2. BACKGROUND

Large dams are increasingly becoming a characteristic component of river basins in Africa. As the 56 countries of the continent-region develop economically and industrialize an increased demand for water has emerged, both for production of electrical energy and for establishment of modern high yield agriculture (usually through flood control and artificial irrigation). Compared to most other continents Africa's water supply suffers from two major shortcomings: 1) a highly seasonal rainfall pattern resulting in most of the usable total annual runoff being available during only a few months of the year, and 2) a low runoff coefficient (due to high evaporation and evapotranspiration) which is aggravated by not infrequent droughts which can span several years and affect large areas. Producing electricity hydrologically is more attractive compared to other processes because it is cheap, safe and reliable. These factors in combination dictate that Africa must store water. Only reservoirs would seem to be able to meet the requirement for an adequate and reliable supply of water for the various users.

Generally, dams radically alter river hydrology upstream and downstream. In effect, they permanently (i.e. for the lifespan of the dam) create novel and artificial types of aquatic environments. The characteristics of these novel environments are determined (either intentionally or coincidentally) by the operation of the dam. The latter attempts to achieve one or more prime objectives (in order of certain priorities). Dam design closely reflects these objectives and places absolute constraints and limits on dam operation. A second major and critical constraint on dam operation is exerted by affluent flow against the dam but this is of course not under human control (except partly in the case of multiple impounding) and is ‘managed’ by statistical prediction.

The novel aquatic environments created by dams possess, as do virtually all aquatic environments, fishery potential. In fact, they always replace existing fishery potentials (or actual developed fisheries). From a cost/benefit perspective it is interesting to determine if impoundment will increase or decrease the overall fishery potential of a river basin (and in some cases the adjacent inshore marine waters). However large dams are never built solely or primarily to achieve an increase in the national fish supply, but rather fisheries usually occupy a secondary position on economic balance sheets when compared to other objectives (Table 1). While it can justifiably be argued that fisheries may not be accurately assessed in such economic analyses, ¹ there is usually little possibility that a dam may or may not be built as the result of the identification of a more or less desirable fishery option.

¹ Especially if losses within the basin due to impounding are not taken into account.

Table 1. Example of a cost allocation analysis of a multipurpose dam project. Note especially footnote (a). From Golze (1977).

Notes:
Item 1. The total annual cost is investment cost (construction cost plus interest during construction), amortized at the appropriate interest rate and estimated useful life period.
Item 2. Annual benefits are adjusted to annual equivalent benefits if there is a growth period for any project function.
Item 3. Alternative annual costs are the costs that would be necessary to provide each function by means of a single-purpose project.
Item 4. The lesser of the annual benefit or the annual cost.
Item 5. A cost incurred only for a specific function - in this example, the cost of power facilities.
Item 6. Item 4 minus Item 5.
Item 7. Calculated by dividing the remaining justifiable expenditure for each function by the total of the remaining justifiable expenditure.
Item 8. Calculated by applying the percentages of Item 7 to the total joint cost, which is Item 1 total minus Item 5 total.
Item 9. The summation of Item 5 and Item 8.

(a) An estimate is not necessary if it is apparent that annual costs would exceed annual benefits.

Of more practical importance is the setting of a management objective to maximize fish production from the novel environments which the dam will create. Since fish production is the resultant of the interaction of an indigenous (or transferred) ichthyofauna with the environmental conditions that it finds itself located in, it follows that manipulations of the environment can strongly affect fish production. Maximizing of fish production must therefore in the first instance be approached by optimizing dam design and operation towards that objective.

Baxter and Glaude (1980) have recently produced a review of the impact of dams in temperate and subarctic environments (mainly Canada). It is most instructive to compare their findings with those presented below for tropical (African) dams. There appears to be only a minimum of overlap between the two sets (i.e. deoxygenation of discharge water), and often what is considered to be an undesirable environmental effect in the temperate situation is either unimportant or even beneficial in the tropical situation (i.e. the creation of a drawdown zone). The transferability of scientific and engineering expertise from one to the other environment is therefore low. Engineers with temperate zone experience accordingly need to undergo an ‘Africanization’ process in order to design and operate dams in a manner fully compatible with tropical conditions. From a fishery perspective the most important point that engineers coming from temperate countries must appreciate is that freshwater fish is an important and in some cases the only dietary animal protein resource for local populations. This contrasts sharply with most temperate countries where freshwater fish is either a highly priced luxury consumer item or else the object of a leisure-time sport industry, but rarely (the case of the Cree tribe in the James Bay area of Quebec, Canada being a notable temperate zone exception) a major dietary animal protein resource. Because of the relatively high and dependable economic return ¹, it is always in the best interests of an African country to fully develop its freshwater fishery potential in all freshwater environments available - either natural or artifical - and this is especially true for the least developed countries.

It is not clear that dam design engineers and dam operators working in Africa are fully aware of the hydrological requirements for maximizing fish production in upstream and downstream environments. The purpose of this paper is therefore twofold: 1) to review some of the major effects that dam designs and operations have on upstream and downstream aquatic environments in terms of general biological production and specifically fish production, and from this review 2) to draw up a series of guidelines for maximizing fish production in impounded river basins for the express use of dam design engineers and dam operators, both in planning new dams in the future and in operating existing dams. To what extent these guidelines can be put into use will depend on the special constraints of each dam situation/project since each is a unique industrial organism.

Initially it was intended that this report be co-authored by an experienced tropical dam engineer. Due to certain constraints this was not possible. Some sections of the report no doubt have suffered as a result. It is hoped that the report will stimulate constructive discussions between dam engineers and fishery ecologists.

¹ A cost/benefit analysis table for an African reservoir would therefore differ radically from the temperate zone example in Table 1.