The underlying assumption in this analysis of a long-term, steady state MSY in the Bangweulu swamps is a strong oversimplification. History shows that all fish populations fluctuate over time, with or without fishing. Nevertheless, almost all fisheries models used for assessing the stocks and evaluation of management options are from necessity built on the simplifying assumption of constant parameters and long-term equilibrium conditions. Only fishing effort and fishing pattern, the parameters we can control, are allowed to vary. The yield analysis presented in this study is only a snapshot of the conditions in the Bangweulu swamps during the two years under study. Long time series over decades of reliable catch and effort statistics and environmental parameters are needed to disclose the inherent dynamics of the system. Though not all gears are studied, the coverage of species and methods is assumed to give an impression of the result of the overall exploitation pattern in the fishery. Weirs and traps, the most important other gears in the floodplain fishery exploit the same set of species and sizes as studied here and are the least selective (Chanda, 1998).
As a snapshot, however, this study does provide some insight in the present exploitation pattern of the fish stocks in the swamps. A first conclusion is the seemingly remarkable adaptability of fishermen to fully harvest the various populations in a multi-species fishery. The present fishing in the swamps is dominated by a combination of gillnets, kutumpula and seines. Although fished with unequal intensity and number of units, and mainly targeting different species, each of these fishing methods contribute about the same amount to the total yield (Tables 10 and 11). In addition, the return rate per individual fisher from each method is surprisingly even and the distribution of fishing gears and mesh-sizes in the area seems well balanced to match the catch rates of each gear (Table 4 and Figure 5). The combination and relative proportion of gears, methods, and mesh sizes are on an aggregated level seemingly finely tuned, and result in a maximized yield at a multispecies level.
How to balance the appropriate fishing pattern in a complex multispecies fishery is still a hypothetical issue in most of the worlds fisheries and, will probably always be. All fishing gears and methods are inherently selective by their design and operation, and different fish species have very different catchabilities due to their habitat preferences and individual behaviour. Therefore, in a mixed multispecies fishery, such as Bangweulu swamps, also a mixture of gears should be utilized to harvest different parts of the community (Misund, Kolding and Fréon, 2002). In this study, long term yields for the artisanal gillnets, kutumpula, and seine fishery were examined under conditions of an unchanged fishing pattern (i.e. proportion of gears and mesh sizes) but with a varying effort. Judging from the perspective of each individual fishing gear (Table 10, middle part) both the gillnet- and the kutumpula fishery seem to be close to the optimal level (effort-factor = 1), whereas the seine fishery can be increased with a factor of more than three. When analysing each gear in isolation, however, the result for one gear does not account for the effects of the other two and thus simulates a single gear fishery. The general trend when all gears are combined (total fishing mortality) is that the present yield from the combination of fishing gears is very close to the calculated potential yield (MSY) (Table 10). Calculated MSY is reached at a factor 1.3 of the present fishing effort. However, very flat-topped yield curves due to high natural mortalities (Table 8) indicate that increased effort will have little consequences for the overall yield (less than 2 percent gain, Table 10). On the other hand, increased effort will disproportionately decrease the daily return (= catch rates) of the individual fisherman. Judging from the multispecies, multigear analysis of the eight most important species, the present fishing pattern and intensity in the swamps is very close to the calculated biological optimum in terms of maximizing the output. All three main fishing methods examined in this study exploit to a large extent a specific and separate part of the fish community (Table 7). In doing so, these methods complement each other and reflect the diversity in the fish community.
The estimated exploitation rates (E) of the various species examined (Table 10 and Figure 7) fall into three clusters: high, medium and low depending on the size of the species - which again is largely a function of their trophic level. The upper theoretical limit for a sustainable harvest commonly is set at an E = 0.5, and for some of the larger predatory species, this value was exceeded in individual gears (Table 9). However, looking at the species in all three gears combined (Table 10), the estimated exploitation rates of the species with a determined MSY within the simulated range of effort range between 0.16 to 0.63 with a mean of 0.36. The upper limit of 0.5 is only surpassed by Hydrocynus vittatus with a value of 0.63. This species yields comparatively little (1%) to the total catch in the overall combination of gears (Table 10). The general trend is that fishing mortalities are below or close to the theoretical maximum for large and medium sized fish species, but that they are negligible for most small sized fish species (Table 9, Figure 7). Overall, the larger predatory species seem to be exploited at rates close to MSY, while the exploitation of the smaller species is decreasing with size. The medium sized fish species - mainly cichlids - contribute the largest proportion to the total yield. Small fish species are mostly under-exploited. Most of the small species contribute relatively little to the total yield (with the exception of T. sparrmanii in stationary gillnets), although they are important in terms of the numbers in which they are caught (Table 9).
