The decline in fish species in Lake Victoria is one of the largest documented losses of biodiversity in an ecosystem. The reduction in species in the lake was attributed to overexploitation through increased fishing capacity, use of illegal fishing gears and poor enforcement of regulations. Introduction of the predatory Nile perch is blamed for the decline of the native species, especially the haplochromine cichlids. The native tilapiines, Oreochromis esculentus and Oreochromis variabilis, declined due to hybridisation and competition with the introduced Oreochromis niloticus. Diversity loss in haplochromine cichlids has also been attributed to hybridisation caused by increased water turbidity, which reduces visibility in recognising conspecifics during breeding. Degradation of the environment through poor farming patterns and waste disposal has led to increased nutrients into the lake, in turn leading to changes in water quality, increased algal blooms and subsequent anoxia which led to frequent fish kills in the 1990s. However, recent resurgence of haplochromines thought to be extinct, disputes the fact that extinction of several species occurred. Though not denying that a drastic reduction in the number of native species occurred, the much hyped extinction could be a result of a lack of adequate information on taxonomy and ecology of the haplochromines as well.

Introduction

Lake Victoria in East Africa is the second largest lake in the world by surface area, covering 68 000 km2. It is shared by the riparian states of Kenya (6%), Uganda (43%) and Tanzania (51%) and has a mean depth of 40 m and a shoreline of about 3 500 km. Until the 1970s, Lake Victoria supported a multi-species fishery dominated by the tilapiine cichlids Oreochromis esculentus (Graham) and O. variabilis (Boulenger) and over 200 species of haplochromine cichlids (Kudhongania and Cordone, 1974; Ogutu-Ohwayo, 1990; Goudswaard and Witte, 1997; Goudswaard et al., 2002). Other important species included the native cyprinid, Rastrineobola argentea (Pellegrin), Protopterus aethiopicus (Heckel), Bagrus docmak (Forskåll), Clarias gariepinus (Burchell), various Barbus species, mormyrid species and Schilbe intermedius (Rüppell). Currently the Lake Victoria fishery is dominated by Lates niloticus (L), R. argentea and Oreochromis niloticus (L). The reduction in fish species in Lake Victoria is the largest documented loss of biodiversity caused by man in an ecosystem (Witte et al., 1999). This paper explores possible factors that led to the decline in fish species in Lake Victoria and outlines possible management strategies to reduce further decline. Information was compiled from published and unpublished literature.

Factors leading to decline in species

Commercialisation of the fishery

Trends in catch show that up to the early 1980s, the Lake Victoria fishery was dominated by endemic haplochromine cichlids (Figure 1). Following the explosion of Nile perch and commercialisation of the fishery, there was a noticeable decline of other species in the lake. The reduction in catches of the native species was attributed to intense exploitation (Kudhongania and Cordone, 1974; Ogutu-Ohwayo, 1990; Witte et al., 1999). The human population around the lake grows at around 3% per annum (Yongo et al., 2005; UNEP, 2006), rainfall is erratic and agriculture is poorly developed, leaving the lake as the main source of livelihood for the surrounding communities. In order to meet increased fish demand for food and export, the number of fishers, fishing crafts and gears has increased in the lake over the years (Figure 2a). Fishers are increasingly using more efficient and illegal fishing gears in the lake (Cowx et al., 2003; Njiru et al., 2006). The legal gillnet mesh size for the lake fishery is 5 inches (127 mm, stretched) or more, but gill nets with mesh smaller than 5 inches are still prevalent (Figure 2b). Banned gears like beach seines and monofilament nets are still used in the lake (Figure 2b). Illegal fishing gears like beach seines disrupt spawners, capture immature fish, and destroy breeding areas, especially those of nest building cichlids and substratum. Increased overexploitation could have affected the recruitment process by the capture of immature fish subsequently leading to decline in catches.

Figure 1.

Capacity trends in Lake Victoria, (a) fishers and fishing gears, (b) illegal fishing gears; Source: Frame survey, 2008.

Figure 1.

Capacity trends in Lake Victoria, (a) fishers and fishing gears, (b) illegal fishing gears; Source: Frame survey, 2008.

Figure 2.

Contribution of the major fish species caught by bottom trawl in Lake Victoria, Kenya. Others include Synodontis, Brycinus, Bagrus and Schible species.

Figure 2.

