The Roundtable met in Cairo, Egypt, on 11, 12 and 13 January 1983. Syntheses of information were presented on reservoirs in Egypt, Ghana, Madagascar, Mozambique, Sudan and Zimbabwe. This addendum to CIFA/83/Inf.5 provides some highlights of the Roundtable and is the forerunner of a more complete report to be completed shortly,
Reservoir fisheries in Africa are important by virtue of the area they occupy, some 30 000 km2 for reservoirs exceeding 10 km2, and for their contribution to overall inland fishery yield in Africa, which amounts to about 10 percent or 150 000 t.
The Roundtable emphasized planning at the early pre-impoundment stages of reservoir development as one of the means for increasing benefits from African reservoir fisheries. Discussions focused on improving capabilities for fishery yield prediction in reservoirs as a planning tool fundamental to estimating the kinds and magnitudes of fishery management and development inputs which would be required in future.
It emerged from individual presentations that some of the large, older African reservoirs are now nearing their maximum capacities for exploitation. In contrast, others remain considerably underexploited despite knowledge of the remaining potential for expansion of fisheries. In the former case attention was drawn to the need for periodic or continual post-impoundment fishery resources evaluation. Information on the status of fishery resources is required for planning for rational fishery management and for further fishery development. In this regard some new and relatively simple methods are available for resource evaluation. A summary of these methods will be included, as an appendix, to the report of the Roundtable.
An immediate (1983) continuation of the CIFA activity on increasing benefits from African reservoir fisheries will include two outputs. One of these will be a compilation of the individual national reservoir fishery syntheses together with an expanded report of the deliberations and recommendations of the Roundtable which will be published in the CIFA Technical Report series. The other will be the dissemination of “A Bibliography on African Reservoir Fisheries” also as a CIFA publication.
The Roundtable decided that a departure from the original plan of operation would better serve the objectives of the activity in increasing benefits from reservoir fisheries. Thus, rather than attempt to produce a comprehensive reservoir fishery planning manual as had been envisaged originally (see CIFA/83/Inf.5, p.2), preparation of short, concise CIFA technical reports which can address specific needs in planning for, managing and developing reservoir fisheries was opted for as a more useful and more readily attainable alternative.
The Roundtable identified a number of priority subjects for study. These, along with brief justifications for them, are listed below. It was judged that each study or review could be completed with about one or two man/months of effort. The results of these studies would be issued as separate CIFA technical papers each of which would make a significant contribution to improving existing African reservoir fisheries or would help in more efficient planning for new ones.
1. A review of small pelagic fishes in African natural lakes and reservoirs and critical evaluation of their present contribution to yield and potentials for exploitation
It has been recognized that pelagic fishes constitute a major fishery resource, either already under exploitation (Lake Kariba) or with a potential for it (Volta, Kainji). The species diversity in clupeid stocks of different reservoirs appear to constitute a major obstacle to application of methods developed on one reservoir, and more input is necessary into the evaluation of both the biology of species as well as into fishing technologies for the optimal use of this fish stock. The only example of introduction of clupeids into an African reservoir has proved that clupeids are eminently suited for large African reservoir conditions, filling there a usually vacant niche.
2. Formulation of a reservoir-specific yield prediction relationship based on updated catch data and morpho-edaphic indices from African reservoirs, and a search for other non-MEI parameters which can be used to increase the accuracy of reservoir fishery yield prediction
The yield-morphoedaphic relationship was recognized as the best tool available for predicting reservoir fishery yield potential before impoundment. However, the original relationship was based on a combination of African lake and reservoir fisheries data. From an ecological and morphological viewpoint the reservoir yield-MEI relationship should be separated from that for lakes. Data made available through the synthesis activity permit the calculation of an updated reservoir-specific MEI which should increase the accuracy and precision of this tool for estimating reservoir fishery yield potential. It was recognized that in addition to an improved yield-MEI relationship, non-morphological, non-edaphic parameters also could account for substantial variations in reservoir fishery yields. Thus such parameters will be sought to more finely tune the reservoir fishery yield prediction process.
3. A review of physico-chemical pre-impoundment studies with the aim at assessing the kinds and information necessary for prediction of reservoir fisheries potential
The present experience has shown that pre-impoundment studies often cover excessive collections of data irrelevant to fish production in future reservoirs.
The Roundtable recognized that the most relevant are the list of fish species, and the morphometry and geology of the future reservoir. A review of such studies should document how much of the pre-impoundment information has actually been used in predictions of fish potential, and what data - if any - should be collected in addition to such studies. The Roundtable discussion has shown that, for example, a better knowledge of the morphometry of lakes, reactive phosphorus availability and flow rates might assist in better predictions of the future fishery potential.
