The Bangweulu perennial swamp (Figure 1) can be characterized as a vast, shallow, oligotrophic, seasonally fluctuating, but predictable aquatic system. The main inflow is via the Chambeshi River that enters the swamps from the east. The surplus water leaves the Bangweulu swamps in its southwestern part through the Luapula River, which later enters the Lake Mweru further north and subsequently connects with the Congo River system. Strong seasonal water level fluctuations with relatively low inter-annual variations (Figure 2) create annual changes in habitat availability (areas of inundation), pathwayas of fish dispersal and pulses of food availability.
The general low level of total dissolved solids in the water, resulting in low conductivity, rank the Bangweulu amongst the most dilute water bodies in Africa (Welcomme, 1972) with very low concentrations of phytoplankton (Table 1). The swamps are dominated by heavy stands of papyrus (Cyperus papyrus) along channels and permanent water bodies, fringed by a zone of Eleocharis sp. and Nymphea sp. Phragmitus pungens (reed) and Eleocharis dulcis is found on sandy shores of channels. Various sedges, spearworts, wild rice and hippo grass (Vosia sp.) are found on firmer ground. The dense emersed vegetation filters nutrients out of the in-flowing Chambeshi waters, especially papyrus, which is able to fix large amounts of nutrients.
FIGURE 1. Map of Lake Bangweulu and the perennial swamps with indication of the two sampling areas at Nsamba and Bwalya Mponda. The fisheries research station is located at Samfya. (Map drawn by Elin Holm).
FIGURE 2. Mean monthly and mean annual water levels of Bangweulu (recorded at Samfya). Mean annual maximum level (March) is 2.46 m (SD 0.60). Mean annual minimum level (October) is1.36 m (SD 0.31).
TABLE 1. Selected geographic, limnological and physicochemical data of the Bangweulu system.
|
Data |
Value |
Source |
|
location |
10°15’ - 12°30’ S |
Bossche and Bernacsek 1990 |
|
permanent swamp area (km2) |
5170 |
Toews 1977 |
|
total lake surface area (km2) |
2735 |
Toews 1977 |
|
floodplain area (km2) |
7101 |
Toews 1977 |
|
catchment area (km2) |
109469 |
Toews 1977 |
|
water depth swamps (m) |
1 to 2 |
own data |
|
mean annual water level fluctuation (m) |
1.2 |
Dept. of Water Affairs |
|
minimum water level as percentage of maximum |
46% |
|
|
Conductivity (mS m-1) |
26.5 to 34.3 |
Bos and Ticheler 1996 |
|
pH |
6.3 to 6.9 |
Toews 1977 |
|
O2% |
40-100 |
Bos and Ticheler 1996 |
|
total alkalinity (mg l-1) |
0.31 to 0.46 |
Bos and Ticheler 1996 |
|
Transparency (m) |
0.6 to 1.7 |
Bos and Ticheler 1996 |
|
Total dissolved solids (mg l-1) |
41.0 to 89.0 |
own data |
|
Water Temperature (°C) |
18.3 to 27.3 |
Toews 1977 |
|
Chlorophyll-a (microgram l-1) |
below detection level of 5 µg l-1 |
Bos and Ticheler 1996 |
Fishery research has been carried out intermittently in the Bangweulu lake and swamps since the late 1930s (Ricardo, 1938; Bertram and Trant, 1991), but little has been published in scientific papers or reports (Toews, 1977; Evans, 1978; Toews and Griffith, 1979). Since the late 1970s research has been limited, particularly in the swamp area. In decreasing order of data available the following information on the fisheries existed before the survey:
|
1) Yield: |
Fluctuates with a slightly increasing trend. Reported annual yield (mean ±SD) since 1952 is 11, 366 ± 2, 370 tonnes (Figure. 3) (Bazigos, Grant and Williams, 1975; Evans, 1978; Lupikisha, Musuka and Mung’omba, et al. 1992, Dept. Fisheries, Zambia). |
|
|
|
|
2) Effort: |
Frame surveys from 1965, 1971, 1973, 1975 (additional frame survey) 1992 and 1996 (additional frame survey) indicate no trends in fishing effort (Table 2). This is remarkable as in various African fisheries the numbers of fishermen follow at least the demographic growth in a country (van Zwieten, pers. comm.). The lack of demographic growth probably has to be attributed to the harsh living conditions in the swamps and the limited amount of islands to settle on. |
|
|
|
|
3) Growth: |
Reported for two species: Tylochromis bangwelensis and Hydrocynus vittatus(Griffith, 1975, 1977), based on scale readings |
|
|
|
|
4) Mortalities, biomass and production: No reliable information available. |
|
FIGURE 3. Reported annual yields from the Bangweulu fishery (1952-2000) together with the mean annual water levels (1956--95). Both trend lines are significantly different from 0 at the 95 percent confidence level. There is no statistical correlation between the annual yield and water level data.
TABLE 2. Recorded effort in the Bangweulu fishery.
|
Year |
No. fishermen |
No. actively involved |
No. canoes |
Source |
|
1965 |
5015 |
|
6 437 |
Anon., 1965 |
|
1971 |
5 193 |
13 878 |
5 475 |
Inoue, 1971 |
|
1973 |
|
|
8 739 |
Bazigos, Grant and Williams, 1975 |
|
1976 |
|
7 696 |
4 500 |
Evans, 1978 |
|
1992 |
4 800 |
10 240 |
5 900 |
Ticheler and Chanda, 1993 |
