In 1858, the British explorer John Speke stumbled onto the southern shore of the lake and proclaimed he had discovered the fabled source of the Nile. In 1875 another British adventurer, Henry Stanley, circumnavigated Lake Victoria to confirm Speke's claim and spent two weeks spinning tales of God and England to curry favor with Mtesa, king of Buganda and ruler of the northern lake region. Then Stanley sent word back to England, calling for missionaries. They came with soldiers and traders. Within 20 years England had taken charge of what became Uganda and Kenya (Germany took over Tanzania), and by 1902 the colonial government had pushed through a railroad from Mombasa on the coast to the lake. Europeans soon denuded vast tracts of forest in Lake Victoria's watershed to plant tea, coffee, sugar, tobacco, and cotton. The human population exploded, and people increasingly turned to the lake not for subsistence but to satisfy a market for fish, especially the tasty tilapia called ngege, in the growing urban centers.
Fishing pressure on the lake began to intensify in 1905 when the British introduced flax gill nets, which soon replaced the local villagers' papyrus nets and fish traps. Catch sizes began to drop; fishermen turned to nets with ever smaller mesh sizes mainly fished in the shallows, thus affecting both the breeding adults and young of many native species. By the 1950s the Ngege was commercially extinct and the Labeo was not far behind. To compensate, British officials decided to stock the lake with new fish species from elsewhere.
The first non-native species of tilapia (Cichlidae), that prospered in Lake Victoria was the Nile tilapia, Oreochromis niloticus which was introduced in the early 1950s. In 1955 the Nile Perch, Lates nicloticus (Centropomidae) was introduced into Lake Kioga (downstream of Lake Victoria), and when a few years later some specimens were found in Lake Victoria, steps were taken to ensure its establishment there.
Despite the fishing pressures, the native haplochromines appeared to be thriving in the 1950s. In fact, they appeared so robust that some colonial administrators favored introducing a big predator like the Nile perch to eat what they considered bony little "trash" fish and "convert the haplochromine biomass" into something more suitable for the restaurant table. Ecologists, fearing the worst for local species, strongly opposed the predator. But in 1954 illicit Nile perch began appearing in commercial catches anyway. Since the it had already been introduced, officials actively stocked the lake with perch in the early 1960s. (Ogari suspects perch were first slipped into the lake by colonial sportsmen, just as homesick British anglers put trout into highland streams throughout eastern and southern Africa.) Still, for the next decade or so, the Nile perch population remained small. A United Nations-sponsored survey completed in 1971 found that haplochromines still made up the expected 80 percent of the lake's fish biomass.
Then came what Pereti Basasibwaki, a Fisheries Research Institute biologist, calls the dark period, the years between 1974 and 1979 when the regime of Idi Amin in Uganda cut off most access to the lake. Until 1978, Nile Perch remained a very small proportion of the commercial catch, less than 5 percent. In 1979, the Kenyan government took over the idle Kisumu fishery facility and work resumed under ICMFRI's auspices. Pilot surveys suggested the lake's fish populations were relatively unchanged: they still appeared to consist of 80 percent haplochromines and less than 2 percent Nile perch. But by 1980 an abrupt change in relative proportions showed up in Kenyan waters, and within two years similar changes were apparent in Ugandan and Tanzanian waters too. Nile perch had suddenly increased to around 80 percent of the biomass, and the haplochromines had dropped to approximately 1 percent. Ngege, already rare, virtually disappeared. The result was that by 1990 the commercial catch had a totally different composition, dominated by Nile Perch (almost 60 percent) and Omena (most of the remaining 40 percent). The haplochromines, and the mixture of other mainly small, native fish had virtually vanished from the commercial catch.
In the early 1980s, however, few government officials in Africa viewed the shift in Lake Victoria's fish fauna as a disaster. The bottom waters of the lake appeared to be a dead zone, devoid of oxygen and fish life. On the other hand, the lake was chockfull of algae--five to ten times more than in the early 1960s. That suggested massive eutrophication, an oxygen-depleted condition caused by high levels of nutrients that encourage the rapid growth of plankton, especially plant plankton such as algae. The decay of plankton in turn depletes water of oxygen.
Water Quality of Lake Victoria : Next Section
Page Author: Dr Rick T Leah, Univ of L'pool - Contents last reviewed15/12/2005