3.1 Tilapia fisheries
Tilapia cichlid fishes are grouped into three genera; Oreochromis, Sarotherodon and Coptodon [33]. More than 90 species of Tilapia can be distinguished by high level of parental care which involves building and guarding their spawning nests to ensure high survival rates for the juveniles [34], [35]. Whereas genus Coptodon displays primitive substrate spawning strategy, Sarotherodon and Oreochromis exhibit advanced mouth brooding strategy [36]. Genus Oreochromis comprises of more than 42 species which are mostly distinguished by the banding pattern in their caudal fin. The genus is among the most heavily exploited taxa, which have been transfered and introduced to many parts of the world primarily for aquaculture and argumentation of capture fisheries [37], [38]. Nevertheless, despite displaying successful spawning strategies, some critically endangered native species of the genus Oreochromis are characterized by slow growth rates and low fecundities which contribute to their dwindling trends (Table 1). For instance, O. niloticus has reported a fecundity of over 8,000 eggs distributed over short spawning intervals [39], while small sized O. jipe exhibits an average fecundity of ~ less than 1000 eggs [40].
Table 1
Native Oreochromis species in the Pangani Catchment considered in this study
Species | Common name | IUCN status | Native range | Introduction status |
O. esculentus | Singida tilapia | Critically endangered | Lake Victoria basin | Introduced in Pangani Catchment |
O. niloticus | Nile tilapia | Not assessed | Nile, W. Africa, L. Tanganyika, Yarkon River | Introduced in L. Victoria and the Pangani Catchment |
O. jipe | Jipe tilapia | Critically endangered | Pangani Catchment | Culture trials in Kenya and Tanzania |
Nile tilapia, Oreochromis niloticus is one of the most commercially important tilapiines that has been introduced all over the world for aquaculture expansion and stock enhancement [38]. The species makes significant contributions to lucrative commercial aquaculture fisheries globally [41]. In Kenya, it has been introduced into lakes Naivasha, Victoria, Jipe and several dams to provide commercially exploitable fisheries. Unlike native tilapia, O. niloticus can adapt well to both fast flowing and slow flowing waters in rivers and lakes but they tend to occupy the shallow inshore waters purposely for feeding and reproduction. In addition, they are known to tolerate high salinity levels, lethal temperatures, low dissolved oxygen and high ammonia concentrations which characterize perturbed ecosystems [42]. Besides high fecundities and being less habitat selective, O. niloticus displays other advanced success strategies such as faster growth rates and genetic dominance. The species has reportedly out competed native O. esculentus and O. variabilis from various ecosystems [43], [44]. This makes O. niloticus a highly invasive species that can occupy and establish in introduced areas and out-compete native species that are endemic to those ecosystems [44]; [45]. The focus on lakes Victoria and Jipe was informed by the reports from the previous studies on the introduction of tilapiines in the two lakes resulting to changes in fish composition and the lake ecology.
3.2 Translocation and Introduction of non-native species
Deliberate transfer and introduction of nonnative fish species across the biogeographical boundaries has been used as fishery management tool globally. According to [46], over 237 species have been introduced globally and more than 50 species introductions within the African continent. The Kenyan freshwater lakes, Victoria and Naivasha have so far recorded the highest number of species introductions of 6 and 7 species respectively, while other freshwater lakes such as Jipe and Baringo have only recorded at most two deliberate introductions (Table 1).
Such introductions are associated with unprecedented economic losses and ecological impacts due to interspecific competition with endemic species, and niche overlap [48]. However, not all introduced Tilapiines species have thrived in the new ecosystems. For instance, Oreochromis spilurus niger failed to establish in Lake Naivasha due to competition from established species, predation by M. salmoides and habitat alteration [49], [50]. Lake Victoria is currently dominated by introduced species, Lates niloticus and O. niloticus, which together with native Rastreneobola argentea form the mainstay of the commercial multispecies fishery [51]. Although the fishery introductions were projected to bring more economic benefits to the riparian communities, the introduction of Lates niloticus and four tilapia species led to decline of native species populations in Lake Victoria [11].
Table 2
Distribution, status and maturity sizes of tilapia in lakes Victoria and Jipe
Oreochromis species | Distribution and maximum known length | Reference |
Oreochromis esculentus (Graham, 1928) | Native to and critically endangered in Lake Victoria; 25.3 cm SL | [46] |
Oreochromis jipe (Lowe-McConnell, 1955) | Native to Pangani basin where it is critically endangered; 34.5 cm SL | [40], [46] |
Oreochromis leucostictus (Trewavas, 1933) | Introduced in Lake Victoria and Naivasha basins; 32 cm TL | [46] |
Oreochromis niloticus (Linnaeus, 1758) | Introduced in Lake Victoria basin; 39.5 cm SL | [46] |
Oreochromis variabilis (Boulenger, 1906) | Native and presently endangered in Lake Victoria basin; 26.7 cm SL | [46] |
3.3 Decline of native tilapia fisheries in Lake Victoria
Riparian communities around Lake Victoria depend on fish as the most affordable source of animal protein [52]. Before the introduction of L. niloticus and four tilapiines, Lake Victoria had a diverse multispecies fishery dominated by over 500 species of Haplochromines which co-existed with the native tilapiines O. esculentus and O. variabilis [11]. Between 1960s and the year 2000, remarkable ecological changes occurred in the lake attributed to the upsurge of L. niloticus, which directly preyed upon O. esculentus populations [53]. Despite the decline of native fisheries, Lake Victoria reported increased in fish landings from the introduced O. niloticus and L. niloticus, but the increase was unsustainable due to reported dwindling trends [43], [54].
