For many years, little attention was given to the development or management of fisheries in reservoirs or other small water bodies. This situation has changed because of the increasing demand for fish and the overexploitation of many inland and marine waters. Many government bodies in the region are now focusing on the management of these small water bodies to make better use of their resources and to give people a little more fish and some extra income. Aquaculture and enhancement of fisheries should therefore be encouraged as a means of improving the nutritional status of the rural population.
The high productivity of small water bodies is largely a result of their large surface area to volume ratios, which is characteristic of most shallow waters (Howard-Williams and Ganf, 1981). Hence their ability to buffer environmental effects which act over the surface area. Diurnal temperature ranges may thus be greater than seasonal ones, and in some cases, such as Lake George, Uganda, and Lake Nakuru, Kenya, there is a diurnal rather than a seasonal cycle of stratification (Ganf and Viner, 1973; Melack and Kilham, 1974). In rather deeper lakes, the pattern of change is seasonal rather than diurnal (Figure 12). In this cycle, nutrients are locked in the hypolimnion during the summer and released at overturn, and there is a marked seasonality in productivity. Some water bodies do not stratify and never develop an anaerobic hypolimnion. An example of such a lake is Lake Manyame, Zimbabwe, which did not stratify during the summer (Cotterill and Thornton, 1985) although Lake Chivero, just upstream, almost always did, although stratification occasionally broke down (Marshall and Falconer, 1973; Thornton and Nduku, 1982b). This is clearly an effect of differences in their surface: volume ratio, exposure to wind and other factors that affect the water temperature. This is typical of the variability to be expected in small water bodies.
As a result of this thermal instability, there is a more rapid exchange of nutrients within the water column and between the water and sediments. Hence the levels of primary productivity tend to be considerably higher in small water bodies than they are in large ones. This is shown by the much higher oxygen production in the waters of the Mazoe and Mwenje dams, two medium-sized dams in Zimbabwe, compared with Lake Kariba, a very large, man-made lake (Figure 13). In Figure 13, note that Lake Chivero is eutrophic due to sewage effluent discharge, and its primary productivity is unusually high.
The end result of this high productivity is that the fish biomass in small waters is much greater than it is in large ones (Figure 141). The relationship between the size of water bodies and fish biomass is much stronger when it is expressed in terms of volume rather than area (Figure 15), reflecting the three-dimensional nature of a water body.
A higher fish biomass will obviously mean a higher potential fish yield. The mean annual yield of fish in a variety of small water bodies in southern Africa was 329 kg/ha, some 3.6 times higher than the average of 17 intensively-fished large to very large African lakes and reservoirs (Table 11). Similar differences can be obtained by using the predictive relationship between fish yield and lake area in Marshall (1984). This model suggests that a water body of 1 km2 should yield 355 kg/ha, compared with those of 10, 100 and 1000 km2, which would yield 200, 112 and 68 kg/ha respectively. The productivity of small water bodies is governed by a wide range of physico-chemical processes and it is difficult to make accurate predictions. Nevertheless, estimates of annual yield range up to 2 000 kg/ha for natural systems and 9 000 kg/ha for intensively managed ones (Table 12). These values highlight the considerable importance of small water bodies in Africa, and estimates of their potential yield range from 1 to 3 million t/yr (Bernacsek, 1984; Bellemans, 1989). In spite of this potential, limnology and fish biology in small water bodies, such as the numerous retention and irrigation dams that have been built throughout southern Africa, has never been adequately evaluated. Instead most attention has been focused on large inland waters, marine fisheries or on large aquaculture projects. This is because the production from individual water bodies is very small when compared to that from larger ones, and few of them are able to support full-time fishers.
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| The dotted line in (b) is the isopleth for pH; above the line the pH of the water was 6.5–7.0, and below it was in the range 7.0–7.5. | |
Figure 12. The seasonal cycle of (a) temperature (°C), (b) oxygen (% saturation), and (c) conductivity (μS/cm) in the Mtsheleli Dam, Zimbabwe.
Figure 13. Primary productivity in relation to depth in four Zimbabwean reservoirs.
