Ecological, environmental and socioeconomic aspects of the Lake Victoria's introduced Nile perch fishery in relation to the native fisheries and the species culture potential: lessons to learn

Authors


E-mail: jbalirwa@yahoo.com

Abstract

Inland fishery ecosystems in Africa are characterized by patterns of overexploitation, environmental degradation and exotic species introductions. Ecological complexity and diversity of aquatic habitats dictate that fishes in general are not evenly distributed in a water body. However, fisheries management regimes tend to ignore this basic principle, assume generalized conditions in a water body, and focus more on ‘desired’ objectives such as maximizing catch. The result is to disregard fish habitat boundaries and anthropogenic influences from the catchment that influence fish production. Overexploitation and environmental degradation disrupt sustainable socioeconomic benefits from the fisheries, create uncertainty among investors, but leave some managers calling for more information with the expectation that the fisheries will recover with time. Open access to the fisheries and full control of fishing effort remain challenges for managers. Exotic species introductions and fish farming can increase production, but such interventions require firm commitment to sound ecological principles and strict enforcement of recommended conservation and co-management measures in capture fisheries. The general tendency to downplay fishing effort issues, other ecosystem values and functions or rely on temperate fisheries models until a new cycle of overexploitation emerges, characterizes many management patterns in inland fisheries. Aquaculture is not an option to challenges in capture fisheries management. Aquaculture should be developed to increase fish production but even this practice may have negative environmental impacts depending on practice and scale. Decades of information on Lake Victoria fisheries trends and aquaculture development did not stop the collapse of native fisheries. The successfully introduced Nile perch (Lates niloticus) has shown signs of overexploitation and aquaculture has again been considered as the option. By reviewing significant trends associated with Nile perch and its feasibility in aquaculture this paper uses Lake Victoria to illustrate ‘special interest management’ targeting selected species of fish rather than the fisheries.

Résumé

Les écosystèmes africains où se pratique la pêche intérieure se caractérisent par des schémas de surexploitation, de dégradation environnementale et d'introductions d'espèces exotiques. La complexité et la diversité des habitats aquatiques impliquent que les poissons ne sont, en général, pas distribués de façon uniforme dans une entité aquatique. Pourtant, les divers régimes de gestion des pêcheries tendent à ignorer ce principe élémentaire, présument de conditions uniformes dans une entité aquatique et visent plus les objectifs « souhaités », comme des prises maximales. Le résultat, c'est que l'on ne tient pas compte des limites de l'habitat des poissons et des impacts anthropiques du bassin versant qui influencent la production de poisson. La surexploitation et la dégradation de l'environnement compromettent les bénéfices socio-économiques durables de la pêche, engendrent l'incertitude parmi les investisseurs et font que certains gestionnaires sollicitent plus d'informations dans l'attente que la pêche se redresse avec le temps. L'accès libre à la pêche et le contrôle total des efforts de pêche restent de vrais défis pour les gestionnaires. Les introductions d'espèces exotiques et les fermes piscicoles peuvent augmenter la production, mais ces interventions exigent un engagement solide vis-à-vis des principes écologiques responsables et l'application stricte des mesures de conservation et de co-gestion recommandées pour la pêche. La tendance générale à minimiser les problèmes des efforts de pêche et les autres valeurs et fonctions de l’écosystème, ou à se baser sur des modèles de pêche tempérés jusqu’à ce qu'un nouveau cycle de surexploitation émerge, caractérise de nombreux schémas de gestion de pêche intérieure. L'aquaculture n'est pas une option pour les défis auxquels fait face la gestion de la pêche. L'aquaculture devrait être développée pour augmenter la production de poisson, mais même cette pratique peut avoir des impacts environnementaux négatifs dus à l’échelle et à la façon dont on la pratique. Des décennies d'informations sur les tendances de la pêche et le développement de l'aquaculture dans le lac Victoria n'ont pas empêché l'effondrement de la pêche originale. La perche du Nil (Lates niloticus), introduite avec succès montre des signes de surexploitation et l'aquaculture a de nouveau été envisagée. En passant en revue les tendances significatives liées à la perche du Nil et la faisabilité de son aquaculture, cet article se sert du lac Victoria pour illustrer la « gestion d'intérêt spécial » qui vise des espèces de poissons sélectionnées plutôt que la pêche.

