5. SELECTIVITY OF GEARS USED IN THE FISHERY

Both in spatial and temporal patterns of operation and by design, all fishing gears and methods are selective. Each fish species has different catchabilities due to species-specific behaviour and habitat preferences. Furthermore behaviour and habitat preferences change over the different life stages that are susceptible to fishing, changing catchability. In a fully developed fishery as Mweru-Luapula a mixture of gears is used to harvest different parts of the fish community. In recent years a large database has been collected through participation of local fishermen in scientific fisheries data collection on most of the gears encountered in the fishery (van Zwieten, Goudswaard and Kapasa unpublished data; for methods see: Kolding, Ticheler and Chanda, 1996; Ticheler, Kolding and Chanda, 1998). Based on an initial examination of this huge database and personal observation of the authors, Table 5 is constructed. The table emphasizes species selectivity of the methods, with some spatial information on the selectivity as a result of the fishing pattern and the total effort in number of gear owners.

As described in the previous paragraph, the three dominant fishing gears in the Mweru-Luapula fishery are gillnets, traps and a type of scoopnet used in the light fishery. The largest output of the fishery is Microthrissa moeruensis (chisense) caught by a limited set of gear types. The dominant gear-type in the initial stages of this fishery - boat seines - required two boats and a crew of seven or eight. Boat seines have been replaced almost completely by the operationally simpler ‘Japan net’ or Chapani. A scoop method requiring one boat with a crew of four that is cheaper both in terms of investment and operational costs and maintenance. The fishery has an important by catch of the fish predators Serranochromis and Alestes. Many different gears and methods mainly based on gillnets target the endemic Oreochromis mweruensis, the second most important species of the fishery. Fishing patterns based on these different fishing methods - stationary, chasing by beating on the water (“kutumpula”) and seining methods (“chibata”; “sikide”) sometimes in conjunction with chasing - are easily interchangeable, and fishermen often switch between them. The two Serranochromis species and Tylochromis mylodon, mainly targeted through gillnets each dominate in different parts of the lake and swamps. Fish traps are dominant in the riverine and swamp area and mainly target clariid catfishes, but specialized trap methods target a large variety of other species. Next to these three dominant fisheries a large variety of specialized methods each target different species or groups of species.

The high diversity of methods used could be complementary to each other in exploiting different parts of the fish community. This diversity in fishing patterns can be considered as adaptations to the large diversity of species in the fishery. Characteristic of the specialization is that all gears require relatively low-investments (traps, hooks and lines, etc.) or, if requiring more investment as in case of gillnets, the gears can be used in a large variety of fishing patterns, thus making them highly adaptable to changing circumstances. All these gears also do not last long, a few years at the most, before they need to be replaced, again contributing to the high adaptability to changing circumstances. The only fishery that requires larger investments, the pelagic light fishery, has converged after initial years of experimentation on a low cost method, both in terms of capital and organization of labour. In the following paragraph we will discuss the result of both this adaptability and the huge increase in effort in particular of the gillnet based fishery.

TABLE 5. Fishing patterns of the Lake Mweru-Luapula fishery as gleaned from fishing effort (numbers of fishermen using a method), spatial patterns (area) and categorical selectivity (target and by catch of species) by gear type and method

¹ Most active methods are illegal. Reported effort through framesurveys for these methods is probably an underestimate;² Mapira = gillnetting at mid-water depths; ³ Kutumpula = chasing fish into the net by beating on the water with a knobbed stick;⁴ Seining = three types of seining are used. The number of fishermen refers to all methods;⁵ Chisense gears = all are light fisheries except chasing. Japan = a net hung in between two loose bamboo poles sticking from the side of the canoe, is lowered under the light and hauled inside by at least 2-4 people pulling ropes and poles. A boat seine is operated from two boats by 7-8 persons; method comparable to purse seine but the net is closed at the bottom only at hauling onto the canoe; Mutobi = handheld scoopnet operated by one person; Chasing = two women walking at kneedepth in the water who haul a piece of netting or cloth in between; ⁶ Previously Chrisichthys mabusi (Rich, 1986).

FIGURE 9. Development of catch rates in the fishery and in experimental surveys with gillnets of dominant mesh sizes in the fishery (76-102 mm in 1970s and 63-76 mm in 1980s and 1990s). Catch rates from the fishery are standardized to 100 kg/m gillnet; catch rates from experimental gear = kg/100 m². Water level is shown as annual deviations of the long term mean. Grey bars indicate years over which biomass-size distributions are calculated (see text)