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Science in Africa
East African Species Introductions and Wetland Management: Sociopolitical Dimensions
The total amount of fresh water available to human beings on earth (in lakes and reservoirs) is only 0.26 percent of the planet's water (UNEP, 1994), hence the great importance attached to this comparatively scarce water resource. Fresh water is a renewable resource made available by the constant flow of solar energy to the earth: Water is evaporated from the oceans and the land (including lakes and reservoirs) and is redistributed around the world. Of the total amount of precipitation on land (110,300 cubic kilometers), about 40 percent (37,400 cubic kilometers) enters the oceans as river discharges. Other sinks include infiltration and uptake by plants. Water supply is therefore one of the limiting factors necessary for advanced economic development, as it plays an important role in various sectors of society. Today freshwater ecosystems in general are facing problems as a result of over-exploitation.
In East Africa freshwater ecosystems are threatened as a result of policies that emphasize exploitation (development) rather than management (and conservation). This policy emphasis could be a result of limited understanding of dynamic freshwater ecosystem integrity in the midst of increasing human pressure. Historically, lake ecosystems have been poorly understood (e.g., in 1887 Stephen Forbes regarded a lake as an ecosystem with a boundary: "On one side you got your feet wet, on the other you did not"). Although it is now generally appreciated that lakes or even rivers cannot be understood in isolation of the drainage basin, and that it is not only fish that constitute the aquatic ecosystems, the traditional government approach in East Africa has tended to ignore the large geographical spread that aquatic ecosystems comprise. For example, while there are government institutions (e.g., departments) dealing with fisheries, these institutions operate independently of others, for example those dealing with water as a resource for drinking. In virtually all countries, there are departments dealing with agriculture and forestry, but there are no equivalent institutions dealing with wetlands. Present demands on aquatic resources (especially human activities) dictate that concerns for the sustainable health of aquatic ecosystems should increasingly take an environmental approach. In this paper, an attempt is made to assess the unresolved consequences of conflicts among conservation, human population pressure, and socioeconomics on freshwater ecosystems of East Africa, with a view to justifying an ecosystem approach for research.
East Africa politically comprises the countries of Tanzania (886,040 square kilometers), Kenya (569,250 square kilometers), and Uganda (236,000 square kilometers), occupying an area stretching between about 30o to 45oE and 15oS to 10oN. Hydrologically, watershed lines between several basins cut across political boundaries; thus, the three countries are a small fraction of the Great Lakes Belt of Africa, which spreads across the five major drainage basins: The Nile, Congo-Zaire, Rift Valley, Coastal, and Zambezi basins. As a result, several other geopolitical entities, such as Congo, Sudan, Rwanda, Burundi, Malawi, and Ethiopia, are part of this diverse hydrological situation. The large-scale tropical controls, which include several major convergence zones, are superimposed upon regional factors, such as lakes, topography, and the maritime influence, resulting in complex climatic patterns. The transition from desert, with rainfall less than 200 millimeters per year, to forests, with annual rainfall exceeding 2,000 millimeters, occurs over relatively small distances and altitudes (Nicholson, 1966). In fact, although geomorphological (topography, tectonics) and hydrological (supply of water) factors have been important in the evolution of aquatic resources and the development of the many characteristic sociocultural systems, several of the water bodies are either associated with political boundaries (e.g., Lakes Tanganyika, Malawi, Albert, Jipe) or are shared between countries (e.g., Lakes Victoria and Turkana, Rivers Nile and Kagera). Within the political boundaries, i.e., the countries, aquatic resource management increasingly revolves around local governments' revenue needs, but the local governments lack structures and methods to ensure integrated resource management. Although this pattern may not be unique to Africa, as it mirrors the situation particularly in Europe and America, the history of deterioration of the North American lakes is well known and the African Great Lakes do not have to be subjected to similar catastrophic changes (Coulter et al, 1986).
Biological and Socioeconomic Interests
The freshwater ecosystems of East Africa, among them lakes, rivers, and wetlands, are an invaluable natural resource base, not only to the region's estimated population of 85 million inhabitants (Rwanda, c. 6m; Burundi, c. 6m; Uganda, c. 19m; Kenya, c. 26m; Tanzania, c. 29m) but also to scientists, naturalists, tourists, and environmentalists worldwide. These diverse but fragile ecosystems are socioeconomically important for drinking, irrigation, hydro-power generation, fisheries, recreation, transport, and human and industrial waste disposal, and are also areas in which agriculture, animal husbandry, and industrial growth thrive. They are therefore among the most densely populated sites in the region (Bootsma and Hecky, 1993) with annual population growth rates of at least three percent typical in the catchment areas (UNECA, 1995). On the other hand, the countries are among the poorest in the world, with a per capita income of less than US$300 at present.