Introduction

Lake Victoria (surface area: 68,800 km2) (Fig. 1) in East Africa is currently known more because of fish exports, its stocked Nile perch (Lates niloticus) fishery, its endangered unique native fish fauna, eutrophication, and the exotic water hyacinth (Eichhornia crassipes). Uncertainty about Nile perch stocks has led to interest in commercial fish farming. The transformation of a 100,000 MT native tilapia artisanal fishery in the 1960s into a 500,000 MT export-oriented Nile perch fishery since the 1990s has not satisfied demand. The preoccupation with fishery yield, demand and markets appear to have influenced management to such an extent that past studies that were commissioned and results disseminated may have often been ignored. This trend characterizes what has been referred to as ‘Special interest management’ (Odum, 1971). Increased fish demand has led to interest in commercial aquaculture including the culture of the Nile perch to sustain fish exports and local needs without equivalent attention to past ecological trends governing capture fishery production.

Figure 1.

 Map of the Lake Victoria region illustrating its connections to other water bodies

Alteration in Lake Victoria's diverse fish communities prior to Nile perch introduction was originally because of overfishing the native stocks (Graham, 1929; EAFFRO, 1953–1967; Benda, 1979; Cadwalladr, 1969). The increased stock of Nile perch has since been accompanied by eutrophication (Hecky, 1993; Mugidde, 1993) and increased fishing pressure (LVFO, 2005). The socioeconomic success of Nile perch introduction to the countries surrounding Lake Victoria (Kenya, Tanzania, Uganda) is currently not doubted but the long-term future of the fisheries is uncertain partly because of not utilizing past experiences including key early observations in effective management. For example, Pringle's (2005) analysis revealed that even as early as 1921 there was evidence of declining fish yields especially of Oreochromis esculentus, but a scientific recommendation to impose a closed season was not acceptable to managers due to ‘the hardship to fishermen of a general closed season’ and lack of information on fish biology. The observed cycle of ‘special interest management’ and lack of information is likely to continue but will not be due to lack of information. This paper reviews significant aspects of the transformation of the Lake Victoria fisheries, Nile perch introductions and biotechnological issues in farming Nile perch.

General characteristics and socioeconomics of African lake fisheries especially Lake Victoria

Sub-Saharan Africa relies heavily on fish as a source of animal protein (FAO, 2004). Capture fisheries have till recently met a per capita consumption (15 kg) that has dropped to 10 kg (FSSP, 2005) while fish catch increased. The trends observed in the Lake Victoria fisheries (overfishing, sequential species loss, introductions and pollution) characterize many inland fisheries in Africa. The fisheries have gone through fundamental and often dramatic structural and environmental changes that are primarily linked to high populations around lakes (Crisman et al., 2003), improved fishing gears and methods, an influx of fishers employing illegal, unregulated and destructive fishing practices, and pollution from catchments. Other characteristics include various forms of overfishing (e.g. reduction in size at first maturity, changes in fish population structure), water quality and fish habitat deterioration.

Fish introductions have either been considered or have been carried out in lakes such as Kyoga, Naivasha, Kariba, Malawi/Nyasa. By the year 2000, Nile perch from Lake Victoria was among the most important export commodities in East Africa. The estimated annual catch is worth at least US$544 million at the fish landings in addition to US$243 million in fish exports in 2003 (LVFO, 2005). However, fish processing factories established in the 1990s have led to rapid expansion of fishing effort. The new fishing regimes now simulate fishing patterns earlier observed in the native fisheries that collapsed. While Nile perch catches have started to decrease (LVFO, 2005; Matsuishi et al., 2006), management interest lies in more information on the abundant but small (<60 mm) indigenous cyprinid, Rastrineobola argentea.

The rapid proliferation of the aquatic exotic weed, water hyacinth (Eichhornis crassipes), in Lake Victoria during the 1990s had negative impacts on fisheries, lake transport, water quality and other uses of the lake. Due to the severity of the infestations (12,000 ha in 1998), Uganda considered the use of herbicides to quickly control the weed (Kayanja, 1998). After the massive collapse of water hyacinth in 1998 due to a combination of the effects of biological control, localized nutrient depletion and hydrological conditions (Balirwa, Mugidde & Ogutu-Ohwayo, 2004), no firm mechanisms were followed to deal with potential resurgence. This approach suggests the tendency to look for short-term solutions to fishery issues. Introduction of exotic fish species was suggested as a solution to reviving the fisheries (Graham, 1929; Anderson, 1961; Pringle, 2005) even though the introduced species may not fully compensate for the lost productivity (Graham, 1929; Fryer, 1960; Jackson, 2000) especially when subjected to intense fishing pressure.

