Disturbance-mediated invasion of Nile tilapia ( Oreochromis niloticus ) in Zimbabwean freshwater ecosystems: distribution, ecological impacts and management challenges

1 Introduction

Freshwater ecosystems are amongst the most heavily modified habitats globally, and biological invasions are now a major pressure on riverine, reservoir and lacustrine biodiversity. Fish introductions are especially complex because many non-native fishes are deliberately moved for aquaculture, fisheries enhancement, biological control or recreational angling. The same species may therefore provide food and income whilst also altering native communities, food webs and genetic integrity. Nile tilapia (Oreochromis niloticus) is a clear example of this tension because its global spread has been driven by both development aspirations and fisheries-management interventions (Welcomme, 1988; Canonico et al., 2005; El-Sayed and Fitzsimmons, 2023).

The global success of O. niloticus reflects its biological and production advantages. It grows rapidly under warm conditions, matures early, breeds repeatedly during favourable periods, tolerates variable water quality and feeds across phytoplankton, periphyton, detrital and invertebrate pathways. These traits explain why the species has become one of the most important cultured fishes in the world, but they also increase establishment probability when fish escape from farms, are stocked into open waters or are moved between catchments without adequate biosecurity (Canonico et al., 2005; El-Sayed and Fitzsimmons, 2023; Zengeya et al., 2013).

In Zimbabwe, O. niloticus is embedded in both aquaculture development and inland fisheries. Fish farming is dominated by Nile tilapia, and Lake Kariba has become central to the supply of cultured and captured tilapia products (Mabika and Utete, 2024). At the same time, its establishment in open waters raises concerns about competition with indigenous and previously established tilapiines, hybridisation with native Oreochromis species, changes in food-web structure and further spread into sensitive catchments (Chifamba and Videler, 2014; Makeche et al., 2022; Zengeya et al., 2015).

This review argues that the invasion of O. niloticus in Zimbabwe is best understood as a disturbance-mediated process rather than as a simple consequence of introduction. Establishment and spread have been facilitated by repeated propagule pressure from aquaculture and stocking, the creation of reservoirs and farm dams, nutrient enrichment and pollution, hydrological modification, and governance weaknesses around the movement of live fish and transport water. Lake Chivero provides the clearest disturbed-system example because long-term eutrophication and wastewater inputs favour tolerant, opportunistic fishes (Magadza, 2003; Tiki and Nhiwatiwa, 2016). Lake Kariba provides a complementary large-reservoir example in which historical farm escapes, continued aquaculture pressure, genetic stock heterogeneity and nutrient-loading constraints interact (Chifamba and Videler, 2014; Makeche et al., 2022; te Velde et al., 2022; Kwikiriza et al., 2025).

The objectives of this review are to: (1) synthesise published information on the introduction and spread of O. niloticus in Zimbabwe; (2) examine the biological traits and anthropogenic disturbances that jointly explain establishment; (3) evaluate documented ecological and fishery effects whilst distinguishing direct evidence from inference; (4) identify governance and biosecurity weaknesses that sustain propagule pressure; and (5) propose operational management and research priorities that balance aquaculture benefits with freshwater biodiversity conservation.

1.1 Conceptual synthesis framework

This review is organised around a conceptual synthesis framework in which propagule pressure and habitat disturbance interact with species traits to determine establishment, spread and ecological outcomes. Aquaculture promotion, hatchery development, broodstock movement, stocking and containment failure create repeated opportunities for introduction. Reservoir construction and hydrological modification create lentic habitats suitable for reproduction, whilst eutrophication and pollution may reduce native-community resistance and increase the availability of algal and detrital pathways that O. niloticus can exploit (Zengeya and Marshall, 2007; Zengeya et al., 2011; Mabika and Utete, 2024; Kwikiriza et al., 2025). Figure 1 summarises these linked pathways and the management requirements that arise from them. This interaction-based framing is consistent with the integrated propagule-pressure, abiotic and biotic framework of Catford et al. (2009), whilst the importance of introduction effort is supported by Lockwood et al. (2005).

The framework does not assume uniform outcomes. In some systems, O. niloticus may dominate catches, simplify community structure or increase genetic risk; in others, native species may persist where habitat refuges, fishing patterns or relative fitness reduce competitive exclusion. Evidence that a locally endemic Tanzanian tilapia maintained faster growth than invasive O. niloticus illustrates why niche overlap alone cannot establish displacement (Champneys et al., 2024). Management must therefore respond to the interaction amongst disturbance, propagule pressure, native-community condition and local governance capacity.