East African freshwater ecosystems also support a high diversity of fauna and flora. In addition, these ecologically diverse ecosystems offer a rare opportunity for scientists simultaneously to study geological, climatological, hydrological, paleolimnological, evolutionary, and ecological phenomena. These lakes include both deep rift-valley systems (e.g., Tanganyika, Malawi) and shallower ones in between the rift systems (e.g., Victoria, Kyoga), which owe their formation to tectonic uplift and a reversal of the old east-west flowing river systems across the region (Beadle, 1981). However, in comparison to the similarly extensive North American Great Lakes freshwater systems, where incomes are much higher, the aquatic ecosystems in East Africa are far less understood and appear to be undergoing ecological change in a much shorter time frame. And while the physico-chemical, biological and climatic-caused changes for these lakes are not necessarily unique, the time scales and magnitudes of motion of these processes are expected to be so, and demand critical analysis if proper management of industrial and other development of these resources is to be properly handled.
Distinctively characteristic of the East African aquatic landscapes are the extensive wetland systems associated with the shallower lakes (Denny, 1985), which reach maximum development in Uganda. It appears that the entire wetland network in Uganda contributes to the hydrological regime of the Nile basin and the ecohydrology of the region, from observation of numerous satellite lakes. These smaller water bodies frequently show the same species assemblages as occur in the larger lakes within the same or adjacent drainage basins. For the variety of reasons given above, scientific studies should adopt a broader environmental outlook than is seen in the majority of existing works on these lakes. As fisheries management has long been the primary concern, this flaw in the research scope is not surprising, and the aquatic ecosystems now manifest indications of the lack of comprehensive management.
Exploitation and Vulnerability of the Fish Communities
Changes in fish communities can be caused by alterations in fishing effort and indiscriminate use of particular gear (e.g., gill nets, seine nets), putting pressure on different species (Craig, 1992). The impact of this equipment may depend on size, habitat, and activity patterns of particular fish. Many species of fish appear to be habitat-restricted, which often leads to rapid local overfishing (FAO, 1983). The history of impacts of fishing on fish communities, especially of Lake Victoria, is well documented (e.g., Mann, 1969; Marten, 1979; Ssentongo and Welcomme, 1985), and in general these studies have shown both species and population structure changes over the period that commercial fishing has been in existence. In many of the East African lakes, but particularly in Lake Victoria, the trend associated with fishing pressure has been an evolution toward small fish sizes, despite attempts at fishing regulations. This evidence shows that human actors can have significant impacts on an aquatic ecosystem, even though fishing may be the primary concern. Some lakes, such as Tanganyika, are dominated by small pelagic species, such as clupeids and cyprinids, with short life cycles and rapid turnover rates (i.e., r-selected); these lakes can therefore produce more protein for human consumption than others, where, for example, haplochromine cichlids with small broods (k-selected) dominate in the shallower plateau lakes (Lowe-McConnell, 1975; 1993; 1996). Hence, in Lake Tanganyika (unlike in lakes Victoria, Kyoga, and Malawi), fishing pressure on the pelagic zone as a whole has had relatively less damaging impact on fish communities and biodiversity in general. Even in Lake Victoria, intense fishing and Nile perch predation pressure on the surviving cyprinid (Rastrineobola argentea) does not appear to have depressed the stocks to the same extent as in the shallow littoral fish communities, many of which stay close to the bottom. The pelagic food webs of the East African Great Lakes exhibit features that suggest continuous, strong biological interactions, as well as strong interactions between biota and their geochemical environment, that make them resilient (Lehman, 1996; Lehman and Bronstrator, 1994).
However, in all lakes, the amount of fish caught has been changing in relation to the increasing riparian human populations. For example, in Lake Tanganyika fish production increased from 73,000 tons per year to 118,000 tons per year over a relatively short period (Greboval and Fryd, 1993). Over a similarly short period, commercial fish production from Lake Malawi increased from 40,000 tons per year (Alimoso et al, 1990) to 67,000 tons per year (Greboval and Fryd, 1993), with the increase attributed to a rapid increase in the population and demand for fish (Tweddle, 1992). The increases in production may appear impressive, as they fulfill immediate socioeconomic objectives, but a major question regarding sustainability of the resources remains. There is a related aspect that does not readily lend itself to immediate concern, i.e., the actual decrease in fish consumption per capita. Often these questions are not addressed during the life cycle of parliaments.