In comparison with the native fishes especially O. esculentus and Labeo victorianus, Nile perch was initially not a popular food fish around Lake Victoria. From the 1980s, there was more concern about the ‘new’ exotic fish on the ecology of native species especially the haplochromines (e.g. Ogutu-Ohwayo, 1985, 1990a,b; Ogari & Dadzie, 1988; Witte et al., 1992a,b). The strong demand for Nile perch from the 1990s in the external market led to suggestions that despite the positive development from an economic benefit and food resource viewpoint, there were concerns related to the distribution of benefits, food security and sustainability (Abila & Jansen, 1997; Okeyo-Owuor, 1999; Balirwa et al., 2004). The increased economic value of the ‘new’ fisheries may have overshadowed attention to the lake's fishery trends and limits to fishery productivity. As Nile perch requirements of the export-oriented fishery began to exceed yield from lakes, fish processing plants started operating below 50% of installed capacity (Table 1). Therefore, it is not surprising that options including culturing Nile perch (Uganda Fish Farming News, 2003), targeting deeper water stocks and exploiting R. argentea (LVFO, 2005) have been advanced.

Table 1.   Fish processing versus operational capacity (tons day−1) in Ugandan-based factories during 2004
FactoryInstalled/actual capacity (tons days−1)FactoryInstalled/actual capacity (tons days−1)
  1. The actual fillet export volume for 2004 = 29,830 tons (Source: Lvemp Aide Memoire April 2004).

120/10920/15
220/101030/15
325/151120/15
430/151220/15
530/1513100/00
620/151420/15
720/101515/00
830/20Total420/185 (44%)

Information needs and tropical inland fisheries management challenges

Lack of reliable information to guide management has sometimes been used as a major reason to take limited or even no action. The research record suggests otherwise. For the Lake Victoria fisheries the best available information includes at least 2000 references ranging from probably unused historical records found in Graham (1929), EAFFRO (1953–1967), associated records of the Lake Victoria Fisheries Service (LVFS), Annual Reports of Fisheries Departments to information summarized by Balirwa et al. (2003, 2004), LVFO (2005) and most recently, Pringle (2005) and Matsuishi et al. (2006). As a follow-up to lack of information since the last century, Pringle (2005) recalls evidence of reported overfishing with imported nets. Following Graham (1929), fisheries management measures especially the 5-inch mesh gill nets regulation in Lake Victoria were designed to conserve declining O. esculentus stocks. Graham's (1929) recommendations were never fully carried out; apart from the 5-inch gill net, mesh size limit intended to conserve the native tilapia fishery, the study had also recommended taking full control of the fishing power, essentially managing the fishing effort, a measure not yet achieved.

The establishment of a research base, the East African Fisheries Research Organisation (EAFRO) at Jinja in 1947, and the LVFS led to data collection and analysis to provide more information for management of the stocks of the native tilapiine fishes (Lowe-McConnell, 1997). As stocks of tilapia were declining, it was also scientifically established that stocks of the anadronomous Labeo victorianus were declining because of seasonal exploitation of gravid spawning females at river mouths (Cadwalladr, 1965). Other anadronomous species that were affected but could have been protected through seasonal closures are Clarias gariepinus and Barbus altianalis but mesh size regulation was the primary management concern.

Management challenges are not unique to Lake Victoria where the native tilapias, large cyprinids and catfishes disappeared from the catch. The fisheries of Lake Albert (where Nile perch is indigenous) and Lakes Kyoga and Nabugabo (where Nile perch was also introduced) have been transformed even under less fishing power, less industrialization, catchment and urban impacts that characterize Lake Victoria. In Lake Albert, the large-sized ‘moon fishes’ (Distichodus and Citharinus) drastically declined in commercial catch by the 1950s while the Lake George-based ‘The Uganda Fish Marketing Cooperation’ (TUFMAC) collapsed after overfishing the tilapia stocks by the 1960s. In all these lakes, the intensification of fishing led to a shift (fishing down process: Balirwa et al., 2003) to small-sized species (e.g. R. argentea in Lakes Victoria, Kyoga, and Nabugabo, and, Neobola bredoi in Lake Albert).

