Oreochromis niloticus (Nile tilapia
) is one of the most cultured tropical fish species in the world. In 2018, its production reached 4.53 million tonnes, which accounted for 75 % of the total farmed tilapia production in the world (
Miao and Wang, 2020). It is one of the most important Cichlid species based on its nutritional and economic role in many tropical and sub-tropical countries
(Sosa et al., 2005). It has a certain number of important characteristics that makes it a special species for aquaculture. One of those key characteristics is the relatively short generation time estimated at approximately 6 months in farm ponds and about 10 to 12 months in natural environment; relative to other species such as common carp (10 to 14 months in ponds and 36 to 60 months in the wild), Clarias (6 to 9 months in captivity and 12 to 24 months in the wild)and trout (12 to 14 months in captivity and 12 to 24 months in the wild), which ensures that the production cycle is completed within a single year (
Kurbanov and Kamilov, 2017;
FAO, 2023). The species is also both planktivorous and omnivorous, making it an excellent fit for low-cost aquaculture (
Tesfahun and Temesgen, 2018). The Nile tilapia has ability of adaptation to various environmental conditions hence, leading to its widespread production (
Fitzsimmons, 2000). It is widely distributed in Africa ranging from the entire Nilo-Sudanian region to the Northern part of the East African Rift-Valley
(Bezault et al., 2011).
In these areas, the massive establishment of aquaculture farms along water bodies (rivers and lakes) has led to escape of fish from the ponds into large water bodies, hence resulting in mixing of escapees with native species (
Ntirenganya, 2019). The repercussions of such mixing have been previously reported by
Firmat et al., (2013) to be competition, hybridization and introgressions between species. The most remarkable incident in the world of fisheries was the disappearance of
Oreochromis variabillis from the main Lake Victoria caused by
Lates niloticus in 1960s (
Boulenger, 1906;
Welcomme, 1966,
Canonical et al., 2005; Angienda et al., 2011).
In Sub-Saharan Africa, conservation and management of the already admixed species might not be successful if the morphogenetic features of the concerned species are not well elucidated. Therefore, considering the various anthropogenic activities, various production systems and environmental conditions in Africa and their influence on phenotypic variation of species, the characterization of
O. niloticus in the major production systems in Africa is highly needed for better management and utilization. The morphometric diversity of the currently raised strains of
O. niloticus has been characterized only in selective environment with varying results (
Appleyard and Ward, 2001). This paper reviews the morphometric variations of farmed Nile tilapia in Sub-Saharan Africa in order to provide a better understanding of underlying factors across the region. In addition, it documents actions needed to preserve and/or sustain utilization of the diversity of Nile tilapia in Sub-saharan Africa.
Geographic distribution of Nile tilapia
The culture of Nile tilapia can be traced to ancient Egyptian times (
FAO, 2009). Introductions of the species are summarized in www.fishbase.org (
Fishbase, 2014).
FishBase (2014) reported the occurrence of Nile tilapia in 102 countries across the world (
Fishbase, 2014, Fig 1).
Tropical and subtropical Africa are the native ranges of Nile tilapia. The species is found in the Nile basin including Lake Albert, Edward and Tana, Jebel Marra, Lake Kivu, Lake Tanganyika, Omo River system, Lake Baringo, Awash River, Suguta River, several Ethiopian lakesand Lake Turkana (
Trewavas, 1983;
FishBase, 2014;
FishBase, 2023). In West Africa, it is naturally found in the Basins of the Senegal, Benue, Gambia, Volta, Niger and Chad (
Picker and Griffiths, 2011;
FishBase, 2023).
Nile tilapia production systems in Sub-Saharan Africa
In Sub-Saharan Africa (SSA), Nile tilapia is cultured in different systems, which include water-based and land-based systems (
El-Sayed, 2006). The choice of the culture system mostly depends on the farmer’s economy and skills, available infrastructureand environmental conditions (
Donbæk et al., 2019). The Nile tilapia production system can also be classified as extensive, semi-intensive and intensive system based on the input utilization
(Omasaki et al., 2016). These Nile tilapia production systems are below discussed with examples in the context of Sub-Saharan Africa (Table 1).
