Fishing on Lake Victoria

Lake Victoria supports Africa's largest inland fishery, with the majority of the catch being the invasive Nile perch, introduced in the Lake in the 1950s.^[1]

History

In 1927-1928

In his official report of the survey, Graham wrote that "The ngege or satu Tilapia esculenta, is the most important food fish of the lake, whether for native or non-native consumption. No other fish equals it in the quality of the flesh. It is convenient size for trade, travels well and is found in much greater numbers than other important fish, such as semutundu (Luganda), Bagrus sp.''.[4] Furthermore, Graham noted that the introduction of the European flax gill-net of 5 inch mesh had undoubtedly caused a diminution in the number of ngege in those parts of the Kavirondo Gulf, the northern shore of the lake, the Sesse Islands and Smith's Sound that were conveniently situated close to markets.^[4]

Before 1954, Lake Victoria's ecology was characterised by enormous biodiversity. It was inhabited by over 500 species of fish, 90% of which were cichlids belonging to the haplochromines.^[5] They are thought to have evolved in Lake Victoria within the last 15,000 years.^[6][7] They are known for their extraordinary ability to evolve rapidly to suit extremely localised and diverse environments, a characteristic termed 'evolutionary plasticity'.^[8][9][10] This ability has made the cichlid species of Lake Victoria an extremely successful fish. Haplochromine species accounted for some 80% of the fish biomass of the lake,^[5] an abundance which led Graham to believe that this species flock could support a trawler fishery of up to 200 boats.^[11] It also meant that Lake Victoria at one time boasted one of the most diverse fish environments on earth.^[12] With such diversity, the cichlids of Lake Victoria managed to exploit virtually every food source available, including most detritus, zooplankton and phytoplankton.^[8]

Haplochromine species are relatively small and bony, and were generally not favoured in catches. Riparian populations preferred the lake's two endemic species of tilapia (Oreochromis esculentus and O. variabilis).^{[Note 1]} Hence, by the late 1940s, British colonial authorities were debating the overall ecological efficiency of the lake. For many authorities at this time, the lake needed a large and efficient predator to turn haplochromine fish stocks into something more economically valuable. The prime candidate was the Nile perch (Lates niloticus). Arguments in favour of introducing a predator were as follows:

1. Evidence from other African lakes suggested that the Nile perch could do well commercially, and if the fish were introduced into Lake Victoria, it would no doubt eat the haplochromines, and hence improve the overall value of the fishery. In addition, it was suggested that by introducing such a fish, fishers might turn their attentions to catching it, so easing some of the pressure on tilapia stocks.^[14] Because the Nile perch can weigh as much as 200 kilograms (440 lb), it was argued, fishers would be encouraged to use large mesh-sizes to capture it.^[14] Furthermore, the perch was said to prefer off-shore habitats, and this would serve to relieve pressure on the in-shore tilapia fishery.^[14][15]
2. In those lakes where the Nile perch was a native it co-existed with tilapia species, and therefore it posed no great danger to commercially important tilapia stocks in Lake Victoria.^[14][15]
3. The potential size of the perch made it a fine sports fish, and this might attract sports fishers and tourists to the lake region.^[15]
4. Finally, archaeological findings close to Lake Victoria suggested that an ancestor of the Nile perch had been native to the Miocene predecessor of Lake Victoria, Lake Karunga.^[16] It was reasoned, then, that if the perch had once been native to the lake, there was little reason why it could not be a native again.^[15][17]

The arguments against its introduction were as follows:

1. Predators can never be as abundant as non-predators because of their nutritional requirements. In other words, the amount of fish a Nile perch has to eat to produce a kilogram of flesh is far greater than the amount of vegetative matter a tilapia has to eat to produce an equal amount of flesh. In ecological terms, the Nile perch is inefficient.^[18] As mentioned above, the fertility of tropical waters depends on the rate at which nutrients (mainly detritus) are brought back into solution. The Nile perch, however, does not eat this detritus, and it was argued that its introduction would do little to improve the over-all ecological efficiency of the lake. If any fish should be introduced, it should be a herbivore or a detritivore.^[19][20]
2. It would be wishful thinking to suppose that the Nile perch would exclusively consume haplochromines.[18]
3. Given the complexity of tropical ecosystems, it is impossible to predict what might happen should the Nile perch be introduced.^[18]

While the argument continued, it was agreed that tilapia stocks needed bolstering largely as a result of increasing fishing pressure on indigenous species, associated with the expansion of the market for fresh fish in the 1940s.