Management of Africa's inland fisheries requires a mixture of basic decisions and the precautionary approach (FAO, 1995) that should reflect the ecology of a tropical lake rather than of temperate marine fisheries. Lake Victoria for example is a primary source of protein food and income in the region. It is not a sport fishery. Fishing is also a culture of many communities, the majority of whom are semi-skilled artisans with basic or no formal education. It is a major challenge to distinguish between inland tropical fisheries and temperate marine environments that are primary sources of mostly predatory food or sport fish. The existence of numerous landing sites (rather than ports or harbours) and landing times (often throughout the day and year) in inland African fisheries makes annual yields difficult to determine accurately. Moreover, not all fisherfolk declare their catch, number and types of gear used, location fished or species caught. In temperate environments, fewer types of fishing craft and gear target fewer species-specific fisheries than in inland tropical fisheries where fishing crafts range from rafts, paddled dugout to variously motored boats. There are other factors such as the relative importance of catch from illegal unregulated and unrecorded fishing in comparison with legal fishing gears and methods. Management of African inland fisheries requires understanding the dynamic biotic and abiotic factors operating within and outside of the fisheries.

The Lake Victoria fisheries prior to species introductions

By the beginning of the 20th century, the native fish fauna of Lake Victoria was sparsely fished with simple traditional methods, with O. esculentus as the most important species (Graham, 1929). Other exploited species included O. variabilis, L. victorianus and several catfishes and momyrids. The decrease in catch per net per night from 50–100 fish in a 50-m long 127-mm mesh gill net to <0.5 fish in the same net by 1970 (Kudhongania & Cordone, 1974) was accompanied by a reduction in mean size and size at first maturity (EAFFRO, 1953–1967). The ‘fishing down’ process also drastically reduced many other native large-bodied fishes such as Bagrus docmak (Benda, 1979). Indiscriminate fishing during breeding seasons virtually eliminated the anadromonus L. victorianus from the fishery (Cadwalladr, 1965, 1969). By the 1960s, most large-bodied fish species in Lake Victoria were overexploited (Benda, 1979; Ogutu-Ohwayo, 1990c). Species introductions in Lakes Victoria and Kyoga, and the promotion of aquaculture were the major response. Trends in the native fisheries of Lake Victoria suggest that without firm management, stock depletion and changes in species richness can be caused by overexploitation which may occur simultaneously with environmental degradation.

Species introductions and the transformation of Lake Victoria fisheries

Nile perch (Centropomidae: Lates niloticus L., 1758) is a predator that supports economically valuable lake fisheries in East Africa (e.g. Victoria, Albert and Kyoga). The species naturally occurs in the Nile system below Murchison Falls (Fig. 1, a natural barrier for upstream environments) as well as in the Congo, Niger, other parts of West Africa and in Lake Turkana. In Lake Victoria, where it is an introduced fish, Nile perch can grow to a length of 2 m and a weight of 200 kg. The fish produces 3–15 million eggs per spawning (Ogutu-Ohwayo, 1985), breeds throughout the year with peaks during rainy seasons, with juveniles concentrating in littoral areas (LVFO, 2005). The species grows to 21–28 cm in 1 year (Ligtvoet & Mkumbo, 1990) but some studies suggest a growth rate of 20–40 cm (Ogutu-Ohwayo, 1994; Gregory & Orukan, 2004).

The distribution of Nile perch in Uganda spread to lakes Kyoga, Nabugabo and Victoria, through stocking with specimens from Lakes Albert and Turkana during the 1950s and early 1960s (Ogutu-Ohwayo, 2004) with the management objective of converting the unexploited but abundant haplochromines into a larger fish of greater commercial and recreational value (Graham, 1929; Anderson, 1961). Due to its predatory habit, concerns about the likely impacts of Nile perch on the native species and its own sustainability in the new habitats were widely debated (Graham, 1929; Fryer, 1960; Jackson, 2000; Pringle, 2005) but recommended studies before Nile perch could be officially stocked were not carried out, with the initial stockings being secretive (Pringle, 2005).

