Fishing on Lake Victoria
[2][3] Other commonly observed species included: Marbled lungfish (Protopterus aethiopicus), Elephant-snout fish (Mormyrus kannume), Ripon barbel (Labeobarbus altianalis), African sharptooth catfish (Clarias gariepinus), silver butter catfish (Schilbe intermedius) and Semutundu (Bagrus docmak).
[4] 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.
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.
[6][7] They are known for their extraordinary ability to evolve rapidly to suit extremely localised and diverse environments, a characteristic termed 'evolutionary plasticity'.
[12] With such diversity, the cichlids of Lake Victoria managed to exploit virtually every food source available, including most detritus, zooplankton and phytoplankton.
For many authorities at this time, the lake needed a large and efficient predator to turn haplochromine fish stocks into something more economically valuable.
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.
"On the face of it...the lake after the debut of Lates [Nile perch] has turned into a fish producer that can only be described in Gargantuan terms".
[24] 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.
[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.
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%.
[Note 2][1] British colonial authorities set out to remedy this through the introduction of exotic species, including tilapia and the Nile perch.
[Note 3] The second impact was the Nile perch's devastation of the haplochromine species flock, its main food source.
The EU has frequently closed its doors to the export for reasons ranging from unsatisfactory hygiene at factories to cholera outbreaks on the lake shores.
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.
[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'.
[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.