In Lake Victoria changes in the commercial fishery have been more phenomenal. In 1989 catches reached a total of 500,000 tons (five times higher than the level reached in 1970s), representing over 27 percent of the total catch from Africa's inland waters (Greboval and Mannini, 1992), but that level is now declining. Unlike the situation in both Lakes Tanganyika and Malawi, the increase in production from Lake Victoria is a result of catches of stocked species, particularly Lates niloticus (Nile perch). Until environmental degradation was recognized as a major problem, discussion was centered on the relative success of species introductions; periodic low catches were attributed to lack of inputs or markets (Kirema-Mukasa and Reynolds, 1991). It is now clear that the effects of fishing can be compounded by introductions of exotic competitors and predators (Ogutu-Ohwayo, 1990), even though only a small fraction (16 percent) of known successful stockings in African lakes were primarily intended to improve the fisheries (Welcomme, 1988; Pitcher and Hart, 1995). In the short term, "successful" introductions also tend to create the false impression that an aquatic ecosystem may be stable or healthy for as long as it yields food (fish).
A typical example is in Lake Victoria, where, following the establishment of the stocked Nile tilapia, a return to the desired mesh size of gill nets was expected to lead to a recovery of the native tilapine fishes (O. esculentus and O. variabilis). The recovery has not occurred. Questions are now being asked whether the failure of these native species to recover is because of recruitment overfishing, competition or hybridization with the stocked tilapines, or combinations of various factors. The confusion is partly a result of looking at a lake primarily in terms of its fishery production potential rather than as an integrated ecosystem with various components, including humans. What seems to have been missed is development of the fisherfolk's capacity to think for themselves, and to include their opinions in attempts to solve their perceived fishery-related problems.
Impact of Fish Species Stockings
Prior to the species stockings of Lake Victoria from the 1950s, the most dominant element of the ichthyofauna were the 300-plus haplochromine cichlids species. Although these mostly small (less than 20 centimeter) bony fishes comprised at least 80 percent of the lake's fish biomass (Kudhongania and Cordone, 1974), occupied a wide range of ecological niches (Ligtvoet and Witte, 1991), and exploited virtually all food resources in the lake (van Oijen et al, 1981; Witte and van Oijen, 1990), they were not important elements of the commercial fisheries' yields. They were restricted to local use, particularly as medicine, ceremonial functions, or simple dietary tastes. The socioeconomically most important species from the main lake was O. esculentus (ngege), followed by O. variabilis, and Graham (1929) described Lake Victoria as a tilapia lake. There were, however, other species of subsidiary and often local importance among the riparian tribes. The target species included Bagrus docmac, Protopterus aethiopicus, and the potamodromous Labeo victorianus, Barbus altianalis, and Clarias gariepinus.
Declines in the traditional tilapia fisheries were followed by introductions of exotic tilapias from 1953, and of the piscivorous Nile perch, Lates niloticus, from 1954 (Lowe-McConnell, 1996). In extensive studies of the species introductions, Nile perch predation was thought to account for the rapid and almost total decimation of the endemic fauna (e.g., Ogutu-Ohwayo, 1990). However, recent findings strongly indicate that part of the species changes are a result of other factors (Bootsma and Hecky, 1993; LVEMP, 1996; Ogutu-Ohwayo et al, 1997). There is now impetus to address Lake Victoria as an ecosystem and to re-examine the influence of human activity driven by socioeconomics as a major factor in ecological change. Stocking of the lake with new species has tremendously transformed the fishery from a multispecies, subsistence and artisanal type to one with a different (reduced species) composition and a much larger market (Greboval and Mannini, 1992). The case of the Nile perch introduction illustrates how fish introductions can cause severe disruption to the ecosystem, but at the same time be beneficial to certain sectors of the population. The potential for economic gain makes introductions for biological management purposes extremely hard to regulate. It also obscures several other aspects that could benefit sustainable management.
The enormous swamplands of East Africa form buffers that provide water quality regulation in receiving waters. They are often dominated by papyrus, and represent a habitat of great ecological importance. Catchment activities, particularly devegetation (biomass harvests), burning, and so on, easily lead to changes in structural components in surface water. In Lake Victoria, just as in North American lakes, eutrophication has become manifest as the fish fauna change (Hecky, 1993). Apart from the perceived "top-down" effects of exploitation and species stockings on species diversity and water quality, catchment impacts on aquatic ecosystems caused by environmental degradation (e.g., of watersheds and airsheds) are directly related to human socioeconomic objectives, and may explain many of the changes in water quality and species assemblages in Lake Victoria.
