3. MWERU-LUAPULA: PHYSIOGRAPHY, PRODUCTION AND PRODUCTIVITY

The Mweru-Luapula fishery is situated in an area that is a clearly defined geographical unit, also recognized by the people of the region who refer to it as the “Luapula valley”. The Luapula river has its origin in Lake Bangweulu, from where a broad (a few hundred metres wide) swamp-like river system flows southward. Where the river hits the plateau it turns to the west and forms the border with the Democratic Republic of Congo (DRC). A steep waterfall, the Mambatuta falls, forms the beginning of a narrower river that meanders to the north to the place where it descends into a series of rapids called the Mambilima falls. This is where the lush and densely populated Luapula valley with its mango trees, cassava fields and the Mweru-Luapula fishery starts. From that point onwards the river broadens into a vast system of floodplains, marshes and permanent lagoons for about 150 km before it flows into Lake Mweru. The lake itself is about 115 km long and 45 km wide. Around the river mouth, the swamp-lake edge and to the Northeast of Kilwa Island, the lake is about 2 m deep. It gradually deepens to 10-14 m in the northern part of the lake. The deepest part of the lake is a narrow trough with a maximum depth of 24 m close to and along the northeastern shore (Bos, 1995). The lake discharges at its northernmost tip in the Luvua River that is part of the Lualaba River system. In the southern half of the lake the shores are formed by beaches, and dambo’s (vlei), while the northern half, where the Kundelungu plateau meets the lake on both sides, is formed by relatively steep hills and escarpments interspersed with small beaches that serve as landing places for boats. A45 km sandy beach forms the north edge of the lake. An almost continuous string of villages from the Mambilima falls in the south along the 300 km shoreline of the river and the lake, usually not exceeding a few hundred metres width, runs to the border with the Democratic Republic of Congo near the Luvua River in the north.

FIGURE 3. A. Annual average water level (thick line) and mean monthly water level (thin line) of Lake Mweru from 1955 to 1998. B. Long term average and minimum and maximum relative change in monthly water levels as a factor of the annual average water level.

Mean annual lake levels range over three metres, while highest minimum and maximum levels recorded at the gauge in Nchelenge in the south of the lake range 6.2 m (Figure 3A). Seasonal lows are reached in January while maximum levels are usually attained in May, with a long-term intra-annual fluctuation of 1.7 m. However, while the size of the seasonal fluctuation is highly variable and ranges almost 2.7 m the timing of the seasonal pulse usually is much less variable, with a shift of one month at the most (Figure 3B). Average water levels exhibited a slight decreasing trend over time, explaining nine percent of the variability. No correlation between water level (de-trended average, minimum, maximum or annual change in mean water level and amplitude) and Zambian demersal catch was found at any meaningful lag.

The proportion of the demersal catch to the Zambian freshwater fisheries output has decreased from up to 40 percent in the 1960s via 20 to 30 percent in the 1970s to around 10 percent since then. However, with the pelagic catch included Mweru contributes around 40 percent of the total catch of Zambia (Figure 1)! The total catch as reflected in the monitoring statistics of the Zambian fishermen of Mweru shows a slightly decreasing trend of 0.3 percent per year over the period of 45 years (r2 = 0.12, p = 0.02). It has a long-term annual average of 8 350 tonnes (Figure 1). Including the DRC the total demersal fishery is estimated from 12 500 (Kimpe, 1964) to 22 000 tonnes per annum. The productivity of the lake based on the “demersal” catch is estimated at 20-36 kg/ha; including the pelagic fishery productivity is 67-108 kg/ha^[6].

FIGURE 4. Development in fishing effort expressed as A. number of fishermen (= male gear owners), boats (plank boats and dugout canoes) and B. gears: chisense nets, gillnets, traps and long lines (number of hooks). ‘Gillnets’ include all methods such as beach-seining, drift netting, driving (“kutumpula”) and various kinds of open-water seining. Similarly three types of chisense nets and at least five types of traps are included in the numbers.