Lake Victoria (Africa) ecosystem

Category: Inland Aquatic Biomes.

Geographic Location: Africa.

Summary: Lake Victoria is the second-largest freshwater lake in the world. It supports millions of people directly, but suffers from pollution, overfishing, and invasive species.

Fabled source of the Nile River, Lake Victoria holds a special place in the hearts and minds of many people, both within eastern Africa and beyond. Lake Victoria is an essential source of sustenance for many species in habitats within its waters and around its shores. It is also vital, in terms of both water and food, for tens of millions of people in the Lake Victoria basin and beyond. Like many lakes around the world, Lake Victoria is also exploited, abused, and polluted constantly through careless and intentional human actions every day.

Lake Victoria’s surface area covers some 26,641 square miles (69,000 square kilometers), making it the second-largest freshwater lake in the world. The catchment area, where the water comes from countless rivers and streams, stretches over 74,500 square miles (193,000 square kilometers) through five countries: Kenya, Tanzania, Uganda, Rwanda, and Burundi. The average depth of the lake is 131 feet (40 meters), with a maximum depth of about 270 feet (82 meters). The volume of water stored is immense, estimated at about 662 cubic miles (2,760 cubic kilometers). This is a significant portion of the entire planet’s freshwater resources that are directly available, not frozen in glaciers or polar ice caps or locked deep beneath subterranean rocks.

Lake Victoria itself is shared by three countries, Kenya, Tanzania, and Uganda. The lake is divided unequally, mainly between Tanzania and Uganda, with a small portion (primarily the Winam Gulf) falling within the jurisdiction of Kenya. In 2015, the immediate basin was home to 35–40 million people, most of whom depended on the lake for their water (in cities like Mwanza, Kisumu, and Bukoba) or for a reliable source of protein in the form of fish taken from the lake.

Effect of Introduced Species

In the 1950s and early 1960s, the ecology of Lake Victoria was changed forever through the deliberate introduction of the Nile perch. What resulted in Lake Victoria is a case of mass extinctions resulting from the introduction of an aggressive invasive species to a stable biome. The Nile perch (Lates nilotica) was present in the Nile and other aquatic systems further north; it had been kept out of Lake Victoria by the steep waterfalls along the river’s course. Therefore, all of the species in the lake had evolved and adapted without ever having to deal with this predator.

The introduction of the Nile perch was done in the northern part of the lake. By 1961, the Nile perch had reached Mwanza Gulf, the southernmost portion of Lake Victoria. Today, it is common throughout the lake, and it is likely to remain a component of the lake’s ecology as long as the lake itself exists. The largest recent mass extinctions of vertebrates, including the loss of cichlids and other fish species in Lake Victoria, have occurred in the decades since the introduction of the Nile perch. Of the lake’s original 500 or so cichlid species, more than half are thought to be extinct.

The Nile perch totally disrupted the ecological balance of the lake. Where hundreds upon hundreds of species of fish, invertebrates, reptiles, birds, and mammals once thrived, the lake’s freshwater ecology in the twenty-first century is dominated by the Nile perch, a single species that seems to have replaced virtually all other piscine predators.

In the open waters, many zooplankton-eating fish were wiped out, and the lake sardine, popularly called dagaa or omena (Rastrineobola argentea), underwent a population explosion through the elimination of its competitors. Another species whose numbers rose in the wake of the Nile perch was the freshwater prawn (Caridina nilotica). Again, this was because the prawn’s population expanded to occupy the niche hitherto filled by cichlids feeding on detritus along the lake bottom.

Throughout its life, the Nile perch is voracious. Young perch prey mostly on insect larvae, crustaceans, and other invertebrates. Older Nile perch feed exclusively on fish, and will eat anything measuring up to a third of their body length.

Spot-necked otters, mammals that relied on hunting cichlids in clear, shallow waters, were also affected. In some places, they managed to change their diet to include the larger fish, but the otters’ large family groups that were once seen hunting in the shallows of the lake declined.

Other invasive species also made their way into the lake. Several tilapia species, including the Nile tilapia (Oreochromis niloticus) and redbreast tilapia (Tilapia zillii), were deliberately introduced by fisheries. These species have also proliferated, and form an important portion of the fish caught and exported from the lake in the 2010s and into the 2020s.

Still other newcomers that changed the lake’s ecology include the Louisiana crayfish, an invasive freshwater snail, and the water hyacinth. A pretty, flowering plant that bears spikes of attractive lilac-hued flowers, the water hyacinth originated in Central and South America. Fed by the nutrient-enriched waters near towns along the lake, and freed from threats that would have been imposed by grazers and parasites in its native habitat, it spread across Lake Victoria with impunity.

The leaf stalks of the water hyacinth are spongy and filled with air sacs, enabling it to float on the water. As it grows, it forms thick mats that soon begin to rot in the lower layers. This depletes oxygen in the water below, in addition to blocking off light. The result is that the native plants, particularly in the littoral zone (from the lake-shore edge to the last rooted, submerged plants), are severely disrupted.