(Data from Mitchell and Marshall, 1975; Machena, 1982)
Figure 14. Fish biomass in relation to lake area in a number of African lakes and reservoirs
Figure 15. Fish biomass in relation to volume in a number of African lakes and reservoirs
Table 11. Fisheries productivity of some small water bodies in southern Africa
| Lake or Reservoir | Area (km2) | Annual yield (kg/ha) | Source | |
|---|---|---|---|---|
| Mean | Range | |||
| Lake Mujunju (T) | 80 | 53 | 46–59 | 1 |
| Lake Jipe (T) | 39 | 77 | 1 | |
| Lake Ikimba (T) | 35 | 1 | 1 | |
| Lake Chivero (1) (Zw) | 26 | 120 | 2 | |
| Lake Babati (T) | 21 | 209 | 23–454 | 1 |
| Hartbeespoort Dam (SA) | 20 | 248 | 3 | |
| Hombolo Res. (T) | 15.4 | 148 | 46–428 | 1 |
| Lake Singida (T) | 12.3 | 181 | 41–338 | 1 |
| Ngwazi Res. (T) | 5.1 | 450 | 1 | |
| Mianji Res. (T) | 4.9 | 76 | 1–198 | 1 |
| Lake Tlawi (T) | 3.2 | 21 | 2–50 | 1 |
| Lake Basuto (T) | 2.6 | 1 066 | 61–1 838 | 1 |
| Lake Kindai (T) | 2.6 | 418 | 127–630 | 1 |
| Lake Rutamba (T) | 2.4 | 231 | 133–329 | 1 |
| Nhumbu Res. (T) | 2.4 | 37 | 1 | |
| Lake Gombo (T) | 1.4 | 550 | 1 | |
| Savory Dam (Zw) | 1.1 | 256 | 4 | |
| Kerenge Res. (T) | 0.8 | 124 | 59–194 | 1 |
| Mgori Res. (T) | 0.8 | 530 | 99–1 876 | 1 |
| Private farm ‘B’ (Z) | 0.8 | 185 | 5 | |
| Malya Res. (T) | 0.7 | 167 | 36–279 | 1 |
| Chamwende Res. (T) | 0.2 | 150 | 1 | |
| Chilanga dams (Z) | 0.25 | 77 | 5 | |
| Private Farm ‘A’ (2) (Z) | 0.14 | 2 857 | 5 | |
| Myombo Res. (T) | 0.02 | 450 | 1 | |
| Magindu Res. (T) | 0.01 | 200 | 1 | |
| Mean of small lakes | 11.08 | 329.0 | ||
| Mean of 17 intensively fished large African lakes and reservoirs | 2 383 (13–8 482) | 90.4 | 9–226 | 6 |
Notes: T = Tanzania;
SA = South Africa;
Z = Zambia; and
Zw = Zimbabwe.
1. Lake Chivero is referred to as Lake Mcllwaine in most of the literature.
2. This lake was known to have been fertilized.
Sources: 1. Vanden Bossche and Bernacsek, 1990;
2. Marshall, 1978a;
3. Cochrane, 1989;
4. Evans, 1982;
5. Gopalakrishnan, 1989; and
6. Hendrson and Welcomme, 1974.
Table 12. Estimated annual fish productivity from various types of water body in the tropics
| Type of water body | Annual productivity (kg/ha) | Data source | |
|---|---|---|---|
| Fish culture ponds | 400–9 300 | 1 | |
| Floodplains | 200–2 000 | 4 | |
| 570 | 2 | ||
| 500 | 3 | ||
| Shallow natural ponds | 50–1 000 | 1 | |
| Shallow lakes | 50–200 | 1 | |
| Shallow managed reservoirs | 30–150 | 1 | |
| (perennial) | 110–300 | 3 | |
| (seasonal) | up to 200 | 3 | |
| Large, slow-flowing rivers | 30–100 | 1 | |
| Deep lakes | 10–100 | 1 | |
| Deep reservoirs | 10–50 | 1 | |
| Small rivers and streams | 5–20 | 1 | |
| 3 | 2 | ||
| Swamps | 5 | 3 | |
Notes: All values from [1] Dunn, 1989, except for
[2] dry season values from Welcomme, 1985;
[3] data from Kapetsky, 1991; and
[4] from Delincé, 1992.