2 Methodology and scope of the narrative synthesis

This article presents a structured narrative synthesis of published literature on Nile tilapia introductions, cage and pond aquaculture, biological invasions, tilapiine hybridisation, fish health and Zimbabwean freshwater ecosystems. The national scope is Zimbabwe, with direct Zimbabwean evidence treated as the primary basis for local conclusions. Southern African studies are used only to interpret mechanisms or risks where local evidence is incomplete, and global reviews provide conceptual and management context.

2.1 Literature search strategy and sources

Literature was identified through searches of Web of Science, Scopus, Google Scholar and PubMed, supplemented by FishBase, FAO publications, regional institutional repositories and backward searching of cited references. Search strings combined taxonomic terms (“Oreochromis niloticus”, “Nile tilapia”, indigenous Oreochromis species and “Coptodon rendalli”), geographic terms (Zimbabwe, Lake Kariba, Lake Chivero-Manyame, Chitsuwa Reservoir, Zambezi and Limpopo), and thematic terms (aquaculture escape, propagule pressure, invasion, hybridisation, eutrophication, cage culture, eDNA, parasite, pathogen and biosecurity). The review considered literature available up to July 2026.

2.2 Inclusion and exclusion criteria

Peer-reviewed journal articles, scholarly books, authoritative technical reports and completed regional dissertations were eligible where they contained evidence relevant to distribution, introduction pathways, ecological mechanisms, aquaculture risks or management. Unpublished manuscripts, papers described only as “under review”, untraceable citations and claims lacking an identifiable source were excluded. Zimbabwean studies were prioritised; regional and global studies were retained only when their inferential role was stated explicitly. English-language sources were used in the present synthesis.

2.3 Evidence synthesis and confidence criteria

Evidence was synthesised qualitatively against the disturbance-mediated framework: propagule pressure, reservoir and hydrological modification, eutrophication and pollution, governance and biosecurity, species traits, establishment, and ecological or fishery outcomes. Evidence was classified as high when a claim was supported by multiple consistent published Zimbabwean studies or by strong direct local evidence corroborated regionally; moderate when based on a single local study or several relevant regional studies; and low or indicative when a mechanism was plausible from global or regional literature but lacked direct Zimbabwean confirmation. Table 1 defines this hierarchy and the interpretive cautions applied. Table 2 summarises the occurrence, principal introduction pathways, invasion concerns, evidence strength and supporting sources for the Zimbabwean freshwater systems and associated pathways considered in this review.

Table 1. Evidence hierarchy and confidence criteria used in the narrative synthesis.

Table 2. Evidence-based synopsis of Nile tilapia occurrence, pathways and invasion concerns in Zimbabwean freshwater systems.

The principal Zimbabwean freshwater systems discussed in the synthesis are shown in Figure 2. The map is intended to orient the case studies and does not imply that all reservoirs or catchments shown have equivalent evidence of invasion.

3 Taxonomy, native range and invasion-relevant traits

3.1 Taxonomy and nomenclature

Nile tilapia, Oreochromis niloticus (Linnaeus, 1758), belongs to Phylum Chordata, Class Actinopterygii, Infraclass Teleostei, Order Cichliformes and Family Cichlidae. It is native to the Nile basin and other parts of tropical Africa but is not native to Zimbabwe or much of southern Africa. Its occurrence in Zimbabwean open waters therefore reflects human-mediated introduction rather than natural distribution (Skelton, 2001; Zengeya et al., 2013; El-Sayed and Fitzsimmons, 2023). Taxonomic consistency is important in this review because older literature used names that have since changed. Redbreast tilapia is referred to as Coptodon rendalli, although older sources may use Tilapia rendalli. Current nomenclature is used throughout except where a historical name is necessary to identify the original source.

3.2 Biological traits that promote establishment

The invasion success of O. niloticus is linked to a suite of traits rather than to one trait alone. The species grows rapidly, reaches maturity early and can spawn repeatedly under warm conditions. As a maternal mouthbrooder, it protects eggs and larvae, improving early survival relative to many broadcast-spawning fishes. These traits increase population growth once founders enter suitable habitats (Canonico et al., 2005; El-Sayed and Fitzsimmons, 2023).

