Lake Malawi's fishes are a source of food for millions and provide a livelihood for thousands by encouraging tourism, fascinating the scientific fraternity, enchanting aquarists around the world and maintaining ecosystem processes in the lake. From a fisheries and resource assessment perspective, the region is data-poor, but there is sufficient peer-reviewed and grey literature on the limnology, fisheries and ichthyofauna of the lake to provide a good overview of the state of the fishery. There are signs of over exploitation and an increasing fishing effort has resulted in decreased catch rates, depletion of larger, more valuable species in the fishery and species changes. The fishery is harvesting stocks that were formerly thought to be under exploited. Previous attempts to manage the fishery have been ineffective and long term strategies addressing overfishing will need to transform the fishery from an open-access to a limited access system. As important as direct intervention in the management of the fisheries, will be the management of catchment processes. Increased nutrient inputs; changes to the phytoplankton composition; sediment loading; nearshore water quality impacts and changing water levels threaten the ecosystem. Introduction of alien invasive organisms is an ever present threat to the ecosystem as well, due to continued development of small scale aquaculture in the region. The overriding causative factor for all these effects is the poverty of the lakeshore communities which do not have the economic privilege of being able to adapt their utilisation patterns.
Lake Malawi (09° 30′ – 14° 40′S, 33°50′ – 33°36′E, 472 m amsl), also known as Lake Nyasa or Niassa, is the 3rd largest lake in Africa and the 9th largest in the world, but it contains more fish species than any other lake. Its fishes are a source of food for millions, provide a livelihood for thousands, encourage tourism, fascinate the scientific fraternity, enchant aquarists around the world and maintain ecosystem processes in the lake. Of particular significance is the rapid speciation leading to the species flocks of cichlids and clariids, which challenge evolutionary biologists (Kornfield and Smith, 2000; Snoeks, 2001; Turner, 2007). As a result there is an abundance of peer-reviewed and grey literature available on the limnology, fisheries and ichthyofauna of the lake (Oliver, 2010).
A synopsis of the physical and limnological characteristics of the lake is provided by Bootsma and Hecky (2003). A summary of morphometric and hydrological data for the lake is provided in Table 1. Topographically, about one third of the lake's 1500 km long shoreline is steep and rocky while two-thirds are gently sloping sandy beaches or swampy river estuaries.
Despite its tropical setting, Lake Malawi is sufficiently far south to experience marked seasonal variations, including a cool winter from May to August, a dry season from September to November and a rainy season extending from late November to April. The lake level rises during the wet season, from rain that falls both on the lake and in the catchment, giving annual fluctuations of level between the extremes of 0.4 m and 1.8 m.
The lake is permanently stratified, being anoxic below depths of about 170–200 m. At shallower depths, water temperatures and lake stratification follow seasonal patterns. From September to December there is a warming of the surface waters and stratification intensifies. By May the upper 60–80 m is homothermal at about 27°C; during the cool windy season the thermocline weakens so that by July it is poorly defined and there is a gradual temperature gradient of 23°C at the surface to 22.5°C at 250 m. Internal waves occur, which may have amplitude of 50 m and a periodicity of 16–30 days (Eccles, 1974).
Lake Malawi is reported to have more species of fish than any other lake in the world and the number of described species increases substantially with every taxonomic survey (Ribbink, 2001). Fourteen families are represented in the lake basin of which the family Cichlidae dominates in terms of species richness, diversity and numerical abundance (Table 2). The Cichlidae within the lake basin comprise two principal phylogenetic lineages: the tilapiines and the haplochromines. The tilapiines consist of the genera Oreochromis and Tilapia. The only representatives of the genus Tilapia are two non-endemic species, Tilapia sparrmanii, which occurs in peripheral lagoons rather than in the lake itself, and Tilapia rendalli. The Oreochromis are represented by a small, endemic species-flock comprising four members, collectively referred to as chambo, and a fifth unrelated species, Oreochromis shiranus shiranus. All remaining cichlids are haplochromines, comprising 39 genera and between 700 and 800 species (Turner et al., 2001). All haplochromines are endemic, except for Pseudocrenilabrus philander in peripheral lagoons. Astatotilapia calliptera and Serranochromis robustus, formerly regarded as non-endemic, are now recognised as distinct lineages (Seehausen et al., 2002 for A. calliptera; S. robustus is awaiting formal publication). The origin and age of the endemic cichlid fauna currently forms the focus of much research and debate (e.g. Won et al., 2005; Genner et al., 2007), but other, more conservative (in terms of speciation) fish families and genera provide clear evidence for the history of fish colonisation of the lake and its inflowing rivers.