Impacts of Wetland Degradation
Wetlands are many things to many people, once they are understood and appreciated as valuable ecosystems. Papyrus swamps are most widespread in the Lake Victoria Basin (Beadle, 1981), where, in Uganda alone, they occupy more than 10 percent of the land (Gumonye-Mafabi, 1990) and form extensive buffer zones around Lake Victoria. Although wetlands are providers of many products and services, and there are reciprocal impacts between humans and wetlands, wetlands management has often been uncoordinated, with only a few commercial objectives being emphasized. Fringe wetlands near lakes and rivers are among the least investigated aspects of these ecosystems, yet habitats associated with lakeshore wetlands provide insight into the origin and structure of fish populations in lakes. In Lake Victoria, ecosystem relationships might result as much from the riverine origin of many species of fish in the lake as from the rainfall pattern in the basin and the slow-flowing conditions in the old river beds, which were colonized by wetlands during the tectonic movements. Thus the effects of swamp vegetation development probably extend some distance toward open water.
It is difficult to draw general rules by which the effects of introduced predators like Nile perch can be predicted. There is one environmental parameter, however, that may be useful as a predictor of introduced predator-indigenous prey dynamics in many aquatic systems: Dissolved oxygen. Papyrus swamps in wetlands, for example, influence habitat availability and quality for fish in two ways: (i) They provide structural heterogeneity in which fish can escape predators, and (ii) their closed canopy minimizes incident light and the mixing of water below, resulting in severe oxygen depletion in the water of the swamp (Chapman and Lien, 1994; Chapman et al, 1994). Some studies of such swamps have shown that substantial loss of nutrients to the interface zone is a result of rain leaching (e.g., Gaudet, 1977). Such functional exchanges add to habitat structural diversity, which may be manifested in the trophic diversity responsible for the high productivity associated with wetland ecotones. In addition, because of the strong links between the swamp buffers and the open water, Lake Victoria could be considered as comprising at least five interconnected zones: The catchment, including the lake's feeder streams, the littoral zone, with or without wetlands as defined by Odum (1971), the sub-littoral, the pelagic, and the profundal zones.
The lakeshore wetlands, in particular, are of considerable significance to the biota and water quality of the lake. By definition these are "areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six meters" (Ramsar, 1971). Considered as such, the definition groups together ecosystems with some common management needs (Dugan, 1990), and would therefore include some of the vulnerable habitats in East Africa's aquatic ecosystems. The ecological functions of wetlands (e.g., as habitats for water fowl, fish, and other wildlife, as regulators of water quantity and quality in receiving waters, areas of high biodiversity, etc.) have for generations been the basis of sustainable socioeconomic returns (e.g., sources of food-fish, drinking water, building materials, medicines, protection from floods, areas of seasonal grazing, etc.). However, the emphasis of "waterfowl of international importance" in the Ramsar Convention signed by 74 countries (Mitsch and Gosselink, 1993) may not adequately address some of the more valuable freshwater ecosystems (e.g. Lake Victoria) of similar importance to Ramsar sites, where water fowl of international importance may not be the major factor determining their global significance. Hence, for example, in comparison to those wetlands with large bird flocks (e.g., Lake George, Uganda; Lakes Baringo, Naivasha, and Nakuru, Kenya), the wetlands of Lake Victoria, which do not attract the same tourist interest, are lacking comprehensive management guidelines and have been less studied.
In general, cost-benefit analyses of increased crop yields, exotic species, or tourist-generated income considered against conservation values cannot produce realistic balances favoring the aquatic environment. As population pressure, with its attendant socioeconomic demands, may be regarded as the primary factor in the degradation of surface water, it may only be a matter of time before wetlands (whether Ramsar sites or otherwise) will be overwhelmed. For both Ramsar-designated wetlands and Lake Victoria, the basic cause of environmental degradation of the water resources is human population pressure. In Lake Victoria wetland loss is a result of conversion by drainage for three main uses: Agriculture (associated with upstream drainage and agrochemical use), urban-industrial establishments, and wastewater influences. Other forces that threaten the integrity and values of wetlands appear to be increasing in intensity, and are associated with the open access regime and a lack of land use planning for wetlands.