The species is also tolerant of environmental conditions that are stressful for many native fishes. It can persist in warm, turbid, eutrophic and moderately hypoxic conditions, although it remains vulnerable to severe oxygen depletion and toxic bloom events. This tolerance gives O. niloticus a strong advantage in urban reservoirs, farm dams and impoundments affected by wastewater or agricultural runoff (Magadza, 2003; Zengeya and Marshall, 2007; Tiki and Nhiwatiwa, 2016).

Dietary flexibility further contributes to establishment. O. niloticus can exploit phytoplankton, periphyton, detritus, macrophyte material, zooplankton and benthic invertebrates. Limpopo River evidence showed strong stomach-content overlap between O. niloticus and O. mossambicus, but stable isotopes indicated finer resource partitioning, with O. niloticus linked more strongly to phytoplankton-based pathways (Zengeya et al., 2011). This matters for Zimbabwe because eutrophic systems such as Lake Chivero are rich in algal and detrital pathways that O. niloticus can exploit (Zengeya and Marshall, 2007; Hamandishe et al., 2021).

3.3 Traits should not be interpreted deterministically

Although O. niloticus is a strong invader, it should not be assumed that all indigenous tilapiines are automatically inferior in growth or reproduction. Lake Kariba work from Zambia found that C. rendalli exhibited high growth-performance and fecundity relative to O. niloticus and O. andersonii, leading the authors to recommend greater consideration of indigenous tilapiines in cage-culture planning around Lake Kariba (Nyirenda, 2017; Nyirenda et al., 2024). This does not negate invasion risk, but it does show that management should be based on species-specific and system-specific evidence rather than on broad assumptions.

The same caution is reinforced by the Tanzanian study of O. korogwe in Lake Rutamba, where native O. korogwe showed faster recent growth than invasive O. niloticus. Champneys et al. (2024) argued that predictions of invasion impact should consider relative fitness and not only niche overlap. For Zimbabwe, this supports a nuanced interpretation: O. niloticus risk is real, but the magnitude of impact depends on habitat condition, fishing pressure, native-species performance, genetic integrity and propagule pressure.

4 Disturbance-mediated pathways and distribution in Zimbabwe

4.1 Historical introductions and propagule pressure

Oreochromis niloticus entered Zimbabwe mainly through aquaculture and fisheries-development pathways associated with research stations, hatcheries, ponds, stocking programmes and broodstock movement. After establishment in culture systems and reservoirs, further spread could occur through deliberate stocking, escape from ponds and cages, flood-mediated movement and unauthorised transfer by farmers or fishers (De Moor and Bruton, 1988; Tweddle, 2010; Chifamba and Videler, 2014; Mabika and Utete, 2024). Lake Kariba illustrates the transition from an initial escape episode to a lake-wide established population, whilst the wider expansion of cage culture creates continuing rather than one-off introduction pressure.

Propagule pressure should therefore not be treated as a single historical event. Kwikiriza et al. (2025) identify cage damage, operational losses, flooding, storm events and other containment failures as pathways by which cultured Nile tilapia can enter African inland waters. In Zimbabwe, O. niloticus is central to aquaculture value-chain development, including hatchery seed production, feed supply, grow-out, processing and cold-chain marketing, but governance, fish-health surveillance and technical capacity remain uneven (Mabika and Utete, 2024). These weaknesses affect containment, broodstock provenance and live-fish movement, and can sustain repeated introductions into connected waters. In invasion theory, propagule pressure includes propagule size, the number of release events and the temporal pattern of arrivals; continuing farm-related releases can therefore sustain establishment and spread (Lockwood et al., 2005; Simberloff, 2009).

4.2 Reservoir creation and hydrological modification

Reservoirs and small dams are important stepping-stone habitats because they create warm lentic environments, modified shorelines and hydrological connections that may favour tolerant fishes. Madzivanzira et al. (2022) reported that O. niloticus was introduced into the newly impounded Chitsuwa Reservoir in 2015 and later dominated abundance, biomass and catch. The result illustrates how reservoir creation and stocking can jointly convert a new habitat into both a fishery resource and a potential source population for secondary spread. More generally, impoundments can function as stepping-stone habitats that increase invasion risk in connected freshwater systems (Johnson et al., 2008).

4.3 Regional distribution risk and catchment connectivity

Ecological niche models indicate that suitable habitat for O. niloticus extends broadly across southern African river systems, although predictions vary with environmental variables and whether models are trained on native or introduced ranges (Zengeya et al., 2013). Broad climatic overlap with O. andersonii, O. mortimeri and O. mossambicus supports a precautionary approach in Zimbabwean catchments containing indigenous tilapiines or strong river-reservoir connectivity (Zengeya et al., 2015). These models indicate potential exposure; they do not demonstrate local establishment, abundance or impact without field confirmation.