Contribution of the major fish species caught by bottom trawl in Lake Victoria, Kenya. Others include Synodontis, Brycinus, Bagrus and Schible species.

Exotic introductions

Nile perch

Nile perch, Lates niloticus (L) was introduced in the 1950s and 1960s mainly to convert the small, bony, but abundant haplochromines, to suitable table fish (Ogutu-Ohwayo, 1990; Goudswaard et al., 2002). The contribution of haplochromines thereafter to the fish biomass in the lake decreased rapidly from > 80% during the 1970s to < 1% by the late 1980s, whereas other native species were occasionally recorded in fish landings (Figure 1, Kudhongania and Cordone, 1974; Ogutu-Ohwayo, 1990; Njiru et al., 2005). In its early existence in the lake, Nile perch predominantly fed on haplochromines (Ogutu-Ohwayo, 1990; Mkumbo, 2002), which were abundant (Figure 3). The voracious predator could have contributed to the decline in the native species. Recent catch statistics indicate a reduction in Nile perch and a resurgence of the native species (Figure 1), revealing that Nile perch played a pivotal role in the catch composition on the lake.

Figure 3.

Diet of L. niloticus in Lake Victoria a) 1968–1977, (b) 1988, (c) 1998–2000. Others include Schible, Brycinus, Clarias, etc. Haplos = haplochromines; adapted from Njiru et al., 2005.

Figure 3.

Diet of L. niloticus in Lake Victoria a) 1968–1977, (b) 1988, (c) 1998–2000. Others include Schible, Brycinus, Clarias, etc. Haplos = haplochromines; adapted from Njiru et al., 2005.

Tilapiines

To compensate for the decreasing catches of native tilapiines (O. variabilis and O. esculentus), in the 1950s, exotic tilapiines O. niloticus, O. leucostictus (Trewavas), Tilapia zillii (Gervais) and Tilapia rendalli (Boulenger) were introduced into the lake to fill the “empty niches” (Welcomme, 1967). The Nile tilapia (O. niloticus) presently dominates the tilapiine fish landings, whereas most of the other tilapiines are rarely caught in the lake. O. esculentus which feeds almost entirely on diatoms and O. variabilis on phytoplankton were out-competed by the more diversified feeder, O. niloticus (Welcomme, 1967). The Nile tilapia diet consists of a variety of food items including algae, fish and detritus (Njiru et al., 2004). The algal composition in the lake has also changed towards toxic and unpalatable cyanobacteria (Lung'ayia et al., 2000), further diminishing the food base for the native tilapiines. Female Nile tilapia produces between 905–7619 eggs and is more fecund than O. variabilis with 323–547 eggs and O. leucostictus with 99–950 eggs (Lowe-McConnell, 1955; Njiru et al., 2006). The introduced Tilapia group, T. zillii and T. rendalli, may not have become established because they lay and bring up their eggs on the lake bottom; the eggs and larval fish could have suffered high predation from then abundant haplochromines (Lowe-McConnell, 1955). The Oreochromine group, which are mouth brooders, offer higher protection to their eggs and fingerlings compared to the substrate brooders of the Tilapia group, giving them a competitive advantage.

Hybridisation of cichlids

Haplochromines

Change in water transparency may have increased hybridisation of haplochromines resulting in loss of diversity (Seehausen et al., 1997). Primary productivity has doubled, algal biomass has increased 8–10 fold accompanied by a shift in algal species composition to a predominance of cyanophytes brought about by abundance in phosphorous (Mugidde et al., 2005). Average values of transparency decreased from 7.3–7.9 m for offshore stations and 1.3–1.5 m in Nyanza Gulf in 1927 (Worthington, 1930) to 1.1–1.4 m and 0.6–1.7 m in the same areas respectively in 1994/1995 (Lung'ayia et al., 2000). Laboratory studies have shown that female haplochromines mate with heterospecific males when conspecific males are not available (Crapon de Caprona and Fritzsch, 1984). Decrease in water transparency has narrowed the light spectrum in Lake Victoria which changed the way male haplochromines perceive colours of females. Reduction of water clarity has caused loss of genetic and ecological differentiation among haplochromine species and has contributed to loss of species diversity (Seehausen et al., 1997).

