Contribution of fish farmers to conservation of endangered Lake Victoria Basin fish species - the case of Oreochromis variabilis and O. esculentus

Abstract

Freshwater ecosystems have suffered greatly from interference from human activities over the past 100 years rendering many fish species extinct, rare or endangered. The need for conservation action is therefore paramount (Cowx and Collares-Pareira, 2002). Both the Mbiru (Oreochromis variabilis (Boulenger)) and Ngege (Oreochromis esculentus (Graham)) are presently listed in the IUCN Red List of Threatened Species as critically endangered (see IUCN, 2000). Both fish species are endemic to Lake Victoria and its satellite lakes. Their disappearance from commercial and subsistence landings of the Lake Victoria fishery has been explained by several authors as due to predation by Nile perch (Lates niloticus (L.)), an introduced predatory species, competition for food and nesting sites from Nile tilapia (Oreochromis niloticus), another introduced species, environmental degradation; including pollution, over-fishing and destructive fishing methods (Lowe-McConnell, 1982; Ogutu-Ohwayo, 1990; Greboval and Mannini, 1992). Before L. niloticus, O. niloticus and Tilapia zillii (Gervais) were introduced into Lake Victoria, the majority of the ichthyofauna of the lake was made up of haplochromines and tilapiines. From as early as 1908, an organized gillnet fishery introduced in Nyanza Gulf three years earlier had intensified, putting a lot of pressure on the native tilapiines, O. esculentus and O. variabilis (Graham, 1929), which were the only large species of commercial value. In less than 20 years, several hundred species had disappeared from Lake Victoria (Witte et al., 1992). The Nile perch upsurge coincided with the eventual total disappearance of these species from the main lake. Until the mid-1960s, Lake Victoria contained large stocks of O. esculentus of relatively large sizes. In the earlier surveys, Graham (1929) recorded O. esculentus of 30 cm total length (TL) in Kavirondo Gulf and 31 cm TL in the open lake and 40–50 cm TL in the southern waters of the lake. Greenwood (1966) continued to record modal adult size 30–32 cm TL for O. esculentus in the lake. Due to increased fishing pressure the fish became smaller and the catches, especially of O. esculentus, which was the most targeted species, reduced (Fryer and Iles, 1972). Tilapias are well known for their abilities to hybridize in the natural environment when native species are in contact with introduced species (Agnèse et al., 1998). Hybridization with O. niloticus is considered as one avenue in which populations of the two species suffered genetic modification and disappearance from the lake (Mwanja and Kaufman, 1995). Welcomme (1967) observed hybrids between O. niloticus and O. variabilis before the latter species disappeared from the lake. It also seems likely that O. esculentus hybridized with O. niloticus, which is presently the most dominant tilapiine species in Lake Victoria. Thus, the best way of conserving these species may be to produce them as pure species in an environment ex situ away from the predatory L. niloticus and the closely related O. niloticus.