The fishery is characterized by generally small mesh sizes (mode at 38 mm) (Table 4). This causes the fishing mortality to rise sharply on most species from around 14-16 cm TL (Table 9, Figure 8). The general impression that exists of a decreased mean size in the fishery during the recent years is not therefore surprising considering such a large output of small specimens in the fishery. The question remains, however, if the fishing pattern has changed towards smaller mesh sizes. Unfortunately no historical data on mesh size distribution exist. Still, there is little doubt that the high fishing intensities in Bangweulu has influenced the stock sizes and that the observed decrease in mean size therefore may be true. The market, however, does not seem to differentiate on fish size: almost everything sells at the same price per unit weight. As the small mesh sizes indeed catch significantly more fish by numbers (Figures 5 and 9), the incentives of changing the fishing pattern to larger mesh sizes might therefore not exist at present. In addition, a number of important, but small, species (T. sparrmanii, P. catostoma, S. mystus, barbus species, etc.) are probably best exploited by the current fishing pattern of small meshed gillnets. In any case, the high number of small fish in the Bangweulu fishery is not an indication of overfishing as the yields have not declined and all the small species are only lightly exploited (Figure 7 and Table 9). The rationale of changing the fishing pattern is therefore also redundant from a biological point of view. On the contrary, increased use of larger meshed nets would only lead to an even higher fishing pressure on the larger species.
The combination of fishing methods and mesh sizes in the Bangweulu swamps harvest all species and all size classes from around 10 cm and upwards, creating an almost unselective fishing pattern. Still, mean exploitation levels increase with fish size due to the increase in exploitation range with size. Theoretically, a non-selective harvesting pattern is ecosystem conserving (Kolding, 1994,: Misund, Kolding and Fréon, 2002;, Jul-Larsen et al., 2003). All species are preyed upon at various rates during their lifespan, and for teleosts the highest mortality is usually during the early life history phase (Bailey and Houde, 1989; Caddy, 1991; Hutching, 2002). Thus in principle, the utopian but optimal exploitation pattern, by which a community structure - that is the relative abundance proportions of the populations -- could be maintained, is fishing each population in proportion to the rate of the natural mortality (M) it is subjected to (Caddy and Sharp, 1986; Kolding, 1994). If maximum yield is an additional objective, then the exploitation level should increase with trophic level: E = 0.5 for top predators and less for lower trophic levels, where E = 0.5-(M2/Z) depending on the predation mortality (M2) (Kolding, 1993, 1994). Although predation mortalities are not known, this is actually the fishing pattern that seems to exist in Bangweulu (Figure 7). As all fishing gears are more or less species and/or size selective, such non-selective exploitation patterns can only be achieved by employing a multitude of gears simultaneously. As demonstrated in this case study, multigear, multispecies artisanal floodplain fisheries that employ a very high gear diversity, often seem to be producing an overall species-, abundance-, and size composition that closely matches the ambient ecosystem structure (MRAG, 1994; Claridge, Sorangkhoun and Baird, 1997, Chanda, 1998;, Hoggarth et al., 1999a, 1999b). On the ecosystem level such an exploitation pattern could be considered unselective across the species diversity range. Many floodplain fisheries, particularly in Asia, seem to have persisted (albeit with natural fluctuations) with a very high and diverse fishing effort for as long as our observations can tell (Misund, Kolding and Fréon, 2002).