4.4 Biosecurity and associated-organism transfer

In this review, biosecurity refers to the regulatory, physical and operational measures used to prevent unauthorised movement or release of cultured fish, strains, gametes, transport water, parasites and pathogens. It includes hatchery certification, broodstock records, quarantine, containment standards, inspection of cages and ponds, controls on live-fish and water movement, mortality management, and surveillance of receiving waters. A value-chain approach can identify critical control points from hatcheries and grow-out facilities through transport and marketing (MacKinnon et al., 2023).

Live-fish movements can also transport parasites and other associated organisms. Shinn et al. (2023) documented a wide diversity of problematic parasites in farmed tilapia and emphasised the fish-health risks created by production and translocation. This pathway expands the disturbance framework beyond the fish itself: escape or transfer can introduce hosts, cultured strains, transport water and associated biota simultaneously. Direct evidence of disease spillover from O. niloticus into wild Zimbabwean fishes is currently insufficient, so this risk is treated as a surveillance and precautionary-management priority rather than a demonstrated national impact. Integrated parasite control and surveillance are especially important where aquaculture facilities interact with wild fish populations (Buchmann, 2022).

5 Case-study evidence from Zimbabwean freshwater systems

5.1 Eutrophication and pollution disturbance: Lake Chivero

Lake Chivero is the clearest Zimbabwean example of a disturbed reservoir in which O. niloticus has become ecologically and economically important. Wastewater and urban runoff from the Harare catchment have produced a long history of eutrophication, cyanobacterial blooms, reduced transparency and oxygen stress. These receiving-system conditions favour tolerant fishes able to exploit algal and detrital resources (Magadza, 2003; Tiki and Nhiwatiwa, 2016; Hamandishe et al., 2021). Catchment-scale nutrient-flow analyses and documented bloom-collapse fish kills support this diagnosis (Nhapi et al., 2002; Mhlanga et al., 2006).

The Lake Chivero evidence does not show that O. niloticus caused eutrophication: external catchment nutrient loading remains the primary driver. Dense tilapiine populations may nevertheless reinforce degraded states through sediment disturbance, nutrient recycling and use of planktonic and detrital pathways. The disturbance-mediated interpretation is therefore a feedback in which eutrophication favours O. niloticus, whilst abundant tolerant fishes may contribute to altered food-web and nutrient-cycling dynamics (Magadza, 2003; Zengeya and Marshall, 2007; Hamandishe et al., 2021). Mesocosm evidence from another tropical system indicates that Nile tilapia can increase dissolved nutrients, suspended solids and phytoplankton biomass, supporting the possibility of fish-mediated feedbacks whilst not establishing their magnitude in Lake Chivero (Zhang et al., 2022).

Management of O. niloticus in Lake Chivero cannot be separated from catchment rehabilitation. Harvesting alone will not correct sewage and urban nutrient inputs; it must be coupled with wastewater treatment, nutrient-load reduction, riparian protection, bloom monitoring and long-term fish-community surveillance (Magadza, 2003; Tiki and Nhiwatiwa, 2016).

5.2 Aquaculture expansion and propagule pressure: Lake Kariba

Lake Kariba provides a different but equally important case. It is a large transboundary reservoir supporting capture fisheries, aquaculture and multiple shoreline livelihoods. Chifamba and Videler (2014) reported that O. niloticus first appeared in fisheries catches in 1989 after invasion from fish farms at Kariba town and was subsequently caught throughout the lake. Its introduction was followed by declining catches of the indigenous O. mortimeri, raising concern about displacement, competition and altered fishery structure (Chifamba and Videler, 2014).

Growth comparisons strengthen the biological basis for concern but also caution against assuming universal superiority. Chifamba and Videler (2014) documented differences between alien O. niloticus and indigenous O. mortimeri. Later Zambian work found that O. niloticus was not superior in every growth or reproductive trait relative to O. andersonii and C. rendalli, whilst noting likely aquaculture augmentation and cage-escape contributions near production centres (Nyirenda et al., 2024). The wider African synthesis of Kwikiriza et al. (2025) confirms that cage aquaculture can maintain continuing escape pressure and threats to congeneric species.