Research initiatives for management on Lake Victoria between the 1960s and 1970s (Lowe-McConnell, 1997) shifted attention to stock assessment with the objective of commercial exploitation of the haplochromine fishes as Nile perch then contributed <1% of the fish biomass (Kudhongania & Cordone, 1974). There were management expectations that the over-represented and under-utilized haplochromine biomass could support industrial fisheries. The short-lived trawl fishery in the Tanzania part of Lake Victoria during the 1970s and 1980s rapidly reduced haplochromine stocks indicating local overfishing (Witte & Goudswaard, 1985; Ligtvoet et al., 1995) and was abandoned. As Nile perch expanded rapidly through the 1980s, fishing techniques and fishing effort adjusted to the ‘new’ sources of marketable fish. By the mid-1980s fish catch had increased to over 400,000 MT, 80% as a result of Nile perch caught using 5-inch gill net.

Bottom trawling in the Ugandan waters of Lake Victoria during 1981–1985 yielded only two of the non-cichlid species Gnathonemus longibarbis and Brycinus found in the 1969–1971 surveys (Okaronon, Muhoozi & Bassa, 1999). Haplochromines in the trawl declined from 91.4% in 1981 to almost zero in 1985, while the contribution of Lates niloticus increased from 5% to 96% over the same period. The mean catch rate for all fish species combined declined from 595 kg h−1 in 1981 to 355 kg h−1 in 1983 and to 150 kg h−1 in 1985 (Fig. 2).

Figure 2.

 Mean catch rates of fish caught in the 4- to 30-m water depth area during bottom trawl surveys (1971–1999) in the Ugandan portion of Lake Victoria (from Okaranoon et al., 1999)

Frame surveys carried out every 2 years since 2000 (LVFO, 2005) have revealed that fishers on lake Victoria increased by 36% in 2002 (from 129,305 in 2000 to 175,890 between 2000 and 2002). In the same period, the number of fishing crafts over the lake increased by 24% (from 42,483 to 52,481), that of gill nets increased by 50% (from 655,053 to 984,084) while the number of long line hooks increased by more than 100% (from 3,496,247 to 8,098,023). The increase in boat power (outboard engine-propelled) of 47% further indicates expansion and intensification of the fishery. By analysing catch and effort data and modelling expected scenarios, Matsuishi et al. (2006) concluded that Nile perch exhibits classic indicators of overexploitation such as decline in total catch over time.

Ecological and environmental changes in the Lake Victoria fisheries

The increase in the Nile perch stock in Lake Victoria was accompanied by a reduction in the native fish species (Ogari & Dadzie, 1988; Ligtvoet & Mkumbo, 1990; Ogutu-Ohwayo, 1990a,b). The haplochromines which were the most abundant species and were expected to form the bulk of the food of the Nile perch, became rare. As a predator which can ingest prey a third of its length (Ogutu-Ohwayo, 1985), the haplochromine fishes that grow to much smaller sizes in length (≤20 cm), juveniles of other fishes and macro-invertebrates are also potential prey of the Nile perch within a year of Nile perch growth.

The 600+ species haplochromine cichlids (Kaufman, Chapman & Chapman, 1997) that comprised diverse trophic levels were reduced by up to two-thirds (Witte et al., 1992a,b). The total colonization of Lake Victoria by the Nile perch in the 1980s led to sequential shifts in the trophic structure of the haplochromine prey base, through other trophic levels such as Caridina nilotica (freshwater prawn), the native R. argentea as well as auto-predation of juvenile perch (Ogutu-Ohwayo, 1985, 1990a,b, 1993; Goldschmidt, Witte & De Visser, 1990; Hughes, 1992; Witte et al., 1992a,b; Goldschmidt, Witte & Wanink, 1993). By 1988, the major types of prey eaten by Nile perch in Lake Victoria had changed to the prawn, Caridina nilotica, Anisoptera nymphs, Nile perch juveniles and tilapiines with very few haplochromines. These remained the main types of prey eaten by Nile perch up to 2000, after which the proportion of haplochromines in the diet started to increase suggesting relatively short-term (two decades) fluctuating impacts of the Nile perch. The dynamic predator–prey balance in conjunction with longer-term influences have contributed to current patterns in life history parameters of the introduced predator. For example, the size at first maturity of Nile perch in the Ugandan waters between 1964 and 1977 was 54 cm for males and 68 cm for females (Acere, 1985). This changed between 1988/92 to 60–64 cm for males and 90–99 cm for females, with the differences attributed to availability of food for the Nile perch (Ogutu-Ohwayo, 1994). There have also been changes in proportion of male to female Nile perch since the species was introduced. Between 1964 and 1967, the proportion of mature females to males in the lake was (60:100) but by 1982, the proportion had reduced to (20:100). A related pattern in the body condition has also been reported (Ogutu-Ohwayo, 2004) prompting the need to continue monitoring the fisheries.