Impact of Exotic Weeds
The water hyacinth, Eichhornia crassipes, is a flowering plant with a native range in the Amazon areas of South America. It is one of the main exotic aquatic weeds (the other is Salvinia molesta) to have invaded East African freshwater ecosystems in the last decades. Water hyacinth is reported to have appeared in Uganda by 1976 (Robson, 1976) but may have been for long before this period one of the ornamental plant species commonly sold in some of the larger urban areas of East and Central Africa. The weed is reported to have been introduced in Egypt much earlier, between 1879 and 1892 (Gopal and Sharmer, 1981), and its presence in freshwater ecosystems in many other parts of the continent may therefore not be surprising. Its widespread occurrence indicates that it was just a matter of time before the weed would invade East Africa's Great Lakes. Although the presence of water hyacinth in Lake Victoria was reported in 1989, it is suspected that the weed may have gained entry to the lake via the Kagera River (from Rwanda) as early as 1981 (Taylor, 1993). Therefore the spread of water hyacinth in East Africa, particularly in Uganda, may have been from several sources.
From the time water hyacinth was observed in Lake Kyoga, Uganda, in 1985 (Twongo, 1991 and 1993), the weed has been an important research issue at the Fisheries Research Institute (FIRI), Jinja, where a lot of insight has been generated into ecological aspects of the weed in the new environments. In Lake Victoria water hyacinth is especially concentrated in Ugandan waters, partly because the prevailing southerly winds blow mats from the mouth of the Kagera River. Increased nutrient loads into the lake also seem to have spurred water hyacinth infestations, particularly in bays receiving municipal sewage. From several ground truthing surveys by FIRI (Twongo et al, 1995), it can be deduced that there are several modes of hyacinth mats occurring in Lake Victoria. These include: Mobile mats; resident shoreline fringe mats; rooted colonies; successional forms found in association with other wetland plants; and mat concentrations in nutrient-rich habitats. The location, size, and form of water hyacinth mats are therefore highly variable, with some mats reaching 300 hectares in cover, others infesting entire bays or drifting about the lake. The main negative effects of the increasing infestations of water hyacinth mats are:
There are also a variety of socioeconomically detrimental effects of water hyacinth. These include:
To the riparian government authorities, the control and management of water hyacinth on Lake Victoria is so critical that many other related issues may be easily ignored. Controlling water hyacinth on Lake Victoria and preventing its spread to other water bodies involves inter-governmental agencies and requires regional cooperation. In addition, the effective control and management of the weed requires large financial inputs and commitment to long-term management capability, which the East Africa countries are unlikely to be able to afford even on a short-term basis. Uncertainty caused by the scale of water hyacinth infestations and the scope of its detrimental effects may explain the sometimes conflicting mechanisms and methods suggested for weed control.
Various water hyacinth control options exist (physical/manual, mechanical, chemical, biological, utilization, and environmental manipulation), but identifying the most efficient, viable and environmentally friendly option combination for Lake Victoria remains elusive. Manual and mechanical control measures have had little impact. Biological control may be regarded as a viable option, especially if systematically introduced in the entire great lakes region and upper Nile basin (Twongo and Balirwa, 1995). However, this option is expensive and takes many years to show impact.
Although the use of chemicals to control the weed has been used in some countries, there are reservations about its perceived environmental, socioeconomic, and political implications. In considering the option, fears have been expressed regarding the contamination of water for domestic and livestock use of the rural communities living close to the lake. In addition, food chain transfers of the potentially harmful herbicides would probably have larger impacts on those communities that are directly dependent on fishing. There would also be uncertainties relating to fish export markets, e.g., whether the Europeans would reject the products on account of chemical use in the lakes. A related political issue is whether or not the other riparian countries (Kenya and Tanzania) would sacrifice benefits of their own exports if chemical application is considered essential to alleviate the water hyacinth burden in only Uganda.
The Changed Ecosystem of Lake Victoria
In order to understand the changes in Lake Victoria from anthropogenic forces, we need first to understand the human demographic changes and patterns and their land-use activities in the catchment. As the population grows it demands more land for agricultural activities, more fishing, and so forth. The countries around Lake Victoria are mainly agricultural (up to 80 percent of the population) and are involved mostly in subsistence agriculture. In most parts of the catchment, terracing and other soil conservation techniques are not practiced. Consequently, soil erosion is a serious problem. In desperation for more productivity from the infertile soil that is left, reliance on agrochemicals is the answer. This soil has already become less capable of holding moisture, which results in the loss of water in the form of run-off. With an increase in run-off, much of the applied fertilizers are washed away, polluting the receiving waters. Forests and other wooded savannah areas have been cut down to make more land available for agriculture, as well as for firewood, charcoal, and building materials.