The genetic picture reinforces this interpretation. Makeche et al. (2022) found that O. niloticus from Lake Kariba culture and capture fisheries could be separated into multiple morphometric, meristic and genetic groupings. Wild populations were genetically diverse and sub-structured, with an absence of panmixia. This indicates that the O. niloticus present in Lake Kariba should not be treated as a single homogeneous stock. Genetic surveillance is therefore essential for distinguishing cultured strains, wild populations, possible strain mixing and any introgression involving indigenous Oreochromis species (Makeche et al., 2022).

Aquaculture carrying-capacity evidence adds a water-quality dimension. te Velde et al. (2022) estimated a phosphorus-based sustainable capacity of 86,900 metric tonnes per year (t yr-1) for O. niloticus cage culture under their assumptions. They projected that continued expansion could account for a substantial share of allowable phosphorus loading unless feed efficiency, mortality handling and monitoring improved. The estimate should be interpreted as model-based guidance rather than a fixed production entitlement, and cage expansion should be linked to nutrient budgets, spatial zoning and lake-wide monitoring.

Published evidence from Lake Kariba therefore establishes three linked dimensions: an invasion history associated with fish farms, differences in performance between alien and indigenous tilapiines, and genetically differentiated O. niloticus stocks in culture and capture fisheries (Chifamba and Videler, 2014; Makeche et al., 2022; Nyirenda et al., 2024). Direct published comparisons of recent fish-community structure between protected and heavily fished Zimbabwean littoral zones remain limited. Claims about contemporary community restructuring should consequently be treated as a research priority until the relevant data are published. An earlier Lake Kariba survey compared fished and protected inshore areas and found that fish abundance and composition were strongly structured by lake morphometry; however, it predates current cage-aquaculture expansion and was not designed to isolate O. niloticus effects (Sanyanga et al., 1995).

Taken together, Lake Kariba shows why the disturbance-mediated framework is useful. Establishment and management are shaped by historical escape and stocking, continued cage-aquaculture propagule pressure, species performance, genetic stock structure and phosphorus loading. Management must therefore move beyond documenting presence and address containment, genetic integrity, nutrient inputs and transboundary coordination whilst sustaining livelihoods in an established fishery (Chifamba and Videler, 2014; Makeche et al., 2022; te Velde et al., 2022; Kwikiriza et al., 2025).

5.3 Reservoir creation and stepping-stone spread: small reservoirs and farm dams

Chitsuwa Reservoir was created through impoundment of the Munyati River in 2015, and O. niloticus was reportedly introduced during the same year to support fishery development. A post-filling survey conducted in April 2018 recorded ten fish species, but O. niloticus already dominated the assemblage, contributing 31.2% of individuals, 66.9% of sampled biomass and approximately 67% of the total catch (Madzivanzira et al., 2022). These results demonstrate rapid establishment and fishery dominance in a newly created small reservoir. However, no pre-impoundment fish-community baseline was available. The authors also noted that O. niloticus might already have occurred in the connected Munyati River. The study therefore cannot establish the precise source of the population or demonstrate displacement of the original fish community.

This evidence broadens the review beyond Lake Chivero and Lake Kariba. National surveillance should include farm dams, irrigation reservoirs, hatcheries, urban impoundments and river-connected small reservoirs because floods, stocking and informal fish transfers may move propagules amongst them (Madzivanzira et al., 2022; Mabika and Utete, 2024; Kwikiriza et al., 2025). Small reservoirs should not be viewed simply as smaller versions of stable lakes. Evidence from two reservoirs constructed on the Munwahuku River in Zimbabwe showed that extensive drawdowns, irrigation abstraction and repeated shoreline exposure produced simplified fish assemblages dominated by disturbance-tolerant and highly mobile species (Nhiwatiwa et al., 2017). Fish populations were partly maintained through recolonisation from connected river reaches, demonstrating how hydrological connectivity may facilitate movement amongst rivers and impoundments. Although O. niloticus was not recorded in these reservoirs, the study provides important contextual evidence that small dams may function as disturbed receiving environments and potential stepping-stone habitats where informal stocking, flooding or secondary translocation could facilitate the spread of introduced fishes.

6 Ecological consequences and disturbance mechanisms

6.1 Competition and niche overlap

Competition risk arises because O. niloticus overlaps with indigenous tilapiines in habitat, diet and climatic suitability. Regional studies show broad potential overlap with several southern African congeners, especially where translocations bring previously isolated species into contact (Zengeya et al., 2013, 2015). This evidence supports risk assessment but does not by itself prove local competitive exclusion.