The increased eutrophication (a doubling of nutrient inputs, increase in algal biomass by 6–8 times) led to anoxic conditions unsuitable for fish in the deeper parts (Hecky et al., 1994; Verschuren et al., 2002) with some haplochromines becoming almost extinct (Seehausen, Van Alphen & Witte, 1997) but anoxic tolerant forms (e.g. Caridina nilotica and chironomids) becoming abundant (Balirwa et al., 2004). The conversion of the fishery from a multi-species system exploiting native fishes to one in which only three species make up almost the whole catch, two of them introduced, is a result of three major events (Balirwa et al., 2003): (i) Fishing intensified over the century by the successive introduction of new technologies – commercially produced, artificial fibre nets, outboard motors, and improved fish processing and marketing systems. (ii) The introduction of five new species of fish influenced the faunal composition and balance. (iii) Increased human occupancy and associated agricultural activity, industrialization, and change in fish composition (i.e. shift from primary to secondary and tertiary consumers), and the denudation of the lakeshore increased the trophic level of the lake from meso- to eutrophic.

Fisheries management is currently complicated by long-term environmental and socioeconomic issues that were not important in the last century but have become apparent through interactions between biotic and abiotic forces. The complexity of management challenges may explain why current fisheries are characterized by interaction between relatively shorter-term (fishing practices and effort), and longer-term anthropogenic influences responsible for degradation of water quality and fish habitats over which fisheries managers may have little control. There is thus limited management integration of ecological, environmental and socioeconomic factors into the dynamics of fish populations. Other factors include co-management, administrative boundaries and policy linkages. In the case of Lake Victoria, demographic factors are important. The basin human population increase from 5 million people in the 1950s to at least 30 million people has led to corresponding demand from the fisheries, catchment resources and new fisheries management challenges that may not be solved by aquaculture alone.

The case for fish farming

In most Sub-Saharan Africa aquaculture dates from the 1950s but its growth has remained insignificant (FAO, 2004). Aquaculture production in the African continent in 2002 was 399,300 MT (1% of the global production, and less than Lake Victoria production); 85% of aquaculture production came from Egypt while East Africa had a combined production of 10,000 MT (0.02%) of the global production (Mushi, Oenga & Mwanja, 2005).

Fish farming appears to have been slow to take off because of socioeconomic and past political conditions as well as competition from capture fisheries. However, in some countries such as Kenya, fish farming evolved from the introduction of sport fishing (rainbow and brown trout) at the beginning of the 1900s, and later after 1920s by Tilapia zillii (for control of aquatic weeds), Astatoreochromis allaudi (for control of bilharzias snails) and Gambusia affinis (for mosquito control); this was followed by small-scale aquaculture through ‘eat more fish campaigns’ (FAO, 2004). This approach may not necessarily lead to more farmed fish production.

Fish farming was introduced in Uganda in 1953 as a means of supplementing declining capture fisheries and to increase protein intake for people (Biribonwoha, 1976). Between 1950s and 1960s, several tilapiine fishes (O. niloticus, O. leucostictus and T. zillii) from Lake Albert, and other strains from central Africa and Israel were introduced in several lakes and dams, and others into aquaculture. Although there are biotechnological constraints (seed, feed, etc.), present production from aquaculture is still less than 5% of total production from all the fisheries. It seems that development of aquaculture as a tool of fisheries management is only effective if applied in relation to capture fisheries production, economics (household factors, markets, exports), demographic (internal fish consumption patterns), sociocultural (fishers and fish farming) and integrated ecosystem management objectives.

Nile perch domestication and cage culture

The rapid commercialization of the Lake Victoria fisheries based on the stocked Nile perch (Lates niloticus), and apparent uncertainty about sustainability of its stocks have been followed by concerns among investors and fishers that Nile perch supplies from Lake Victoria at current levels may not be sustainable (Balirwa et al., 2004; LVFO, 2005). As Nile perch requirements of the export-oriented fishery began to exceed supply from capture fisheries with fish processing plants operating below 50% of installed capacity (Table 1), the feasibility of cage culture of Nile perch has been advanced (Uganda Fish Farming News, 2002). However, unlike three species (Nile tilapia, catfish and carp), much less is known about the feasibility and potential of domesticating Nile perch because its ecology in stocked systems as reviewed has only started to be recognized.