Introduced tilapia species were unable to establish themselves in the lake between the 1950s and 1963. Following unusually heavy rainfall in the early 1960s, however (the so-called 'Uhuru Rains'), the lake-level rose considerably, flooding large areas of shore-line, and opening up new breeding areas to fledgling tilapia stocks, so creating the opportunities for these exotic species to compete with indigenous stocks.[1] Of the six exotic species introduced, two – Oreochromis niloticus and O. zillii – were to firmly establish themselves in the lake, so that by the early 1980s, these two species comprised the mainstay of tilapia catches. Of the indigenous species, O. esculentus was extirpated from the lake as a consequence of competition with introduced tilapiids, while O. variabilis populations declined significantly.^[22]

The Nile perch is a substantial

The introduction of the Nile perch had a decisive impact on haplochromine stocks which it favoured as its prey, affecting both their abundance and diversity. It is believed that the contribution of this species flock to the fish biomass of the lake has decreased from 80% to less than 1% since the introduction of the Nile perch,[5]^[25] and that some 40% of the haplochromine species were driven to extinction in the process.^[26][27][28] This may well represent the largest extinction event amongst vertebrates in the 20th century^[8] and has been called the "most dramatic example of human-caused extinctions within an ecosystem".^[29]

Freed from their evolutionary predators, populations of the diminutive endemic silver cyprinid Rastrineobola argentea (omena in Luo, mukene in Ganda and dagaa in Swahili), flourished, developing into huge shoals. In turn, pied kingfisher (Ceryle rudis) populations, that had hitherto fed on haplochromines, exploded in response to this new food source.^[30] Similar and other impacts have propagated throughout the ecosystem.

Given its 'evolutionary plasticity', it follows that haplochromines should adapt rapidly to the new environmental conditions generated by the Nile perch 'boom' and eutrophic conditions. Evidence from studies on the lake does suggest that this is happening.^[31] One haplochromine, H. (Yssichromis) pyrrhocephalus, a zooplanktivore, was almost driven to extinction by the Nile perch. This species has recovered alongside increased exploitation of the Nile perch, at a time (the 1990s) when water clarity and dissolved oxygen levels had declined as a consequence of eutrophication.^[32] Over a period of just two decades, H. pyrrhocephalus responded to raised hypoxia (oxygen deficiency) by increasing its gill surface area by 64%. Head length, eye length, and head volume decreased in size, whereas cheek depth increased. The former morphological changes may have occurred to accommodate this increased gill size. Other morphological changes suggest adaptations in response to the availability of larger and tougher prey types.

By the end of the 1940s, fish stocks were under severe pressure. The market for fish grew since railways were available to carry fish to the coast and settlements in between. Improved fishing boats and nets were introduced.

In 2007, Lake Victoria Sailing Company (LVSC) petitioned the Ugandan government to gazette the first Lacustrine Protected Area (LPA) in Ugandan Lake Victoria that is enforced by the UPDF-Fish Protection Unit (FPU). This protected area is to encourage fishers to form fishing cooperatives and allow fish populations to replenish. [38]

In 2015, Victory Farms was founded by two Americans, and soon became the fastest growing fish farm in sub-Saharan Africa.^[39]

Economic impact

With such high demands for Nile perch, the value of the fishery has risen considerably. Labour inflows into the fishery have increased along with growing demand. In 1983, there were an estimated 12,041 boats on the lake. By 2004, there were 51,712, and 153,066 fishermen.^[34] The fishery also generates indirect employment for additional multitudes of fish processors, transporters, factory employees and others. All along the lakeshore, 'boom towns' have developed in response to the demands of fishing crews with money to spend from a day's fishing.^{[Note 4]} These towns resemble shanties, and have little in the way of services. Of the 1,433 landing sites identified in the 2004 frame survey, just 20% had communal lavatory facilities, 4% were served by electricity and 6% were served by a potable water supply.^[34]

The impact of perch fishing on the local economy is the subject of the 2004 documentary Darwin's Nightmare.

Controversy

The Nile perch fishery has proved controversial, not least between conservationists keen to see the preservation of the lake's unique ecology; and others who recognise the fish's importance to regional economies and poverty alleviation. In the 1990s, one group of thinkers argued that the export of this fish represented a net loss of fish proteins to riparian fishing communities, hence explaining high incidences of malnutrition amongst them.^[41][42][43] 'Empirical evidence ... vividly show that the growing export of the Nile perch and the commercialisation of the dagaa are undermining the survival of households'.^[44] Malnutrition amongst these communities are indeed high; one study estimates that 40.2% of children in fishing communities are stunted.^[45] Rates of childhood malnutrition are, however, lower around the lake than they were in the agricultural hinterland. At fish landing sites, 5.7% of mothers were found to be chronically malnourished.^[45]

The three countries bordering Lake Victoria – Uganda, Kenya and Tanzania – have agreed in principle to the idea of a tax on Nile perch exports, proceeds to be applied to various measures to benefit local communities and sustain the fishery. However, this tax has not been put into force, enforcement of fisheries and environmental laws generally are lax, and the Nile perch fishery remains in essence a mining operation so far.

Notes

References