out locally. Over 80 percent of the fish landed by local fishermen is smoked while the remainder is sold fresh. No salting or sun-drying of fish is carried out in Kainji Lake.
An experimental pilot fisheries development scheme is underway at two stations on the lake, one at Shagunu at the mid-western shore of the lake and the other at the south-western shore. The objective is to provide fishing inputs to a group of fishermen who are expected to sell their daily catches to the scheme operators and 20 percent of the sales is deducted as repayment for the loan in kind given to the fishermen.
Commencing with a minimum of input, the scheme could not handle the bulk of the fish initially supplied by the fishermen at Shagunu. Tables 29 and 30 show the production charts for the fishermen at Shagunu within a year's period. Tables 31 and 32 show a similar chart for the fishermen at Monai for a period of two months. The reasons for the fluctuations in the landings are still under investigation. Based on the preliminary model (Ita and Eyo, 1981) a conceptual scheme has been proposed to take care of the production in the lake at 8 stations and cater for a minimum of 200 fishermen at each station with an estimated average catch per boat of 10 kg/day.
The financial analysis for the proposed model is given in Tables 33 and 34. The estimated return on investment (ROI) will be worked out to encourage proposed investors. The prices of the various species of fish at source are shown in Tables 31 and 32 while the current costs of revolving inputs to 100 fishermen are shown in Table 35. The objective of this development plan is to bring all the fishermen under control by free registration as proposed in Ita (1981a) and therefore facilitate the management of the system.
Adeniji, H.A., 1975 Drought: Its effects on water management and plankton abundance in Kainji Lake, Nigeria. In Proceedings of the I.W.R.A. Conference on Water for Arid Lands, Teheran, Iran
Adeniji, H.A., 1976 Effects of the outflows from the Kainji Lake on the dissolved oxygen and water temperature in the river Niger just below the dam. Paper presented at the Symposium on Lake Volta, Legon, 6–10 September 1976
Adeniji, H.A., 1978 Diurnal vertical distribution of zooplankton during stratification in Kainji Lake, Nigeria. Verh.Int.Ver.Theor.Angew.Limnol., 20:1677–83
Ajayi, T.O., 1972 Biological studies on the family Bagridae (Pisces Siluroidea) in Lake Kainji, Nigeria. M.Phil.Thesis, University of Ife
Akintunde, E.A., 1976 The biology of Tilapia and Sarotherodon species of Lake Kainji, Nigeria, with special reference to Sarotherodon galilaeus. M.Sc.Thesis, University of Ife, 200 p.
Anthonio, Q.B.O., 1970 Fish marketing survey in the Kainji basin: Yelwa area study. Ibadan, Nigerian Institute of Social and Economic Research
Anthonio, Q.B.O., 1973 The traditional marketing organization for smoked fish in the Yelwa area of the Kainji Lake basin. In Kainji, a Nigerian man-made lake: Kainji Lake studies Vol. 2: Socio-economic conditions, edited by A.L.Mabogunje. Ibadan, Nigerian Institute of Social and Economic Research, pp. 71–89
Arawomo, G.O., 1976 Food and feeding of three Citharinus species in Lake Kainji, Nigeria. J.Fish.Biol., 9:3–10
Babalola, M.O., 1976a Fresh fish marketing around Kainji dam. Niger.Trade J., 23(3):41–5, July/Sept.
Babalola, M.O., 1976b, Fresh fish marketing around Kainji dam. Niger.Trade J., 23(4):10–5 (October/December)
Babalola, M.O.,n.d. Preliminary investigations into fishery problems around Kainji Lake. Report to Kainji Lake Research Project. (Unpubl.)
Bakare, O., 1968 Food and feeding habits of non-cichlid fishes of the middle Niger with particular reference to the Kainji reservoir basin. M.Sc. Thesis, University of Ife, 110p.
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Bazigos, G.P., 1971 Frame survey at Kainji Lake Nigeria. Rome, FAO, FI:SF/NIR/24, Statistical studies, 1:80 p.
Bazigos, G.P., 1972 The yield pattern at Kainji Lake, Nigeria. Rome, FAO, FAO/UNDP/SF/NIR/24, Statistical studies, 2:24 p.
Bazigos, G.P., 1974 The recent trends of the yield pattern at Kainji Lake. Rome, FAO, FAO/UNDP/SF/NIR/24, Statistical studies, 3:14 p.
Blake, B.F., 1977a The effect of the impoundment of Lake Kainji, Nigeria on the indigenous species of mormyrid fishes. Freshwat.Biol., 7:37–48
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Daget, I., 1961 Report on the Kainji dam project (fisheries). CSA/CCTA Fourth Symposium on Hydrobiology and Inland Fisheries, Fort Lamy, 61:69–76
Ekwemalor, A.I., 1975 Frame survey of Lake Kainji. Report to Kainji Lake Research Institute. 33 p. (Unpubl.)
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Ekwemalor, A.I., 1978 Frame survey of Kainji Lake, April 1978. Unpublished report to Kainji Lake Research Institute
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Ibeun, M.O., 1979 Kainji Lake Basin, Nigeria: a multi-disciplinary bibliography, 1957–1978. 72p.
Imevbore, A.M.A., 1970a The chemistry of River Niger in the Kainji reservoir area. Arch.Hydrobiol., 67:412–31
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Imevbore, A.M.A. and F. Adeniyi, 1977 Contribution on the role of suspended solids to the chemistry of Lake Kainji. In Interactions between sediments and freshwater, edited by H.L. Golterman. The Hague, Dr. W. Junk b.v. Publishers, pp.335–42
Imevbore, A.M.A. and O. Bakare, 1974 A pre-impoundment study of swamps in the Kainji Lake basin. Afr.J.Trop.Hydrobiol.Fish., 3:79–93
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