Putting all the above-mentioned aspects together, the impression surfaces that in the Bangweulu swamp fishery a well balanced way to exploit the stocks in all their diversity, using a variety of fishing methods, has evolved. The combination of gears and effort has created an exploitation pattern that appears to have maximized yield from the community without causing deep structural changes. Two of the fishing methods used - seines and kutumpula - are technically illegal but without these methods less than one third of the present and potential yield would be realizsed (Table 10). Actually, without these methods two of the most important species in terms of yields (M. macrolepidotus and particularly T. rendalli) would hardly be exploited (Table 9). The stocks do not seem to be overfished in a biological sense but there may be little room for expansion under the present overall fishing pattern (Table 10). On the other hand, there is no evidence of any significant changes in fishing effort over the past 30 years (Table 2). Furthermore, from a journey in the late 1930s Bertram and Trant (1991) write every dry spot in the swamps is inhabited and fishermen crowd together on tiny patches of floating papyrus beds. This descriptive record gives the impression that the fishing effort in the swamps has remained pretty constant for the past 70 years. It is therefore doubtful whether overfishing has ever been a problem in the Bangweulu swamps. The people of the Bangweulu swamps have always been fishing and they have had plenty of time to develop a fishing pattern that would suit the local conditions. From a biological point of view, effort or gear regulations do not seem to be a key issue at present and the catch per unit effort would, in terms of an economic break-even, probably be a regulating factor by itself (Beverton, 1990; Kolding, 1994). From an economic point of view, however, the fishery in the Bangweulu remains a marginal activity in which there is little room for expansion. If the number of fishermen would increase, it will become more and more difficult for individual fishermen to make a living out of it.
This study is a snapshot and should not be seen in isolation, as the Bangweulu swamps are not a more or less independent ecological entity. The swamps are part of a larger complex with a number of major lakes (Lake Bangweulu, Lake Walilupe, Lake Chifunabuli, Lake Kampolombo) and the Chambeshi and Luapula as major rivers. Both on a biological level and on the level of human interventions, many interactions between the different areas take place. There are water and nutrient flows between the swamps and major lakes and rivers, fish migrations take place, fishermen have a pattern of seasonal migrations between the different areas, etc. Therefore this study of the Bangweulu swamps, which can serve as a bench mark in future stock monitoring and assessments, should be followed by a study on the fish stocks in the open waters. As was the case for the swamps, hardly anything is known about the status, the exploitation patterns and the potential of the open water fishery at present. It is recommended that a full stock assessment through a similar set up with involvement of local fishermen in data collection is carried out for the open water fishery.
Simple data collected by local fishermen in combination with data on the size and structure of the fishery (obtained from Frame Surveys) can be used to estimate total annual yield and assess the fishing pattern. Our results give the impression that the fishers in the Bangweulu swamps have adapted their methods remarkably to fully harvest the various fish stocks and sizes. The combination and relative proportion of gears, methods, and mesh sizes seems to be finely tuned to maximize output without over-exploiting the stocks. In the Bangweulu swamps a well balanced exploitation pattern appears to have evolved in which a variety of fishing methods are being used and the fish resources are exploited in all its diversity. The fishing pattern has most likely evolved over time from trial and error based on individual catch rates. Individual return rates, rather than biological overfishing, might therefore be the most important regulating factor of the future fishery.
With the findings presented here it becomes apparent that the nation wide fisheries regulations, laid down in the Fisheries Act, do not suit the specific conditions prevailing in the Bangweulu swamp fishery. To be able to effectively exploit the diversity of species in the swamps, a variety of fishing gears are required, using small and medium meshed nets. Methods now used to exploit the different species, kutumpula and seines are prohibited and so is the most commonly used mesh size (38 mm stretched mesh). Further analysis is needed to investigate the impact of mesh size regulations, but at the present stage we feel it safe to conclude that kutumpula and seines are not detrimental to the fishery. On the contrary, these methods are harvesting parts of the fish community that otherwise would have remained untapped. The results show the importance to actually investigate the specific selectivity and impact of different fishing methods before making uniform regulations on their use. Many fisheries regulations still in force can be traced back to the colonial administration from the first half of the previous century (Malasha, this volume 2003) and have often proved to be fairly ineffective (Chanda, 1998). Striking a balance between enforcement of management regulations, and leaving room for fishers who are simply trying to survive is not easy. Therefore, the Department of Fisheries should, with the present adoption of the concept of co-management, consider directing all its management efforts towards monitoring and improving the working relation with the fishing community to be able to give advice based on sound scientific investigations.
The Fisheries Act of Zambia should be revised to leave room for differentiation between the various fisheries in the country. In this way specific regulations, adapted to the prevailing conditions in each individual fishery can be designed. For the Bangweulu swamps, kutumpula and seine netting should be legalized and until further analysis is done on the effect of the smallest mesh sizes the Department should tolerate these methods. Instead it should concentrate its efforts on developing and implementing the co-management concept in the fishery acknowledging that the local fishermen in this fishery to a large extent know what they are doing.