Tilapiines

Tilapia are known to interbreed both under natural and artificial conditions (Welcomme, 1967). Hybrids of O. variabilis x O. niloticus, and O.esculentus x O. niloticus occurred under experimental conditions. Studies in northern waters of Lake Victoria found such hybrids (Balirwa, 1992). A characteristic feature of all hybrids is the dominance of O. niloticus morphological features. Therefore for all practical purposes, the native tilapias of O. variabilis and O. esculentus have been swamped up and the common tilapia in Lake Victoria is some form of O. niloticus (Balirwa, 1992).

Ecological changes

Water quality

Water quality in Lake Victoria has declined greatly in the past few decades, owing chiefly to eutrophication arising from increased inflow of nutrients into the lake (Lung'ayia et al., 2000; Mugidde et al., 2005). Increased input of nutrients is attributed to changes in farming patterns such as deforestation, monoculture cultivation up to the lake's bank, which encourages soil erosion. Further nutrient load is through entry of untreated industrial and municipal waste water into the lake. Enrichment of the water has led to massive blooms of algae especially of the toxic blue-greens (Lung'ayia et al., 2000; Mugidde et al., 2005). Collapse of these blooms led to reduction in dissolved oxygen, at times dipping below 1.9 mgl− 1, a level which is lethal even to the more tolerant cichlid fishes (Mhlanga et al., 2006). Further, there has been a loss of about 30–50% of the oxygenated waters volume in Lake Victoria since the 1960s, which has reduced the fish habitat (Mugidde et al., 2005). Low dissolved oxygen concentrations and probably phytotoxins contributed to occasional fish mortality observed in the Nyanza Gulf of Lake Victoria (Ochumba, 1990). Clearance of fringing swamps around the lake has reduced their buffering capacity (Mugidde et al., 2005). Reduced marginal vegetation has impacted negatively on recruitment and survival of most Lake Victoria fish which depend on the fringing zones during their early stages of development (Njiru et al., 2006). For example, the disappearance of water lilies and other aquatic weeds reduced the nursery grounds for O. esculentus, while decline of plants such as Potamogeton pectinatus and Ceratophylum demersum favoured by T. zillii drastically reduced its feeding niche (Welcomme, 1967).

Enforcement of regulations

Management of Lake Victoria by the three riparian states of Kenya, Uganda and Tanzania is coordinated regionally by the Lake Victoria Fisheries Organization (LVFO) but implementation of agreed measures is through their respective Fisheries Departments/Divisions. Rules and regulations on how to conduct fishing in the lake to minimise detrimental fishing practices are very explicit, their low compliance is attributed to laxity and weakness in their enforcement (Njiru et al., 2008). The Departments/Divisions of fisheries do not have the capacity to police the entire lake. There are incidences of corruption where fisheries officers are compromised to allow illegal fishing gears and methods to continue to be used in their areas of jurisdiction. This is manifested in the high numbers of illegal fishing gears still operated on the lake (Figure 2b).

Management strategies

Capacity enforcement

The majority of the native fishery of Lake Victoria occurs in waters of 0–10 m deep which apparently is the most heavily fished area (Witte and Van Densen, 1995; Njiru et al., 2005, 2006). In order to sustain the fishery the ban on illegal fishing gears and methods should be enforced. Licensing schemes and extension campaigns should be put in place in anticipation of the need to restrict the open access to the fisheries. The LVFO developed a RPOA – IUU (Regional Plan of Action on Illegal, Unreported and Unregulated Fishing) and RPOA-Capacity to guide initiatives to eliminate fishing illegalities and to control fishing effort. A 200 m from the shore no-fishing zone can be adopted to reduce fishing in the shallow nursery and the breeding areas of the lake. The zone will be easier to monitor than discrete areas in the lake created as water parks either as breeding or nursery grounds.

The policies and regulations governing Lake Victoria resources were different in each country (Ntiba et al., 2001; Van der Knaap et al., 2002; Njiru et al., 2005) but substantial efforts for harmonization have been going on under LVFO (LVFO, 2004; 2007). For, example, the use of monofilaments was banned in Tanzania and Kenya but allowed in Uganda but during the LVFO Council of Ministers Session in February 2009, a joint communiqué was signed to ban monofilament nets in the three partner States. The same session harmonized the minimum mesh size for R. argentea to be 10 mm instead of 5 mm nets. However, fishing for R. argentea is prohibited only in Kenya between 1st April– 31st August. So far, the minimum mesh size of gillnets in Lake Victoria is 5 inches but there should be an agreement regarding minimum mesh size for certain fishes such as Brycinus, Synodontis and Schilbe species which are relatively small in size. Though the regulations of all the three countries are similar, the penalties are very different. There is therefore an urgent need to harmonize all the regulations and penalties in the Lake Victoria fishery and treat the lake as one ecosystem.