Sufficient demand for Nile perch to support increased supply exists, but questions about Nile perch culture remain. There is no proven record of artificial reproduction (from spawning to hatchery and seed production) technologies known. Relying on wild-caught juvenile Nile perch, Uganda's Fisheries Resources Research Institute (FIRRI) conducted experiments between 2003 and 2004 to determine the feasibility of culturing Nile perch in ponds (Uganda Fish Farming News, 2003; Gregory & Orukan, 2004). Growth rate in ponds was rapid in the first 4 months (Fig. 3) when perch were fed tilapia fry. Other results indicated that Nile perch can survive low (5 mg l−1) dissolved oxygen levels, can be crowded in cages in water at least 0.5 m in depth and grow from 20 g (10.0–12.0 cm) up to 1 kg (40–45 cm) in 1 year. However, the weaning of wild cultured Nile perch from live feed (tilapia fry, Caridina shrimps, tadpoles, insects, Rastrineobola) to dead but fresh (fish, minced meat or pellet fed) proved difficult (Gregory & Orukan, 2004).

Figure 3.

 Nile perch growth in experimental earthen ponds at Kajjansi, Uganda (from Gregory & Orukan, 2004)

There are lessons to be learned from the Nile perch experiments. The use of Nile perch from juveniles captured from the wild contravenes legislation on immature fish and could, if authorized, lead to increased use of illegal seines that also capture the young of other fishes. The technology for breeding of Nile perch is not available and is a major challenge because of incomplete understanding of the causes underlying the reported changes in sex ratio with age (e.g. Ogutu-Ohwayo, 1994, 2004). Without an artificial feed, reliance on cultured wild fish such as tilapia or Rastrineobola, would add to production costs. Even if Nile perch cage culture were to be profitable, there would be need to resolve environmental, sociocultural aspects including fishers’ perceptions and acceptability. A major issue to be considered is the wide dissemination of Nile perch culture technologies, and the likely result of their finding access to nontarget water bodies where the predator is not indigenous. Their escape into new water bodies could lead to similar impacts on native species as has happened in Lakes Victoria and Kyoga.

Conclusions

Capture fisheries in Africa play an important socioeconomic role in local economies but because of high population growth, overfishing in lakes and rivers, and exports to international markets, there are declining trends in local fish consumption patterns. Fishing pressure compounded by other challenges ranging from exotic species introductions, native species loss to eutrophication caused by urbanization, catchment degradation and under-utilized capacity in fish processing plants calls for committed integrated management of aquatic ecosystems and systematic development of aquaculture technologies. Over-dependence on an international export base built around Nile perch from Lake Victoria provides opportunity for rapid commercial culture of Nile tilapia, Carp and Catfish as initial culture species that could also provide alternative income, livelihood and food security. The main factors to consider include: the fish gap between supply from capture fisheries and demand, and the intensity and location of aquaculture systems.

Apart from the challenge of improving management (including co-management) systems for capture fisheries as part of integrated ecosystems, there is need to improve assessments of capture fisheries by considering fish catch and fish population parameters as only part of production factors. In addition, it is essential to apply strict enforcement of recommended management measures in order to increase and sustain socioeconomic benefits from capture fisheries and fish farming. The application of monitoring and environmental impact assessment tools in the sector provides a firm basis for implementing management measures and increasing yields using two major tools: ‘Code of Conduct for Responsible Fisheries’ (FAO, 1995), and ‘Technical Guidelines for Responsible Fisheries (Aquaculture)’ (FAO, 1997).

Acknowledgements

This review was considered at a time when there appeared to be among user groups contradicting opinions on the status of fish stocks in Lake Victoria with Nile perch culture being considered as the option to uncertain fish supplies. Colleagues at FIRRI provided valuable inputs to this review. Ms M. Nsega, J. Namara and I. Wafula greatly assisted with the literature reviewed. Nile perch experiments conducted at FIRRI's Kajjansi Aquaculture Research and Development Centre were supported with USAID and DFID funds, and were led by Mr Rick Gregory and Mr Sam Orukan.

Ancillary