Trophic evidence provides a more detailed mechanism. In the Limpopo River, O. niloticus and O. mossambicus had high stomach-content overlap, but stable-isotope signatures suggested finer separation in the carbon sources supporting each species. This means that competition may not always be obvious from stomach contents alone; isotopes, habitat use and resource availability are needed to interpret whether species are truly competing or partitioning resources (Zengeya et al., 2011). For Zimbabwe, similar work is needed in Lake Chivero, Lake Kariba and small reservoirs to determine whether O. niloticus is replacing native species through exploitative competition, habitat-mediated advantage or simply dominance in already disturbed environments.

Competition is therefore probable but context-dependent. Broad overlap increases encounter potential, whereas displacement depends on density, habitat complexity, fishing pressure, food availability, recruitment, genetic integrity and water quality. This interpretation avoids both unsupported assumptions of harmless coexistence and deterministic predictions of inevitable exclusion (Zengeya et al., 2015; Champneys et al., 2024). Experimental shelter competition also shows that O. niloticus can dominate a threatened congener under particular habitat conditions (Champneys et al., 2021).

6.2 Hybridisation, strain mixing and genetic integrity

Hybridisation is one of the most serious long-term risks of O. niloticus introductions. Oreochromis species can hybridise when brought into artificial contact, and genetic introgression may erode locally adapted lineages even when ecological displacement is not immediately visible. This risk is especially relevant for catchments containing O. mortimeri, O. mossambicus, O. andersonii or O. macrochir (D'Amato et al., 2007; Firmat et al., 2013; Deines et al., 2014; Zengeya et al., 2015). Genome-scale evidence indicates both ancient and recent hybridisation across Oreochromis, including recent admixture associated with introductions and aquaculture escape (Ciezarek et al., 2024).

The Lake Kariba evidence shows that genetic management must consider both interspecific hybridisation and intraspecific strain mixing. Makeche et al. (2022) identified multiple morphometric, meristic and genetic groupings in cultured and captured O. niloticus. Cultured strains, escapees and established wild stocks may therefore differ in growth, genetic diversity and fish-health risk, requiring species confirmation, strain-level baselines and provenance records.

Morphological identification alone is insufficient for long-term management. Hybrids or mixed strains may not be reliably detected in the field, especially after several generations. A national genetic baseline for indigenous tilapiines and cultured O. niloticus strains is therefore a priority, supported by periodic screening of hatcheries, farms, reservoirs and wild-capture samples (Deines et al., 2014; Makeche et al., 2022).

6.3 Food-web, nutrient and habitat feedbacks

Oreochromis niloticus can influence food webs through grazing on phytoplankton and periphyton, detritivory, sediment disturbance and nutrient recycling. In eutrophic systems, these mechanisms may reinforce algal dominance or alter energy pathways from plankton and detritus to fish biomass. However, these effects should not be separated from catchment nutrient loading, which remains the primary driver of eutrophication in systems such as Lake Chivero (Magadza, 2003; Zengeya and Marshall, 2007; Hamandishe et al., 2021).

Aquaculture adds another ecosystem feedback. Cage culture releases phosphorus and organic matter through uneaten feed, faeces and mortalities. In Lake Kariba, te Velde et al. (2022) showed that carrying capacity depends strongly on feed conversion ratio, nutrient uptake efficiency and mortality processing. Thus, the environmental effect of O. niloticus aquaculture is not simply a function of tonnes produced; it is also a function of management quality and spatial placement of cages (te Velde et al., 2022). The review therefore treats water quality and invasion risk as potentially reinforcing. Nutrient enrichment can favour O. niloticus, whilst dense cultured or wild populations may affect nutrient cycling and turbidity. The strength and direction of these feedbacks remain system-specific, and management must address both propagule pressure and the degraded habitat conditions that favour tolerant invaders (Magadza, 2003; Zengeya and Marshall, 2007; te Velde et al., 2022).

6.4 Parasite and associated-organism co-translocation

Aquaculture-related translocation may move more than the target fish. Parasites can accompany broodstock, juveniles and transport water, and escaped cultured fish may create contact between farm-associated parasite assemblages and wild hosts. Shinn et al. (2023) reviewed problematic and pathogenic parasites of farmed tilapia globally, including monogeneans and protozoan ectoparasites of importance to production and biosecurity.