The lake's fish fauna reflects its complex geomorphological history. Much of the fauna has East Coast affinities. Geological evidence suggests that river systems in the Central Region of Malawi formed the headwaters of the Rovuma River prior to the development of the Malawi rift valley (Crossley and Crowe, 1980). Further links with the East Coast River systems result from the proximity of the Upper Shire River, draining Lake Malawi, to Lake Chiuta and Lake Chilwa, which are headwaters of the Rovuma system. Lake Malawi species derived from the East Coast fauna are species of the genera Barbus (e.g. B. atkinsoni), Bagrus, Marcusenius (bicuspid toothed species), Labeo, Oreochromis (species with four anal spines), and also, according to Banister and Clarke (1980), possibly related species of Labeobarbus in the Rufiji and Lake Malawi.
Other evidence links elements of the fauna with the Upper Zambezi system. The South Rukuru River, isolated from the lake by a series of falls over the rift escarpment, has a fish fauna distinct from that of all other rivers flowing into the lake (Tweddle and Skelton, 2008), probably as a result of river capture from highland tributaries of the Luangwa River system. The geological history of the Luangwa is complex and it has been suggested that it formerly flowed north east into an eastward flowing proto-Congo River (Stankiewicz and de Wit, 2006).
A number of species found in streams and floodplains on the northern lakeshore have strong affinities with the fauna of the Upper Zambezi, e.g. Pollimyrus castelnaui, Barbus bifrenatus, Clarias stappersii, Tilapia sparrmanii and Pseudocrenilabrus philander, while the predominantly Zambezian Hippopotamyrus ansorgii occurs in the North Rukuru River in the Nyika Plateau foothills (Tweddle and Skelton, 2008). Other species with Zambezian affinities such as Serranochromis robustus have colonised the whole lake. A suggested former link between Lake Malawi and the Lower Zambezi River via the Shire River (Banister and Clarke, 1980) was discounted (Tweddle and Skelton, 2008) because of the youthful nature of the linking Shire River rapids and the marked differences in fauna above and below the rapids (Tweddle et al., 1979). Ruo River links with the lake were also discounted (Tweddle, 1979; Tweddle and Skelton, 1998).
Lake Malawi supports what is arguably the world's richest, multispecies freshwater fishery. For example, more than 200 species have been recorded from single fishing localities during recent surveys of the artisanal and industrial fisheries in southern Lake Malawi (Weyl et al., 2005).
Figure 1 summarises the limnological conditions, last formal estimate of fishing effort indicators, potential and estimated yield and fishery status. Despite the importance of fishing to the lakeshore communities of the three riparian countries, none of the countries monitors the fisheries with the accuracy and effectiveness that is needed to develop well informed management strategies (Darwall and Allison, 2002). Only Malawi has a continuous time series of catch and effort data that can, at best, be used to highlight overall trends in the fishery (Figure 2).
The lake's fisheries are characterised by diversity in species composition, fishing techniques and mode of utilisation (Banda et al., 2001). Although the export of live fish as ornamental fish provides an important source of employment and revenue to the local economy, the majority of Lake Malawi's fish is harvested for food by industrial and artisanal fisheries. In Malawi, the total biomass of the catch from these two sectors has been stable since the mid 1980s, fluctuating around 31,000 t y−1, 80% of which is landed by the artisanal sector (Weyl, 2003).
The artisanal sector comprises many highly commercialised, small-scale ventures that support village economies (Ganter et al., 2001). The artisanal fishery uses beach seines, open water seines locally termed chilimira nets, gill nets, traps and longlines. Their fishing vessels range from plank boats with or without engines to dugout canoes (Weyl et al., 2005). In Malawi, monitoring data show a continuous growth of the artisanal fishing sector (Figure 2). Between 1999 and 2003, the number of gill nets almost doubled and the number of open water seines increased by 25%. There has been a similar growth of effort in Tanzania (Irvine et al., 2002) and Mozambique (Halafo et al., 2004). At least 44,000 t y−1 of fish are harvested annually from the lake as a whole.
