This case study examines the regime shift caused by the "Nile perch boom" in Lake Victoria, Eastern Africa. The Nile perch, an introduced species, remained a minor component of the lake's cichlid-dominated fauna for more than two decades after its introduction. This regime was maintained by feedbacks between the cichlids and phytoplankton, preventing the lake from oxygen depletion, as well as feedbacks between the cichlids and Nile perch juveniles, controlling the Nile perch population. Human population growth indirectly caused both increased fishing pressure on the cichlids and nutrient input to the lake, eventually leading to the regime shift in the 1980s. The new regime is especially upheld by feedbacks between the Nile perch and cichlids as the latter are the preferred feed of the Nile perch. The ecosystem service impacted the most by the regime shift was fishery, changing from a local and more gender-equalized system to a large-scale international fishing business with local inequalities. For management interventions, the focus is not on reversing the regime shift, but rather on improving social conditions in the lake surroundings.
Type of regime shift
- Cichlid dominated state to Nile Perch dominated state
- Freshwater lakes & rivers
- Small-scale subsistence crop cultivation
- Large-scale commercial crop cultivation
- Extensive livestock production (natural rangelands)
- Timber production
Spatial scale of the case study
- Local/landscape (e.g. lake, catchment, community)
Continent or Ocean
- Eastern Africa
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Key direct drivers
- Vegetation conversion and habitat fragmentation
- Harvest and resource consumption
- External inputs (eg fertilizers)
- Adoption of new technology
- Species introduction or removal
- Environmental shocks (eg floods)
Key Ecosystem Processes
- Primary production
- Nutrient cycling
- Aesthetic values
- Knowledge and educational values
- Food and nutrition
- Health (eg toxins, disease)
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
- Social conflict
Spatial scale of RS
Time scale of RS
- Contemporary observations
Confidence: Existence of RS
- Well established – Wide agreement in the literature that the RS exists
Confidence: Mechanism underlying RS
- Well established – Wide agreement on the underlying mechanism
Cichlid dominated regime (1950s – 1980s)
The regime was characterized by a lake of high biodiversity dominated by different species of native haplochromine cichlids (Balirwa et al. 2003; UNEP 2010). Cichlids span over many different trophic levels and represent nearly all different functional groups in Lake Victoria (UNEP 2010). In this regime the lake was less eutrophic compared to the current regime and thus less turbid. Nile perch was already introduced to the lake but populations remained low.
In the lake basin there were many local communities living on subsistence farming and small-scale fishing (Swallow 2009). The native fish species provided food for the local communities and the fishing for native cichlids was non-commercial but still population growth in the region put a large pressure on the native fish stocks (Seehausen et al. 1997).
Nile perch-dominated regime (From 1980s)
In this current regime the system has much reduced biodiverse lake with high abundance of only three species, the dominant Nile perch Lates niloticus, Nile tilapia Oreochromis niloticus (Balirwa et al. 2003), and a native cyprinid Rastrineobola argentea (UNEP 2010). The cyprinid is competing for the same resources as some cichlids (Matsuishi et al. 2006) and thus became very abundant once the cichlids disappeared. This low diversity makes the system vulnerable to disturbances (Balirwa et al. 2003; UNEP 2010). Eutrophication is an ever increasing problem resulting in algae blooms, turbidity, and hypoxia (Balirwa et al. 2003), all of which are undesirable conditions for the native species.
The fishing of Nile perch created a global market for fish export. The fishing industries involved many from the local communities and there was an increase in income for those in the Nile perch business. However, the changes brought many social problems emerging from unequal distribution of the increased income (Geheb et al. 2008).
Drivers and causes of the regime shift
The apparent extinction of several hundreds of cichlid species was caused by Nile perch predation (Goudswaard et al. 2008). However, this event was preceded by drivers that weakened the cichlid population and opened up the field for the Nile perch. One of the main drivers was the increased pressure on the Lake Victoria cichlids by fisheries. At the beginning of the 1980s, cichlids accounted for more than 90 percent of the catch (Balirwa 2007). The other main driver was the increase in nutrient input from the Lake Victoria surroundings, leading to instances of algae blooms and hypoxia. The deoxygenation created an environment which was not suitable for the cichlids, many of which inhabited lower water columns where the oxygen deficiency was most acute (Hecky et al. 1994). The increase in nutrient inputs also caused turbidity which contributed to the reduction of reproductive capacity among cichlids (Seehausen et al. 1997).
The underlying driver of both of these developments is the human population growth around the lake, accelerating fishery activities and off-shore developments such as deforestation and expansion of agriculture which fed the lake with nutrient run-offs (Ibid.). Another cause of the demise of the cichlids was the heavy rainfall events during the 1960s. This external shock destroyed littoral vegetation around the lake which had served as important spawning and nursery grounds for native species. This enabled another species, the Nile tilapia, to conquer some of these areas and outcompete cichlids (Kaufman 1992). At this time the system was resilient enough to avert a shift but these events were precursors to later developments.