For Zimbabwe, this is an evidence gap rather than a documented ecosystem-level impact. Hatchery and cage-farm health screening should therefore be linked to wild-fish surveillance, and transport-water controls should be included alongside escape prevention. Future studies should distinguish parasite introduction, amplification of endemic parasites and disease caused by environmental stress.

7 Ecosystem-service trade-offs

Oreochromis niloticus creates a direct ecosystem-service trade-off. It supports fish production, employment, affordable protein and aquaculture investment. These benefits are important in Zimbabwe, where aquaculture has been promoted to address fish-supply gaps and livelihood needs, and where Lake Kariba has become central to the tilapia value chain (Mabika and Utete, 2024). Global reviews similarly show that Nile tilapia has made major contributions to aquaculture growth, rural livelihoods and food systems across several continents (El-Sayed and Fitzsimmons, 2023).

The trade-off is that the same species may reduce native biodiversity, alter community structure, increase genetic risks, reinforce degraded water-quality states and create dependence on a non-native fish in modified systems. Deines et al. (2016) argued that tilapia introductions involve competing ecosystem services rather than a simple good-bad classification. This is a useful framing for Zimbabwe because the species is already socially and economically important in some systems but remains risky in others.

The appropriate management question is spatial and contextual: where can O. niloticus be cultured under strict containment, where is it already an established fishery requiring adaptive management, and where should further spread be prevented? Indigenous tilapiines may provide alternatives in biodiversity-sensitive catchments, but their culture performance and conservation implications require case-specific evaluation. Nyirenda et al. (2024) support greater consideration of indigenous tilapiines around Lake Kariba, whilst Mabika and Utete (2024) identify the policy, training, fish-health and governance capacity needed for sustainable aquaculture.

8 Management implications

Eradication is unrealistic where O. niloticus is widely established, socially important and connected to capture fisheries or aquaculture. Management should instead combine prevention of further spread, protection of high-value native-fish areas, genetic and fish-health surveillance, water-quality rehabilitation, stakeholder education and adaptive co-management. This approach is consistent with calls to incorporate invasion science explicitly into regulatory frameworks for established feral tilapia populations (Costa-Pierce, 2003). To make the recommendations operational, Table 3 classifies actions by urgency, feasibility, implementation scale and responsible actors.

Table 3. Operational decision matrix for Nile tilapia invasion management and aquaculture biosecurity in Zimbabwe.

8.1 Operational priorities and responsible actors

Containment is the immediate priority. Hatcheries, cages, ponds and live-fish transport should be governed by permits, enclosure standards, flood-risk screening, cage-maintenance records, broodstock provenance, mortality controls and penalties for unauthorised release. Requirements should be strongest near connected waters containing indigenous tilapiines (Makeche et al., 2022; Mabika and Utete, 2024; Kwikiriza et al., 2025).

Zimbabwe also needs a catchment-based risk map. Catchments containing indigenous tilapiines, protected areas, hatchery clusters, river corridors and farm-dam networks should be ranked by likelihood of introduction and ecological consequence. Regional niche models provide an initial climatic screen, but local occurrence, connectivity and habitat data are required before management decisions are made (Zengeya et al., 2013, 2015).

Established fisheries should be managed adaptively rather than ignored. Monitoring should include catch composition, size structure, recruitment, genetic status, water quality and trends in indigenous species rather than relying only on yield or catch per unit effort. Published Lake Kariba evidence on growth, stock heterogeneity and nutrient loading provides a starting point, but comparable long-term community datasets remain a priority (Chifamba and Videler, 2014; Makeche et al., 2022; te Velde et al., 2022).

Habitat restoration and nutrient-load reduction are also invasion-management measures. In eutrophic systems, wastewater treatment, phosphorus reduction, riparian restoration and sediment control can reduce conditions that favour tolerant invaders. In Lake Chivero, these actions are essential because fishery management cannot substitute for catchment rehabilitation (Magadza, 2003; Tiki and Nhiwatiwa, 2016). For Lake Chivero, quantified catchment nutrient flows demonstrate the importance of sewage-treatment works and other external nutrient sources (Nhapi et al., 2002).