Surplus production models fitted to Malawi's catch and effort data estimate potential yield for the artisanal fishery at 30,000 t y−1 (Weyl, 2003). These results, which are based on models that ignore multi-species and multi-fisheries interactions, serve as a rough estimate for the potential fish yield and not as management targets. If potential inshore yields for Tanzania and Mozambique are similar to those obtained from the central and northern regions of Malawi, then potential inshore yield approximates the 44,000 t y−1 that are presently harvested.
Industrial purse seining for chambo was initiated in 1943 in southern Lake Malawi (FAO, 1976). Experimental trawling in the mid 1960s led to the establishment of a pair-trawl fishery in 1968 to harvest demersal cichlid stocks in the shallow parts of the south east arm that were not exploited by the artisanal fishery (Tarbit, 1972). The pair trawl fishery, based on small, 38 HP open decked boats, was affordable and developed rapidly. By 1984, there were 20 operational pair-trawl units in the southern part of Lake Malawi. Despite this early success, and attempts to develop this fishery in central and northern Lake Malawi, only eight units were active in 2001 (Banda et al., 2001).
Stern trawlers were introduced into the fishery for bottom trawling in 1972 and for mid-water trawling in 1976 (FAO, 1976; Turner, 1977a). By 2001, six stern trawlers operated in the southern part of the lake, three of which are capable of trawling at depths in excess of 80 m (Banda, 2001).
State of freshwater fisheries science in the country
State of knowledge of freshwater fishes
It is virtually universal that growth of fisheries precedes the development of solid scientific knowledge of the resources that are being exploited. Consequently, pristine baselines against which to compare change are rare. Because the thorough knowledge necessary for informed management decisions is seldom available, adaptive management is the dominant process. Such difficulties are compounded within multi-species systems, particularly those where the closely related taxa of species-flocks are notoriously difficult to distinguish, as are the cichlids of the African Great Lakes. The species composition of catches, the original size of individuals within a species, the natural representation of species within communities, the interrelationships of members of the communities and hence their ecological roles, life-history, growth, resilience, genetic diversity as well as the ecological and evolutionary responses of populations and communities are all changed by exploitation. Whereas fisheries scientists focus on issues related to catch, effort, size and seasonality, the more fundamental, less obvious concerns that relate to the dynamics of communities which are driven away from the natural order by anthropogenically imposed impacts of fishing are seldom explored and are little understood. Yet, it is such understanding that might ultimately determine the success or failure of fisheries in sensitive ecosystems.
Knowledge of the fish, fisheries, ecology and evolution is expanding for Lake Malawi but, the state of knowledge of the fish, their taxonomy, their ecology, evolutionary pressures and fisheries is still inadequate, particularly in more remote areas. Also, those who have the knowledge are too few and many of those who are knowledgeable are not resident at the lake and not able to share or use their knowledge to benefit the lake-shore people or to ensure the sustainability of the resources upon which such people depend. Competent cichlid fish taxonomists are frighteningly rare and declining in number in the riparian countries and elsewhere. The need to build capacity, to retain that capacity at the lake and to provide the infrastructure and career paths for qualified people is pressing.
State of knowledge on freshwater fisheries
Mainly as a result of Malawi's long history of investment in fisheries assessments on the lake, the state of knowledge on the fisheries is adequate for decision making based on single species approaches and the response of mixed catch yields to harvesting. Directed research programmes were reviewed by Tweddle (1991), Jackson (2000) and Kachinjika (2001).
Initial surveys focused on the larger Oreochromis species, taxonomic studies, descriptions of traditional fishing methods, experimental fishing and limnological investigations (Bertram et al., 1939; Lowe, 1952; Fryer, 1959; Jackson et al., 1963). Experimental trawl surveys have been undertaken since the 1960s (Tarbit, 1972; FAO, 1976; Tweddle et al., 1995; Banda et al., 1997) and a catch and effort monitoring system was initiated in the early 1970s for all fisheries (FAO, 1976). This monitoring system comprises a 30-year time series for Malawi and forms the backbone of the fisheries assessments (FAO, 1993; Weyl., 2003) showing that the inshore stocks, particularly those of larger species, such as the chambo, have been and still are under increasing pressure and that there is overexploitation of some resources (Figure 2). Similar series are, unfortunately, not available in Tanzania or Mozambique.