There are many mushrooming towns along the shores of the lake. These, together with the already established towns, throw their industrial and domestic wastes into the lake. Textile, sugar manufacture, beer brewing, food processing, and other manufacturing industries operate within the towns, releasing untreated or partially treated waste effluents into the lake. Mining activities within the catchment have raised levels of specific heavy metals, e.g., methyl mercury (Ramlal et al, 1998), in the environment and in fish. Very often, the mining companies use the existing adjacent wetland systems as part of their wastewater treatment structures (Kansiime et al, 1994). Many have had their treatment capacity surpassed, and others have been degraded. As a result, wastewater effluents in the run-off from the catchment empty into the lake and contribute to the observed ecological changes in the lake.
Eutrophication, an aquatic environmental degradation factor, may be defined simply as a process whereby water bodies become progressively enriched with nutrients (mainly nitrogen and phosphorous), with a resulting excess production of plant (usually algae) biomass. Ecological functioning and floral and faunal balances become grossly disturbed. The increased reposition rates of nitrogen, beginning in the 1920s, and phosphorous, beginning in the 1950s (Hecky, 1993), were likely the result of Lake Victoria watershed and airshed disturbances. Although phosphorous has generally been regarded as the nutrient that limits primary production in many temperate lakes (Vollenweider, 1970), in Lake Victoria, the comparatively low nitrogen and phosphorous concentrations, even in nearshore areas (Balirwa, 1998), suggest that additional nutrients could be controlling algal growth. Experiments with culture media under stabilized methodological protocols with known lake water concentrations and algal combinations could facilitate further insight.
Comparison of historical limnological observations with modern conditions in Ugandan waters of the lake indicates that the present lake is quite different from what it was in the 1950s and 1960s. Oxygen conditions have been altered, with surface waters now being supersaturated with oxygen, while hypolimnetic oxygen concentrations have fallen (Hecky et al, 1994). Phytoplankton productivity has increased by at least a factor of two, and biomass by as much as a factor of four to five, both inshore and offshore (Mugidde, 1993). The increases in productivity have been accompanied by qualitative changes in phytoplankton composition, with greater representation of blue-greens, especially the filamentous nitrogen fixing cylindrospermopsis, Anabaena, Lyngbya, and Microcystis (Balirwa, 1998), indicating a change in the nitrogen/phosphorous ratio, which may be favoring nitrogen-fixing forms. The cause may be a combination of nitrogen mobility from the degraded landscape and atmospheric deposition from biomass burning.
Changes in diatom dominance from Melosira to Nitzchia have coincided with increased depletion of soluble-reactive silicon, which is about ten-fold lower than recorded in the 1960s (Hecky and Bugenyi, 1992; Hecky, 1993). However, the relatively higher silica content in interface habitats at the shore (Balirwa, 1998) in comparison to samples from the 1960s (Hecky and Bugenyi, 1992) may explain the differences in horizontal diatom distribution patterns. The differences could also be a result of unique patterns in ecological structure of the previously unsampled wetland-dominated habitats. In addition, changes in grazing pressure, as shown in studies of benthic communities (e.g., Steinman, 1996), could also alter algal species diversity. Changes in diatom and blue-green algal counts may therefore be a result of changes in ratios of several nutrients that may be selectively limiting. The decrease in the lake's transparency (or increase in turbidity) is probably due to a higher chlorophyll concentration, which has resulted in a shallower euphotic depth, causing in turn a loss of the photosynthetic zone (Mugidde, 1992). Higher biomass levels, accompanied by more frequent algal blooms, can result in high oxygen demand during decomposition, which has often resulted in fish kills (Ochumba and Kibaara, 1989) and increasing deoxygenation of the hypolimnion (Hecky et al, 1994).