8.2 Technical and feasibility considerations for eDNA and genetic surveillance

Environmental DNA (eDNA) can provide sensitive early detection, but implementation requires validated species-specific assays, contamination controls, inhibition testing, replicated sampling and access to suitable molecular laboratories. Assays must be tested against indigenous tilapiines to avoid cross-amplification. Warm, eutrophic or turbid waters may contain inhibitors, whilst transported or persistent DNA can produce detections that do not represent an established population. Operational guidance for invasive-species programmes emphasises fit-for-purpose assay selection, validation, quality assurance and capacity planning, whilst contamination reviews underscore the need for negative controls and transparent interpretation (Morisette et al., 2021; Sepulveda et al., 2020). Environmental DNA (eDNA) should therefore complement rather than replace gill-netting, electrofishing where appropriate, market and catch monitoring, morphological examination and genetic analysis of captured fish. Regional university and agency partnerships may make initial surveillance feasible whilst national laboratory capacity is strengthened.

9 Research gaps and future priorities

A national distribution, health and genetic-status survey is the most urgent research need. Current knowledge is concentrated in Lake Chivero, Lake Kariba and a small number of reservoirs, whilst many farm dams, hatcheries, irrigation reservoirs and river sections remain undocumented. Surveys should distinguish indigenous species, cultured O. niloticus strains, established wild populations and possible hybrids, and should record parasite and disease indicators where feasible (Makeche et al., 2022; Shinn et al., 2023; Zengeya et al., 2015).

Long-term community monitoring should measure species composition, relative abundance, size structure, recruitment, habitat condition, macrophyte cover, water quality and fishing pressure. Published time-series and comparative data are presently insufficient to attribute community change consistently to O. niloticus rather than to fishing, habitat modification or eutrophication. Standardised monitoring is needed to separate these interacting drivers.

Food-web and ecosystem-function studies should combine gut contents, stable isotopes, plankton monitoring, nutrient fluxes and sediment-disturbance measurements. Lake Chivero and Limpopo studies demonstrate why multiple methods are needed to distinguish dietary overlap from actual competition and to evaluate fish-mediated feedbacks in eutrophic systems (Zengeya and Marshall, 2007; Zengeya et al., 2011; Hamandishe et al., 2021).

Aquaculture-impact studies should validate phosphorus carrying-capacity estimates against farm production, feed conversion, mortalities, hydrodynamics and observed nutrient trends. Escape surveillance should also quantify the frequency, scale and causes of containment failure around Lake Kariba and emerging production areas (te Velde et al., 2022; Kwikiriza et al., 2025).

Socio-economic research should quantify trade-offs amongst aquaculture income, food supply, capture-fishery livelihoods, native biodiversity and regulatory costs. Decision tools are needed to identify where contained O. niloticus culture is acceptable, where indigenous species should be prioritised, where open-water spread must be prevented, and how established fisheries can be managed without creating new invasion pathways (Deines et al., 2016; Mabika and Utete, 2024).

10 Conclusion

The invasion of O. niloticus in Zimbabwe illustrates a broader African freshwater-management dilemma: a species promoted for food security and aquaculture can become an ecological, genetic and biosecurity risk when moved into open waters without adequate containment. The evidence reviewed here shows that outcomes are produced by the interaction of repeated propagule pressure, receiving-system disturbance, reservoir connectivity, species traits and governance capacity.

The disturbance-mediated framework integrates evidence that would otherwise remain as separate case studies. Lake Chivero shows how eutrophication and pollution can reduce ecological resistance; Lake Kariba links historical escape, ongoing cage-aquaculture pressure, stock heterogeneity and nutrient loading; and Chitsuwa Reservoir demonstrates the importance of smaller impoundments as establishment and stepping-stone habitats. Regional evidence strengthens concern about niche overlap and hybridisation, whilst also showing that impacts depend on relative fitness and local habitat. Parasite and associated-organism transfer is a credible biosecurity pathway, but direct Zimbabwean evidence remains a priority for research.

Zimbabwe should therefore prioritise prevention over attempted eradication: certify hatcheries and cage facilities, control live-fish and water movement, establish genetic and health baselines, combine eDNA with conventional surveillance, rehabilitate eutrophic catchments, educate stakeholders and coordinate management across shared basins. In already established fisheries, adaptive co-management should protect livelihoods whilst monitoring native species and preventing secondary spread.

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Footnotes

Edited by: Crispen Phiri, Marondera University of Agricultural Sciences and Technology (MUAST), Zimbabwe

Reviewed by: Hugo Verreycken, Research Institute for Nature and Forest (INBO), Belgium

Julio Alejandro Franco Ortega, University of Guanajuato, Mexico