4. Study of the feasibility of the enhancement of reservoir fisheries through dam design and reservoir operation
Reservoir fishery yield potential is affected by dam design and by reservoir operation. The amount of drawdown, for example, can affect littoral biological production as well as the efficiency of fishing. Evidence was presented which suggests that there is leeway in reservoir operation for optimizing fisheries without jeopardizing the reservoir ability to produce power or water for irrigation. This should therefore be applicable to existing reservoir fisheries. It also follows then that design modifications in dams which result in improvement of fishery potential also are a possibility. Such modifications in the design could be entered in the early planning stages. It is the objective of this study to examine the extent to which fisheries interests can be accommodated in reservoir operation and dam design. It is envisaged that the study would be carried out by a fishery biologist/limnologist and an engineer experienced in the design and operation of large dams.
5. A review of the experience from flooding totally uncleared areas, partially cleared ones, or those without vegetation, and its implications for fisheries
Pre-impoundment clearance of terrestrial vegetation is expensive, and in cases of larger areas to be impounded it is impossible to carry it out. Experience has shown that the presence of drowned trees enhances fish productivity, but reduces fish catchability. A compromise is sometimes sought to decide on the area to be cleared prior to flooding. An evaluation of the present experience from different environments should assist in a better formulation of proposals during the pre-impoundment stage of future reservoirs.
6. Aquatic macrophyte development implications for reservoir fisheries
Some African reservoirs undergo two phases in aquatic macrophyte invasion: the first one is characterized by a rapid expansion of floating macrophytes, sometimes capable of carpeting many square kilometres of surface area; the second phase characterized by low numbers of floating macrophytes, and by the presence of a belt of littoral submerged and emergent macrophytes. Solid cover of plants during the first phase may have a negative impact on the fish population also making fisheries difficult or impossible. Littoral plants generally greatly enhance fish production. There is an urgent need to evaluate both the positive and negative impacts of macrophytes on fish production and fisheries, to be able to formulate recommendations for future reservoir projects
In addition to the priority activities listed above the Roundtable reached some conclusions as follows:
(a) Management of savannah reservoirs vs. management of tropical forest reservoirs
Savannah reservoirs have similar climatic characteristics and similar fauna and thus form a loose “management/development” set. In contrast, tropical forest reservoirs are relatively unproductive and are depauperate in fish fauna. Thus, for the latter introductions and in some cases continual stocking may be the most appropriate fishery management methods.
(b) Communications between resource managers, planners and developers
In one case effective communication of resource potential by fishery managers to planners/developers has resulted in a rational fishery management policy before resources were seriously threatened. Elsewhere in African reservoir fishery situations lack of such effective communication and transformation to policy has resulted in overexploitation of fishery resources, or threatens to do so as reservoirs approach their sustainable yields. Conversely, at least in part, ineffective communication between resource evaluators and developers has had the result that ample reservoir fishery resources have remained markedly underexploited. Clearly, lines of communication need to be opened and implementation of policy has to be effected if available reservoir fishery benefits are to be realized.
(c) Large reservoirs vs. small and medium-sized impoundments
Of necessity with one exception, the Roundtable has focused on large reservoirs. This is because they have received the most scientific attention and therefore the fishery information from them is most complete. Nevertheless, smaller impoundments are numerous and important locally for the fish they supply. These smaller reservoir systems may be more productive than their larger, better studied counterparts and relatively more easy to manage. A start towards attaining greater benefits from them may be made through compiling information on their physical and biological characteristics as well as their fishery resources.
(d) Management of cascade reservoirs
Cascade reservoirs, where more than one reservoir occurs on the same river, are becoming more and more common in Africa as maximum use is being extracted from the available water resources. The implications for fisheries are diverse. For example, it seems that such reservoirs are likely to be very different from each other and therefore there is little opportunity for comparable fishery situations to develop from one to the other. On the contrary it will be necessary for close pre-impoundment studies to see how the upstream reservoir may affect the fishery characteristics of the other so that negative impacts can be minimized while potential positive interactions can be taken advantage of in fishery planning.
(e) Stock differentiation and management in reservoir sub-basins
In comparison with large lakes, large reservoirs are less homogeneous ecosystems. For example, parts of reservoirs directly influenced by major inflowing rivers differ from the more lacustrine sections. Therefore overall reservoir predictions may not be adequate for resource evaluation or fishery management in specific sub-basins. For instance, the upper reaches of reservoirs maintain populations of originally riverine fishes which are migratory while the rest of the lake develops truly lacustrine fish populations.
(f) Fishery implications of the early, high-productive phase of reservoirs
Within a few years after dam closure reservoirs productivity peaks. Planners should take this into account. Advantage may be taken of this early high fish production phase, if fishermen are highly mobile. However, if they are not, care should be exercised that the fishing inputs (capitalization, manpower) are not significantly greater than those necessary for sustained fish production in the later, more stable phase of the reservoir.