Recent research has focussed on the assessment and development of offshore demersal (FAO, 1982; Irvine et al., 2002) and pelagic fisheries (Menz, 1995; Turner et al., 2000). These directed research programmes and biological research on many of the major commercial fish species have made available sufficient biological and fisheries data that can be used to guide management and apply more complex stock assessment models (Tweddle and Magasa, 1989; Thompson and Allison, 1997; Irvine et al., 2002; Kanyerere et al., 2005).
In 2001 an international panel of experts identified research gaps that constrained the management of the fisheries in Lake Malawi. They concluded that continued fisheries and biological monitoring and taxonomy were vital and that “the paucity of knowledge on the social aspects governing the behaviour of the fishing community was. a severe ‘bottleneck’ in the implementation of management measures” (NARMAP, 2001).
Current status of freshwater fisheries
Attempting to assess the status of one of the most species rich freshwater fisheries in the world is complicated by multi-species interactions in the fishery and the paucity of species-specific data. To define the status of Lake Malawi's freshwater fisheries we look at criteria for overfishing at both individual species level and at the ecosystem levels (Murawski, 2000; Allan et al., 2005). Available data indicate decreasing catch rates, biomass depletion and declining diversity, most notably in southern Lake Malawi where the concentration of fishing effort is greatest.
Decreasing catch rates
While total yield from the Malawi section of the lake has been fairly stable around 30000 t y−1, this yield is maintained through increasing effort; as a result, the catch per fishing gear or catch per unit effort (cpue) is declining (Figure 2). Such a response is a strong indicator of overfishing.
Depletion of high value fish
The most pertinent example of species specific biomass depletion is the decline of the fishery for the endemic Oreochromis(Nyasalapia) species flock (Lewis, 1990; FAO 1993; Bulirani et al., 1999; Banda et al., 2005). Chambo are the most valuable component of the fishery with prices at least twice as high as that of comparably large haplochromine cichlids (Weyl, 2003) and have been considered fully or over-exploited since the first assessment over 60 years ago (Lowe, 1952).
Prior to 1980 chambo cpue was primarily influenced by fluctuating lake levels, but post 1980, a strong negative correlation with increasing fishing effort was shown (Tweddle and Magasa, 1989). Chambo were caught mainly in southern Lake Malawi by artisanal gill nets and beach seines, with industrial purse seine catches contributing between 20 and 40% of the total 5000–9000 t y−1 catch until 1992 (Turner, 1994). The fishery crashed between 1992 and 1999 when annual catch decreased from 5000 t y−1 to about 2000 t y−1 (Figure 2). All subsequent assessments indicate overfishing (Bulirani et al., 1999; Weyl, 2001). Banda et al. (2005) noted no change to the collapsed status of chambo in southern Lake Malawi.
Comparative measures of biodiversity in the context of Lake Malawi are complicated by the high diversity of haplochromine cichlids and the paucity of species level fisheries assessments. Changes in the species composition in the pair trawl fishery in commercial area A in southern Lake Malawi (Turner, 1995; Turner et al., 1995; Weyl et al., 2005) provide an excellent example of species loss and replacement as a result of fishing.
This area was traditionally exploited by artisanal gill net and beach seine fishers targeting chambo (Turner, 1995). Haplochromine cichlids were considered underexploited (Tarbit, 1972) and the pair trawl fishery was developed to harvest these stocks (Banda et al., 1996). Soon after the initiation of pair trawling in 1968, the depletion of larger, slower-growing, late maturing species was noted (Turner, 1977a; 1977b). A reassessment of the fishery in 1991 demonstrated continued changes as a result of trawling (FAO, 1993; Turner et al., 1995). Despite effort limitations in this fishery (Banda et al., 1996) the catch composition in pair trawls continued to change and by 2001, two other major target species (Lethrinops stridei and Lethrinops sp. ‘oliverii’) had disappeared from the pair trawl catch and the cpue of eight other target species had declined, not only due to the effects of trawling but also to the impact of the growing artisanal fishery (Weyl et al., 2005).