Alternative livelihoods

Even with the best management systems in place, the supply of fish within the lake basin available to the local community will be insufficient to meet the ever growing demand due to a rapidly increasing population (Abila, 2000; Yongo et al., 2005). Kenya, Uganda and Tanzania have a combined aquaculture annual production of less than 10 000 mt which is less than 0.02% of the global production (Mushi et al., 2005). The possibility of aquaculture contribution is enormous because the East Africa region is endowed with adequate photoperiod, water bodies, wetlands and suitable native aquaculture species (Mushi et al., 2005). LVFO has developed a Strategy and Investment Plan to guide the development of aquaculture and to attract investors. Other economic activities such as ecotourism and horticulture would reduce pressure on the Lake Victoria fisheries.

Pollution

Decline in water quality and the change in phytoplankton community in Lake Victoria towards dominance of toxic cyanobacteria in the past few decades, is chiefly due to eutrophication arising from increase of nutrients (Lung'ayia et al., 2000; Mugidde et al., 2005). These nutrients originate from agricultural land, unregulated effluents from factories and untreated sewage. Reduction in nutrient emissions can be through improved agricultural practices, such as reduced clearance of vegetation to avert soil erosion and transport of nutrient to the lake, reduced slash and burn vegetation as well as forest fires started deliberately to clear the forest for agriculture, which increases phosphorous in the atmosphere (Mugidde et al., 2005). Upgrading the industrial and municipal treatment systems to include tertiary treatment of waste using constructed wetlands can reduce nutrient flow into the lake. Laws deterring effluent discharges should be reviewed and weakness in enforcement strengthened. Public sensitization should be heightened to curb pollution.

Co-management

A monitoring, control and surveillance scheme properly implemented should be linked into community based management approaches because government enforcement has not previously worked properly (Ntiba et al., 2001; Njiru et al., 2005). A study conducted by SEDAWOG (1999) found that beach communities formed patrol units to monitor their fishing grounds and as a measure to keep off thieves and illegal gears. These were very effective. Recently Lake Victoria Fisheries Organization (LVFO) through a EU funded project (IFMP) has mobilized the formation/reformation of 1069 community-based structures along the lakeside referred to as Beach Management Units (BMUs). Each of the BMUs is responsible for a part of the lake near their settlement. The BMUs have obtained legal status and are operating in collaboration with the Fisheries Officers and Local Goverments in managing the lake resources. It is hoped that the BMUs formation will translate to good fishing practices and conservation of the resources of the lake and the ecosystem at large.

Funds

Though Lake Victoria generates up to $500 million US annually from fish catches (Yongo et al., 2005), there are no substantial funds from the riparian governments assigned for research and management of the lake; most of the funds for these purposes originate from donors. Donor funding is inconsistent and sometimes comes with specific objectives which might not be of immediate concern to the management of the lake. However, the LVFO Council of Ministers has approved creation of ‘Lake Victoria Endowment Fund’ aimed at conserving and ensuring sustainability of the Lake Victoria Fisheries. The revised Fisheries management Plan for 2009–2014 includes a strategy to form a Fisheries Trust Fund of which a percentage will be contributed from the taxes levied on fishing. This fund will then be used for management and demand driven research.

Conclusions

Various factors have acted singly or in conjunction in reduction of species in Lake Victoria. To further arrest the decline, there is a need to treat the lake as one ecosystem. Stakeholders including various government ministries and NGOs working around the lake and its basin should work collectively with an aim to protect and conserve the lake and its environment. The Nile perch definitely played a major role in the disappearance of the native Haplochromine species but various other factors affected their demise also. It should be noted, however, that the taxonomic research and description of the Haplochromine species flock was underway when the Nile perch boom occurred. It will remain unknown how many distinct species occurred in Lake Victoria before the Nile perch and how many became “extinct” after its establishment. With the high fishing pressure exerted on the Nile perch, some haplochromine species thought to be extinct have reappeared in catches. It is recommended that proper taxonomic studies be conducted in order for fisheries managers to have a complete inventory of the fish biodiversity in the lake.

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