Some new approaches to studies of the changes in Lake Victoria could increase our understanding of the impacts of various factors, both historical and recent, on the ecology of the lake. For example, in order to assess when these changes occurred, sediment cores (from Kenya) had to be analyzed (Ogutu-Ohwayo and Hecky, 1991). The structure, composition, and abundance of benthic macrofauna are correlated with substratum type and depth (Mothersill et al, 1980; Okedi, 1990) as well as with vegetation, distance from the shore, and season (Balirwa, 1998). Although less investigated, present patterns in invertebrate communities may indicate disruption in the trophic status of the lake. For example, in some studies (Mbahinzireki, 1994; Mwebaza-Ndawula, 1994; Witte et al, 1995) it has been suggested that a reduction in the intensity of the grazing food chain (principally haplochromines) could have led to an increase in the abundance of some invertebrates, such as mollusks and insects. However, observed changes in invertebrate community structure and species abundance could also be caused by changes in water quality, such as dissolved oxygen (in the case of Caridina and chironomids), elemental ratios (e.g., carbon and nitrogen, correlated with growth and fecundity of secondary trophic levels; see McMahon et al, 1974) or increasing infestation by water hyacinth (which could be correlated with both chironomid and mollusk densities).
The diatom and chlorophyte communities began to change as early as the 1920s, coinciding with increased carbon and nitrogen deposition at the site. These trends continued up to the 1950s, when silicon deposition began to increase at the site. The modern diatom assemblage and nutrient deposition rates were established by the 1970s. All these changes preceded the Nile perch population explosion, although the dramatic changes in the 1960s were coincident with the introduction of the Nile perch and exotic tilapines. These changes were also coincident with the record high water levels of the lake in early 1960s. Independent evidence of increased phosphorous input comes from comparing the chemical composition of the rain in Uganda in 1960 and 1990, where a three-fold increase is apparent. The dominance of heterocystous, filamentous cyanobacteria indicates that nitrogen fixation has increased dramatically in the lake since the 1960s, although denitrification in the deep low-oxygen waters of the lake largely prevents this influx from accumulating in the sediment record. The increased flux of silicon to the sediments, together with that of phosphorous, indicates that widespread basin disturbance, probably related to increasingly intense land use, is causing the eutrophication of the lake. Even though these observations have been limited in both time and space, they serve as indicators of the sustainability of the lake at different perturbation levels. They could also be among a set of parameters (both biotic and abiotic) that, under standardized methodology, would serve as indicators for continuous monitoring.
Socioeconomic Effects of Stocked Species in Lake Victoria
Traditional fishing communities such as those in the Lake Victoria basin generally belong to the poorer segments of the population. The low social and political status of these communities and the comparatively low level of priority given to the sector show the complexity of the fishing industry, which is a continuum of interacting linkages between immediate concerns (production, processing, marketing, supply, maintenance) and other rural and urban sectors (institutions, services, socioeconomic and cultural aspects). Raising the standard of living of these traditional fishing communities is extremely complex. Very often, priorities among the inter-linked sectors differ widely. Whereas regulations are part of the management regime associated with aquatic resources, the emphasis in the crop sector encourages expansion of acreage. In comparison to crops or animal husbandry, the low level of priority given to the fisheries sector may not in itself indicate government's inability to give the communities the desired attention; rather, it is probably a result of the complexity of the water environment as a production system, with a considerable degree of natural variation and uncertainty, in which only the fishery interest is clear.
There are conflicting accounts of the socioeconomic benefits and costs of the stocked species in the region. Prior to the Nile perch boom, the lake fisheries were exploited primarily by small-scale fishermen, numbering about 50,000 and operating about 12,000 fishing vessels (Butcher and Colaris, 1975; Abila and Jansen, 1997). The decentralized ownership of canoes and nets restricted outlets for fish, 80 percent of the fishermen derived their primary income from fishing, and local people depended mostly on fish for their protein needs (Jansen, 1977; Abila and Jansen, 1997). It was a self-regulating, open access system, free from outside interference.
Since the introduction of the Nile perch, there has been an increase in the number of people employed directly in the fisheries sector (300,000 people) or in related activities. The development of markets is also considered to have led to processing plants (at least 10 in each riparian country) targeting the export market. The business approach from these investments is presently producing much-needed foreign exchange earnings (around US$100 million) for the countries in the region.
However, there is also some cause for concern. For example, present export volumes, which may account for 20 to 30 percent of production, may rise to as much as 60 percent; yet, the stability of Nile perch catches is uncertain and may decrease. This trend would certainly lead to an increase in the price paid by local consumers, and fish consumption per capita may have already gone down. It also appears that the required investment in fishing is now beyond the ability of small-scale fishers, many of whom have abandoned the activity. These segments of the community tend to look beyond the lake for alternative livelihood, often in agriculture. As land is scarce, encroachment on the surrounding wetlands may be one of the indirect consequences of the "success" of stocked species. Other observed negative effects have included the disappearance of consumer choice and preferences (e.g., for the catfishes, cyprinids, and endemic tilapines)
The Lake Victoria ecosystem has undergone and is undergoing ecological changes. Socioeconomic transformations, fueled by increasing populations and development needs, are part of the cycle of changes. These changes are manifested in the observed transformations of the fisheries, other biota, and water quality, and whatever the magnitude, the changes cannot be ascribed only to species stockings. There are clear indications of catchment degradation, which is likely to increase the deterioration of water quality. Increasing infestations by water hyacinth, especially in the productive littoral zone, adds further uncertainty to the socioeconomic sustainability and environmental health of the natural resource base.