Extensive and semi-intensive fish production systems, commonly used by smallholder farmers in developing countries, involve culturing fish in ponds or small bodies of water with minimal inputs (
Hernández-Mogica et al., 2002). In Kenya, Nile tilapia production primarily occurs in earthen ponds, although Recirculating Aquaculture Systems (RAS) are also used
(Omasaki et al., 2017). RAS offers advantages such as reduced land and water requirements, but it requires a stable energy supply and careful biosecurity management. In Zambia, various systems such as ponds, raceways, hapas, tanksand cages are used while in the DR Congo, Nile tilapia farming is mostly extensive for household consumption, with a few semi-intensive and intensive systems in periurban areas (
Fagbenro and Adebayo, 2005;
MINPE, 2021). Intensive culture systems, including earthen ponds, tanksand Recirculating Aquaculture Systems (RAS), are spreading in Sub-Saharan African countries, with stocking densities ranging from 50,000 to 100,000 fish/ha and yields of 25 to 40 kilograms per cubic meter. Challenges faced by farmers include the lack of quality seed and feed (
El-Sayed, 2013;
El-Sayed, 2017).
Variation factors used to morphometrically characterize Nile tilapia in Sub-Saharan Africa
Morphometric characters have been widely used to differentiate the various populations of Nile tilapia across Sub-Saharan Africa. Different variation factors have been used to study the morphometric characteristics of Nile tilapia populations in Sub-Saharan Africa (Table 2).
With regard to the sex,
Amoussou et al., (2017) found significant differences between male and female Nile tilapia, with males exhibiting higher body weight, total lengthand standard length (p<0.05). Environmental factors also influenced morphometric traits, as fish from Ouémé River and Couffo River had higher total and standard lengths compared to those from Lake Toho (p<0.05). With regards to the population type,
Fakage et al., (2019) observed distinct morphometric traits between natural populations (Lake Kivu, Ruzizi River) and a cultured strain (Nyakabera strain) in DR Congo, with discriminant traits including Head Length, Body Height, Eye Length, Eye Diameterand Anal fin length.
Vreven et al., (1998) noted morphological differences between Nile tilapia populations from different regions, with Nile populations closer to East African populations than West African populations.
Makeche et al., (2020a, b) identified significant morphometric differences among Nile tilapia strains from Zambia (Yalelo Fishery, Fwanyanga Fishery), including total length, body weight, standard length, body height, head length, pre-anal distanceand pre-ventral distance (Table 2).
Nile tilapia strains
Studies conducted across the Sub-Saharan Africa revealed the existence of different strains of Nile tilapia in natural as well as artificial environments. A summary of their results is presented in Table 3.
Actions needed to preserve the diversity of Nile tilapia in Sub-Saharan Africa for appropriate aquaculture breeding programs across the region
Conserving diversity is a global challenge. For Nile tilapia, both in situ and ex situ conservation methods are feasible. Long-term in situ conservation should be complemented with
ex situ efforts.
In situ conservation
According to
Pullin and Capilli (1988) the best strategy for tilapia conservation is to maintain their original habitats. Toward this the documentation of the status of the species genetic diversity is of great importance. For tilapia, documentation has just begun through databases such as FishBase that already comprises a tilapia strain registry. In Sub-Saharan Africa, Malawi and Ghana are examples of countries with responsible attitudes and important fish populations. Malawi ensures preservation of the Lake Malawi ecosystem and Ghana has created a nature reserve on an ecologically important sector of the Volta catchment (
Pullin and Capilli, 1988).
Ex situ conservation
So far, only live fish and sperm banks are possible. Examples of institutions maintaining live fish and sperms banks include ICLARM. The later collects African strains for the purpose of establishing a national breeding programme in the Philippines. ICLARM’s strategy consists on collecting
O. niloticus subspecies from different river basins. Despite the fact that the
ex-situ conservation method plays a critical role in the preservation of Nile tilapia diversity it should be recognized that the establishment and maintenance of collections are costly.
As part of strategies that are suggested for preservation of the diversity of Nile tilapia especially the conservation of indigenous populations,
Lind et al., (2012) suggested zoned aquaculture systems based on large water body catchements. On the other hand,
Mbiru et al., (2015) and
Mapenzi and Mmochi (2016) proposed the use of all-male hybrid populations.
Researchers have proven that the attempt to improve population diversity through hybridization mostly results in a reduced fingerling production associated with potential reproductive incompatibilities between crossed species (
Popma and Lovshin, 1995). In view of this,
Moses et al., (2020) suggest that decision related to the importation of GIFT as a main seed source in tilapia and Nile tilapia particularly, should be taken based on data originating from growth performance, population genetics and potential biodiversity threats for the wild native populations.
According to
Leung et al., (2002), the control of invasive species through prevention is the cost-effective means of species biodiversity conservation. Towards this it is suggested that all land-based facilities should be constructed with infrastructure that can resist the effect of floods or tidal currents (
Hinrichsen, 2007); physical barriers surrounding the facility are also of great importance to prevent invasive species (
Novinger and Rahel, 2003). Additionally, as a way of reducing the risk of escapes, facilities should be constructed at appropriate gradient and water level frequently monitored to assess flood threat (
Hinrichsen, 2007).