Deep water demersal fisheries
Sustainable harvests from deepwater (>50 m depth) demersal fisheries in Malawi are in the region of 10,000 t y−1 for all trawlable areas (Banda et al., 1996; Banda and Tómasson, 1997; Kanyerere, 1999; Banda, 2001). There is evidence of locality-specific over fishing in southern Lake Malawi (Turner et al., 1995) and stern-trawl yield has exceeded the recommended sustainable harvests in the southeast arm of the lake since 2001. Development opportunities are, therefore, limited to the south west arm as well as the central and northern regions (Kanyerere, 1999; Banda, 2001). However, there is cause for caution as surveys in southern Lake Malawi have shown considerable overlap between artisanal and trawl fisheries (Weyl et al., 2005) and there is evidence that the artisanal gill net fishery is now operating at depths greater than 50 m (Weyl et al., 2000).
The pelagic ecosystem was last assessed using acoustic methods during the UK/SADC 1991–1994 assessment project (Menz, 1995). The estimated pelagic biomass was 168,000 tonnes with a sustainable exploitation potential of 33,000 t y−1 (Menz, 1995). The primary components of this offshore stock are pelagic haplochromine cichlids from the genera Rhamphochromis and Diplotaxodon and the small cyprinid Engraulicypris sardella.
This pelagic fishery is considered exploitable. Turner et al. (2000), on the basis of large population sizes and genetically homogenous stocks of Diplotaxodon and Rhamphochromis, concluded that the exploitation of adults on the relatively modest scale practised by the artisanal fishery was unlikely to pose any serious threat and that local stock collapses were likely to be replenished by populations from the rest of the lake. Engraulicypris sardella or ‘usipa’ has a high reproductive output, high natural mortality rate and a density independent larval survival which leads to a highly productive fishery requiring limited management intervention (Thompson et al., 1996). The usipa fishery does, however, fluctuate widely in response to environmental variables which may have major impacts on the fisheries for haplochromine cichlids.
The small scale usipa fishery is based largely on light attraction and the use of open water seine nets. During good usipa years this fishery focuses on this highly abundant cyprinid (Lewis and Tweddle, 1990) but in poor usipa years, the fishers have two choices, migrate or switch target species. In an assessment of this fishery during a “poor” usipa year, Weyl et al. (2004) recorded 75 species in the catches and showed that in some areas truly pelagic species contributed only 35% of the catch.
Pelagic fishes may also be exploited at higher levels than previously thought. In an assessment of the artisanal fisheries in the shallow area A, Banda et al. (2002) showed that truly pelagic species contributed 36% to the total catch. Thus, although the pelagic stocks are unlikely to be in present danger of overfishing, they cannot be considered unexploited. If the proportion of pelagic species in area A is used as a proxy for the fishery as a whole, the lake-wide pelagic fish yield may already be in excess of 28,000 t y−1. Further, the last assessment was more than 15 years ago and the development of a fishery based on such outdated data is not recommended.
In the context of Lake Malawi there are two aquaculture sectors: smallholder and commercial aquaculture. As a result of strong legislation prohibiting the use of alien species, both sectors are based on indigenous tilapiine cichlids. Smallholder aquaculture is based on modest earthen ponds in the lake's catchment. A recent survey of more than 400 smallholder fish farmers estimated that smallholder aquaculture production was in the region of 50–120 t y−1 (Andrew et al., 2003). This sector is unlikely to alleviate pressure on fisheries as optimistic projections indicate an output of, at most, 1750 tons by 2025 (Andrew et al., 2003). Commercial aquaculture is currently limited to a single cage culture operation in the southeast arm of Lake Malawi that in 2009 produced 400 tons and the anticipated production at full operational level is 3,000 tons (Shipton, 2006).
Current status of freshwater habitat
Collectively the work of Bootsma and Hecky (1999) and others show several correlations which suggest that water quality of the lake is deteriorating. As human activities in the catchment have increased, so too have erosion and consequent sedimentation. The inflow of sediment affects water clarity and light penetration, which reduces photosynthesis and productivity in areas of the inflowing plumes. On settling, the sediment mantles impact negatively on demersal algae and fauna in both the sandy and rocky habitats, sometimes burying completely the photosynthetic algae, stopping primary and secondary demersal productivity and smothering the habitats for the infauna of the algal mats. Effectively, the food resources upon which fishes of those habitats depend are lost. Local reduced productivity is a consequence. On the other hand, the inflow from rivers carries nitrogen, phosphorus and other nutrients into the lake, increasing productivity and eutrophication. This is a larger scale phenomenon and it can be concluded that the lake as a whole, but particularly the southern region and areas around major rivers, is becoming increasingly eutrophic. Such eutrophication is accompanied by a change in algal species composition which has been tracked over time (Kling, 1999), and the more beneficial algae are being replaced by potentially harmful blue-green algae.