When the countries around Lake Victoria gained independence more than 35 years ago, most basic aspects of the colonial policies and laws governing natural resources remained intact and continued to operate for some time. Even today, many laws and policies governing natural resources and the environment are outdated and either encourage full exploitation of the resources (e.g., reclamation of wastelands including wetlands, hunting, bush burning, etc.) or lack in-built mechanisms to regulate human activity (e.g., chemical use in water bodies, or intensive agricultural practices). In the case of fishing, the original guidelines were made to protect stocks of only tilapia (O. esculentus), and little attention was paid to other species. This approach seems to support the view that, for the most part, development programs have tended to emphasize fisheries-oriented options in order to raise the living standards of the fisherfolk, with such objectives as increasing catch and access to resources (improvements in boat and gear designs), lowering the costs of inputs, and encouraging industrial scale exploitation. While beneficial in some ways, these goals must be balanced with the concept of sustainability of the complex ecosystem.
It is only relatively recently that attempts have been made to draw up environmental action plans. In general, however, the existing environmental and natural resources laws and policies have very often lacked the involvement of the local people. Present laws are limited in scope, reactive rather than anticipatory, poorly implemented and enforced, and uncoordinated.
Fishery management has been traditionally defined as the implementation of a number of measures for the regulation of fishing efforts in order to achieve a given objective. The aims of fisheries management policy have generally been biological (maximum sustainable yield, MSY), social (maximum job creation), or economic (maximum economic yield) (Breuil, 1997). In the past, however, the aim of fisheries management in most African countries was to achieve MSY. This emphasis on MSY has many drawbacks, one of which is that the concept does not take into account socioeconomic features, which play a determining role in inland fisheries dynamics. The reasons why people over-fish should be considered.
Every environmental policy should be based on the principle that a country has two major assets with regard to economic development: Man-made capital (equipment, infrastructure, factories, and technologies) and natural capital, or heritage. In order to measure a country's development level, economists rely on macro-economic indices, such as GDP, taking into account man-made capital only. In some cases, increases in the man-made capital occur to the detriment of the natural capital. This trade-off generally leads to an impoverishment of future generations, despite indications of an apparent progress. This kind of development is not sustainable, since each generation impoverishes the next one.
Lake Victoria is of great economic worth to the three riparian countries and of great scientific and cultural significance to the global community, mainly with respect to its unique waterborne biodiversity. It is suffering severely from three major global environmental concerns: Degradation of water quality because of pollution from land-based activities; introduction of non-indigenous species (both fish and plant weeds); and excessive exploitation of living resources. It is also facing the typical consequences of these problems: Potentially irreversible environmental damage, hardships among the poor, and serious health concerns.
The World Bank International Development Association (IDA), the Global Environment Facility (GEF), and the national governments of the riparian countries are funding the Lake Victoria Environmental Management Programme (LVEMP). The assistance will act as a catalyst for the three countries to develop a better understanding of how the lake functions, to learn how the actions of their respective populations in the basin affect the lake environment, and to work out jointly with one another how to implement a comprehensive approach to managing the lake ecosystem to achieve global environmental benefits. This coordination and harmonization of activities will be overseen by the Lake Victoria Fisheries Organisation (LVFO), a body established for that purpose and set up to replace the Committee for Inland Fisheries of Africa (CIFA), which operated under the United Nations Food and Agricultural Organization (FAO).
LVEMP notwithstanding, there will still be a critical need to continue investigations along ecosystem lines. It is also essential to develop skills (especially in methodological protocols) and related infrastructure by which indicators of sustainability can continuously be monitored. There is, for example, an urgent need to move away from the spot investigations of the past toward broader systemic approaches. In addition, recent technological developments (e.g., remote sensing and data logging capability) offer practical possibilities for new multidisciplinary research focusing on the ecosystem concept, aided by enhanced basic studies in paleoclimatology and paleolimnology.
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