The O. variabilis is currently red-listed by the IUCN among the critically endangered species which are on a verge of extinction from their native range in the Lake Victoria catchment following exotic introductions [38]. The O. niloticus is an opportunistic omnivore with a diversified feeding that exploits a wide range of food sources such as insects, algae, fish, molluscs and detritus [55]. Therefore, the proliferation of E. crassipes is likely to have provided additional feeding and breeding grounds resulting to increased catches of exotic O. niloticus during the periods of infestation [55]. Consequently, the fishery composition of Lake Victoria shifted from the dominance of native tilapiines O. esculentus and O. variabilis in early-1960s [43], [56] to O. niloticus. It is possible that that the displacement of O. esculentus to small satellite lakes such as Lake Kanyaboli, where it currently occurs as separate and isolated stocks contributed to this decline [54]. Several factors have contributed complex interactions between native and exotic tilapias, which account for the decline of native tilapiines in Lake Victoria [57]. For instance, the observed dietary overlap between adult O. leucostictus and O. variabilis, and juvenile O. esculentus and the three exotic tilapiines, which were introduced into Lake Victoria [58]. There is also niche overlap between O. variabilis and the three introduced tilapiines [11]. The declines of native species have reportedly caused disruption of food chains and nutrient cycling which have consequently accelerated eutrophication in Lake Victoria [59].
3.4 Other causes for decline of native fisheries
3.4.1 Effects of Euthrophication
Excess nutrient loading has also resulted to change in the phytoplankton community from small-sized diatoms to unpalatable large-sized Cyanobacteria, Microcystis spp. which sometimes produces harmful algal blooms (Lung’ayia et al. 2000). This might have reduced the food sources for O. esculentus that feeds almost exclusively on diatoms [57], and O. variabilis that feeds mainly on small sized phytoplankton [37]. The declines of O. esculentus and O. variabilis populations has been accelerated by the change of algal community from the dominance of small sized diatoms which can be manipulated by native tilapia with a small gape size to blue green algae which can only be fed by exotic O. niloticus [23]. Besides, the proliferation of Water hyacinth, Eichhornia crassipes favoured the occurance of populations of introduced C. zilli that feeds on the floating macrophyte over native tilapiines [44].
3.4.2 Interspecific hybridization
Increased interspecific hybridization between O. niloticus and the native O. esculentus species makes it difficult for Lake Victoria to support self-sustaining populations of native tilapia. There has been reported interspecific hybridization between O. variabilis and O. niloticus [57]. Oreochromis niloticus does not occur naturally with native tilapiine species and has been reported to hybridize with native tilapia due to lack of complete genetic isolation between the exotic and native tilapia [37].
3.5 Lake Jipe Ecology and fisheries
Lake Jipe fisheries sector initially supported slightly more than 5,000 people [60]. In addition, the supports a a large biodiversity and provides refugia and breeding ground for both resident and migratory birds, and has been recognized as a wetland of international importance under the Ramsar Convention [31]. The lake also supports a high diversity of wildlife that contribute significantly to socio-economic development through tourism and is protected as national reserve under the Tsavo West National Park in Kenya and Mkomazi Game Reserve in Tanzania [61]. Despite being a highly significant ecosystem, there is a lack of uniformity in cross border management and conservationist strategies due to inadequate scientific information to support policy formulation. This is compounded by lack of integrated water resource management plan to guide largescale water abstraction for various uses such as urban, agricultural and domestic uses. Lake Jipe faces many ecosystem challenges such as accelerated runoff, increased siltation, deterioration of water quality, a collapsing fishery and altered hydrology [60]. Several hydrological transformations have resulted from the diversion of River Lumi instead of flowing into the lake. The lake riparian land and its catchment are being used for agricultural development while the waters of the drainage basin are diverted for domestic supply and expansion of irrigated agriculture.The current ecological changes are linked to lack of adequate community involvement in management and conservation of Lake Jipe ecosystem. This results to poor land use has led to degradation of the lake catchment and deterioration of water quality during erratic rains characterized by high levels of siltation and increased salinity during drought, resulting to virtual disappearance of the foating-leaved macrophytes such as the water lilies Nympaea species and the pigmy Geese [62]. Due to decline in the lake water depth, a large proportion of the inshore area is dominated by emergent macrophytes such as Typha domigensis and a decrease of the area of open waters [62]. Thus, plant succession has been observed by disappearance of water lilies and dominance of Typha sp. as the main wetland plant dominating the inshore areas. Catchment degradation has been aggravated by unsustainable farming activities resulting to siltation.
3.6 Challenges to Lake Jipe Fisheries
In contrast to O. niloticus and O. esculentus, the Kenyan Coastal native Oreochromis jipe species is limited in distribution to a native range of less than 100 km² in the Pangani Catchment of Kenya and Tanzania and makes marginal contributions to livelihoods [38]. Oreochromis jipe and O. esculentus have reportedly a high potential for commercial aquaculture because of many characteristics shared with O. niloticus and being the most important species in terms of size [46]. The back side of sharing common characteristics with O. niloticus is increased rates of inter-specific hybridization which makes it difficult for lakes Victoria and Jipe to support self-sustaining populations of O. jipe and O. esculentus respectively [63]. Nevertheless, the two native species currently coexist with exotic O. niloticus in Lake Jipe. The O. jipe has been reportedly displaced and hybridized by both O. niloticus and O.esculentus. For instance, [64] reported that O. jipe species was displaced from Lake Jipe by O. esculentus, which invaded the lake from Nyumba ya Mungu reservoir downstream. In addition to O. esculentus invasion, siltation led to the proliferation of emergent macrophytes such as Typha domingensis and Cyperus papyrus which culminated into the loss of inshore habitats, also contributing to remarkable decline and subsequent abandonment of the O. jipe fishery [31].