The process is exacerbated by atmospheric deposition of nutrients, amounting annually to almost 50% of allochthonous input, mainly as a consequence of biomass burning within the catchment and the subsequent settling of material in the lake.
The three riparian countries have strong legislation with regard to fisheries and have institutions that have a direct mandate for research (Malawi Fisheries Research Institute [MAFRI], Tanzania Fisheries Research Institute [TAFIRI], Mozambique Fisheries Research Institute [IIP - Instituto de Investigação Pesqueira]) and fisheries management (Department of Fisheries, in Malawi and Tanzania respectively, Institute for Small Scale Fisheries Development [IDPPE- Instituto Naçional de Desenvolvimento da Pesca de Pequeno Escala], and National Directorate of Fisheries in Mozambique). Legislation mainly stipulates allowable fishing gear and the requirement for licensing of fishing gear and boats. Only the industrial fishery is effort limited. For the artisanal fishery effort limitation and exclusive rights are not included in legislature. In Malawi, however, fisheries policy incorporates Participatory Fisheries Management as a central theme; it legitimizes local community participation in management and provides the legal framework to implement co-management strategies (Allison et al., 2002).
What happens on land, within catchments, impacts on the lake and hence on biodiversity and normal ecological processes. It is difficult for Malawi, with its present large population size of relatively poor people in a natural resource-based economy, to reverse catchment trends. The worst hit fish faunas are the riverine and potamodromous species, including the endemic cyprinids, Labeo mesops and Opsaridium microlepis, which depend upon river health for their survival. Another endemic Opsaridiummicrocephalus, is also predominantly potamodromous, although there is evidence of breeding in the lake (Tweddle, 1995). Declining abundance of these species since the 1930s has been linked primarily to the degradation of river catchments and subsequent alterations in flow regime and sedimentation (Skelton et al., 1991). Fish communities close to river mouths and in the lake as a whole are also affected by activities within the catchments. Causes of erosion and flow of sediment, loss of breeding sites in rivers and changes of water quality in rivers and lakes need to be confronted and reversed. Socio-economic, political and cultural issues need to be factored into any problem resolution. In certain areas of Malawi the anthropogenic impacts are so deeply entrenched and of such long-standing that recovery of the catchment may be unrealistic, unachievable goals. In Mozambique and Tanzania, however, where riparian populations are smaller, there are still opportunities to develop terrestrial and aquatic parks, in which good husbandry of the catchment is possible. The existing communities who live in such areas should be involved in such a manner that the profits to them of conservation outweigh the losses of their existing unsustainable resource use on land and in the water.
Examples – biodiversity conservation
Successful resistance to alien fish invasion
The ill-advised introduction of alien fish has historically been considered a quick fix to problems in fish production but has often had disastrous consequences for biodiversity (Welcomme and Bartley, 1998). During the era of fisheries expansion in the 1950s, suggestions were made to introduce the Tanganyika sardine Limnothrissa miodon and tigerfish Hydrocynus vittatus into Lake Malawi (Jackson, 2000 [Ch 7: EAFRO, controversy and the big predators]), an idea resurrected (for the sardine) in the early 1980s (Turner, 1982). Fortunately, as a result of heavy opposition by the scientific community (Iles, 1960; and later Eccles, 1985; Barel et al. 1985) no direct introductions were undertaken. However, the threat of accidental introduction of alien fish introduced for aquaculture or recreational angling purposes in the catchment area of the lake is very real. Largemouth bass Micropterus salmoides have previously been stocked in the catchment but have not become established in the lake.
In recognition of the potential danger to the biodiversity and fisheries, Malawi has specific legislation requiring a permit for the introduction of any non-indigenous fish (Government of Malawi, 1997 [Part XI section 41(1) c]). As a result, no alien species are promoted for aquaculture in Malawi and species such as the common carp, Cyprinus carpio, that was previously introduced, were contained outside the Lake Malawi catchment. Fish introductions within the lake are limited to some instances of in-lake translocations of haplochromine cichlids (Stauffer et al., 1996; Genner et al., 2006) and the accidental introduction of the lungfish Protopterus annectens brieni by an ornamental fish trader (Tweddle, 1989; Weyl, 2004).