Fisheries

The fisheries of Lake Victoria form a large, dynamic, and thriving economic sector for the region, especially in Kenya, Tanzania, and Uganda. Entire towns and cities, like Mwanza, Tanzania, and Kisumu, Kenya, rely on fishing, fish processing, and exporting as the main drivers of their economies.

In the decades following the introduction of the Nile perch and tilapia, the introduced species reproduced rapidly but were not yet important to the lake fisheries. However, by the 1980s, the number of Nile perch exploded and came to dominate the fish catch, along with the Nile tilapia and the lake sardine (dagaa and omena). Ecologists labeled the population explosion an irruption, as when numbers swell rapidly.

By 2020, the fish catch from Lake Victoria was about 151,000 metric tons annually. Huge profits have been, and continue to be, made off the lake, with 2020 revenue reported at about $US297 million. The rapid expansion of the fisheries, of course, came at a high environmental price.

Environmental Threats

Based upon all the evidence, the fate of the Lake Victoria biome is gloomy. The fishing boom on Lake Victoria lasted into the 2020s, leading to overfishing. Fishing trends included fish killed by poisoning from commercial synthetic pesticides that are poured into the water at night by unscrupulous fishers. These poisons cause the fish to rise to the surface as they die slowly. The larger tilapia and Nile perch are incapacitated but still alive; they are then pulled from the water to be processed fresh. Overfishing and illegal harvesting of fish has become such a large problem, that the International Union for Conservation of Nature issued a 2018 report stating about 76 percent of the fish and crustacean species in the lake could face extinction as a result.

The wholesale export of fish and the extinction of cichlids also have some human consequences. Most of the small communities along the lake shore do not have the fuel, motorized boats, or nets needed for catching the large tilapias and Nile perch. This trade is dominated by foreign-owned companies with fleets of trawlers.

Traditionally, women were able to access protein from cichlids year round, and much fishing could be done with simple techniques in the shallows. After the introduction of the Nile perch, this system changed. In the twenty-first century, many poor women traded sex for fish, which, in the light of the AIDS epidemic, became a serious health risk.

Deforestation in the catchment area of the lake severely reduced the amount of water flowing into the lake. Soil erosion from the loss of forest cover is washing increasing amounts of silt into the lake, which makes the water murky. This disrupts the balance between sunlight penetrating the water and the growth of algae, which is important for many fish and other species. Pollution from towns on the lake shore (Mwanza, Bukoba, and Kisumu), including raw, untreated sewage, also enters the lake daily.

Burning and clearing of the papyrus wetlands that fringe the lake deprives the water of a vital filtration system that cleanses and oxygenates it. Sheltered nests for many young fish, birds, and insects are also wiped out through this practice, which is driven by the high population pressure for more agricultural land.

Overly enriching the water with tons of raw sewage and fertilizer runoff led in many areas to large pockets of anoxic, or zero-oxygen, water, often called the dead zone. This zone, once limited to the deepest, darkest parts of the lake, spread rapidly.

Despite all these ecological problems, Lake Victoria remains one of the most dynamic and productive of the African great lakes. There are challenges in managing it sustainably in an area with a rapidly growing population. Countries sharing the Lake Victoria basin and the Nile River basin are engaged in discussions to better manage this lake, which is important for many people.

Bibliography

Bennun, L. and P. Njoroge. Important Bird Areas in Kenya. Nairobi, Kenya: East Africa Natural History Society, 1999.

Daghar, Mohamed. “Illegal Fishing in Lake Victoria Endangers Livelihoods and Species.” ENACT Africa, 2 July 2019, enactafrica.org/enact-observer/illegal-fishing-in-lake-victoria-endangers-livelihoods-and-species. Accessed 18 Aug. 2022.

"Lake Victoria." Encyclopedia Britannica, 2018, www.britannica.com/place/Lake-Victoria. Accessed 9 July 2018.

Martins, D. J. “Differences In Odonata Abundance And Diversity In Pesticide-Fished, Traditionally-Fished And Protected Areas In Lake Victoria, Eastern Africa (Anisoptera).” Odonatologica 38, no. 3 (2009).

Onyango, Horace Owiti, et al. “The Lost Coin: Redefining the Economic and Financial Value of Small-Scale Fisheries, the Case of Lake Victoria, Kenya.” Social Sciences & Humanities Open, vol. 4, no. 1, 2021, doi.org/10.1016/j.ssaho.2021.100221. Accessed 18 Aug. 2022.

Pringle, R. M. “The Origins of the Nile Perch in Lake Victoria.” BioScience 55 (2005).

Weston, Mark. "Troubled Waters: Why Africa's Largest Lake Is in Grave Danger." Slate, 27 Mar. 2015, www.slate.com/articles/news‗and‗politics/roads/2015/03/lake‗victoria‗is‗in‗grave‗danger‗africa‗s‗largest‗lake‗is‗threatened‗by.html. Accessed 9 July 2018.