How the regime shift worked
Under the cichlid dominated regime, Lake Victoria was an ecosystem with high species diversity. By eating basically everything in the lake these cichlids performed tight internal recycling (Kaufman 1992) and the pelagic planktivorous and detritivorous cichlids occupied a significant role in protecting the system against the excess production of phytoplankton (Goldschmidt et al. 1993). This allowed allowed resilience against increased nutrient input. The dominance of the cichlids also suppressed other species, including the Nile perch. The cichlids fed upon Nile perch larvae and competed with Nile perch juveniles for space and food (Goudswaard et al. 2008). These dynamics might explain why it took the Nile perch decades to accomplish its virtual takeover of the lake; as long as the cichlids were abundant, the Nile perch reproduction was suppressed (Ibid.).
However, due to the combined threats of Nile perch predation, overfishing, and the increasing nutrient inputs, this changed. First, the overfishing severely reduced the cichlid population in certain areas. Once the cichlid stock was reduced the Nile perch juveniles were able to colonize the areas. From there on, the Nile perch population could grow, invade neighboring areas, reduce the cichlids, and open up new areas for the recruitment of Nile perch juveniles (Ibid.). In this new regime, the Nile perch was the dominator and cichlids then dominated. When the cichlids started to disappear, this situation was opportunistically exploited by other species, including native shrimps. In the absence of the cichlids the shrimp population grew massively and became the most important source of food for the Nile perch (Kaufman 1992, Goldschmidt et al. 1993).
The reduction of cichlids, in combination with the spike in nutrient input, in turn had detrimental effects on the nutrient recycling, resulting in hypoxia. Those cichlids which had inhabited the lower water columns, and had therefore avoided Nile perch, were forced to migrate to shallow waters to escape the hypoxic waters. In these shallow waters they became the target of Nile perch predation (Hecky et al. 1994).
Impacts on ecosystem services and human well-being
Both before and after the regime shift, Lake Victoria has been providing important provisioning ecosystem services, primarily fisheries. Before the shift, these fisheries were run by small-scale fishermen and operators; women were largely involved in the business (Abila & Jansen 1997), and the fish was for local consumption. However, the Nile perch boom diminished the variety of native fish, and turned this food source into economically more valuable forms of fish meat (Geheb et al. 2008). Foreign investment brought processing plants to the lake's shores (Kaufman 1992), boosted fish export, and eventually turned the artisanal fishery into a large-scale international enterprise. Another provisioning ecosystem service, namely the provision of freshwater suffered from decreased water quality through the cichlids' disappearance (Kaufman 1992).
Cultural ecosystem services were gained in terms of (fishing) tourism but presumably lost in terms of ecosystem/biodiversity knowledge, and aesthetic values. Since the regime shift, the majority of local community members have been carrying out economic activities linked to the Nile Perch fishery. The livelihood for many has improved (Abila & Jansen 1997). However, income from the fishing industry has been unequally distributed (Geheb et al. 2008). Due to social gender inequalities mainly men have been drawing profits from the fisheries boom while women and children have often been victims of malnutrition which is high along the lake's shores (ibid.). Many women have been forced into prostitution which in turn has increased the number of HIV/Aids infections after the Nile perch boom (Appleton 2000).
The Nile perch was introduced in Lake Victoria to improve the commercial fishing values (Balirwa et al. 2003). The social effects from this have had huge and diverse impacts on the local communities around the lake resulting in, on one side, a billion dollar export industry, and on the other side, malnutrition, unequally divided income, and increased spread of HIV/Aids (Béné & Merten 2007, Appleton 2000). Nothing was done to prevent the shift from the cichlid dominated regime to the Nile perch dominated regime in the 1980s. Currently there are many different projects that work across the Lake Victoria Basin. The Lake Victoria Basin Commission (LVBC) is an institution of the East African Community which coordinates the policy and management agenda for the region (LVBC 2011). The program works for local community participation, poverty reduction and environmental concerns. Health issues, especially HIV/Aids (LVFO 2011) and human well-being are the most important parts of many different projects working in the lake surroundings. Some projects in the Kenyan part of the Lake Victoria basin mentioned by Swallow 2009 are the TransVic project byWorld Agroforestry Center, the Western Kenya Integrated Ecosystem Management Project and Vi Agroforestry (Swallow 2003 and 2009). These are all focusing on land degradation and agroforestry as land use management methods which both reduce the nutrient input to improve the lake conditions and work for poverty reduction (ibid.).
A return to a regime with low abundance of Nile perch is not the focus of management and action plans (Schindler et al. 1998, Kitchell 1997) since the economic value of the Nile perch is so important. The ecological focus is rather on restoration of biodiversity, the lake ecosystem services, and the sustainability of the fisheries in the lake. A sustainable fishing of Nile perch can be reached by the control of fishing practices(Seehausen et al. 1997a). A study by Schindler et al. in 1998 show that by using a minimum mesh size of 5-inches the Nile perch cannibalism and the predation of native species can be reduced by 44% at the same time as the catches of Nile perch is only reduced by 10%. The re-emergence of native cichlids is an important step in the lake management to restore biodiversity in the lake system. By continuing the fishing of Nile perch the species is held at a controlled level giving the cichlids an opportunity to return to the system. For this it is also of great importance to improve the lake conditions by reducing the leakage of nutrients into the lake e.g. through investments in sewage treatment and restoration of wetlands in the basin area (Balirwa et al. 2003).
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