The risk of accidental introductions from aquaculture facilities remains. African freshwater aquaculture is dominated by the Nile tilapia, Oreochromis niloticus, which continues to be promoted for aquaculture development in Africa. Indigenous aquaculture species have lower growth performance and it is, therefore, likely that there will be increasing political pressure for the approval of the use of this species. This is particularly relevant when one considers that O. niloticus has already invaded several lakes and river systems in Mozambique and Tanzania (Canonico et al., 2005; Weyl, 2008).
Controlling effort in the fishery
The history of the fishery for Malawi's national fish, the chambo, is a typical example of problems facing inland fisheries worldwide. Chambo has been the focus of much research and attempted management interventions (Tweddle and Magasa, 1989; Lewis, 1990; FAO, 1993; Banda et al., 2005). Effort limitation in Malawi's fisheries has, however, not been implemented as a management strategy. By the time the chambo project (FAO, 1993) recommended that effort limitation was necessary to rebuild chambo stocks in Lake Malombe and the southeast arm of Lake Malawi, the fisheries had already collapsed (Tweddle et al., 1995). Despite this, the reduction of fishing efforts was not considered feasible and alternative methods, based on mesh size limits, minimum size allowances, closed seasons and gear size limitations were suggested (FAO, 1993). These measures were not however adequately implemented. The Chambo Restoration Strategic Plan (CRSP) of the Government of Malawi now recognises that the “restoration of the stocks to significant levels must involve reduction of the fishing effort…”(Bulirani, 2005). The CRSP also lays out comprehensive strategies for restoring the stocks including: stock enhancement; brush parks; protected areas; co-management and re-seeding. It remains to be seen whether these strategies will be implemented.
Recent developments to expand the offshore seine fishery (World Bank, 2000), for example, may negate CRSP goals to limit effort on chambo. In southern Lake Malawi this fishery harvests chambo (Weyl et al., 2004) and single fishery approaches are inappropriate given the large overlap in target species between fisheries (Weyl et al., 2005). Further, evidence of trophic shifts as a result of fishing (Irvine et al., 2002), raise the question whether the chambo stock will “rebuild” in an environment where density dependent intra-specific interactions are likely to reduce the availability of vacant niches.
As in many other freshwater fisheries (Allan et al., 2005), increasing fishing effort in Lake Malawi has resulted in decreased catch rates, and the depletion of larger, more valuable species has led to an overall decline in the value of the fishery. When viewed in conjunction with evidence of species changes resulting from fishing, it is apparent that the fishery is over utilised. There is also increasing evidence that the artisanal fishery is harvesting stocks that have long been thought to be under exploited. Long term strategies, therefore, need to consider this shift in artisanal fishing effort. As previous attempts to manage the artisanal fishery, first through top-down technical measures and later by attempting to limit effort through assigning property rights under a co-management framework, have been largely ineffective (Turner, 1996; Allison et al., 2002), it is unlikely that there will be a change in status quo in the short-term. Long term strategies addressing overfishing will need to transform the artisanal fishery from an open-access to a limited access system.
As important as direct intervention in the management of the fisheries will be the management of catchment processes. Bootsma and Jorgenson (2005) list increased nutrient inputs and changes to phytoplankton composition; sediment loading; near shore water quality impacts and changing water levels as threats to the Lake Malawi ecosystem. The causative factors are deforestation, suboptimal agricultural practices, biomass burning and climate change. We add the ever present threat of the inadvertent introduction of alien invasive organisms to this list. The overriding causative factor for all these effects is the poverty of the lakeshore communities who do not have the economic privilege of being able to adapt their utilisation patterns.
The authors would like to thank the conveners Frits Roest, Martin van der Knaap and Mohiuddin Munawar and the Aquatic Ecosystem Health and Management Society for inviting this paper, Jose Halafo for providing current effort estimates for Mozambique, and Anthea Ribbink for her careful editing of this manuscript. Two anonymous referees are thanked for their valuable comments on the manuscript.