Oreochromis Esculentus Research Articles

Community Diversity and Dynamics of Fish Assemblages in Lake Kanyaboli, Western Kenya

ABSTRACTAlthough Lake Kanyaboli provides a sanctuary for fish species that are considered extinct (e.g., Oreochromis esculentus, Oreochromis variabilis and Haplochromis spp.) in Lake Victoria, comprehensive data on species diversity and abundance are lacking. This study, conducted over a year (February 2020 to February 2021), addressed this gap by assessing the temporal variation in fish diversity, abundance and catch (biomass) in Lake Kanyaboli. The primary fish data collected in the lake were supplemented with secondary data, and physico‐chemical variables were used to correlate with fish assemblages. Fish catches decreased over time from 1981 to 2020, with the highest catch observed in 1981 (250 mt). Fish species composition data showed heterogeneity over the years, ranging from 9 to 15 species. O. esculentus, haplochromines, Oreochromis niloticus, Protopterus aethiopicus and Clarias sp. were present in all the reviewed years (from 1981 to 2020), whereas Coptodon zillii was non‐existent post‐1981 results. The February 2020 to February 2021 survey recorded 14 species dominated by Cichlidae (10 species), whereas Protopteridae and Anabantidae recorded one taxon each. Interestingly, Bagrus sp. was recorded in the current study, suggesting fish movement from the Yala River to the lake. The annual fish catch in the lake comprised tilapias (50%, O. esculentus, O. variabilis, O. niloticus and Oreochromis leucostictus), Clarias sp. (23%), P. aethiopicus (20%), haplochromines (7%), Cyprinids (0.03%) and Anabantidae (0.01%). There were no significant monthly differences in fish abundance and fish catches. The decline in fish catch in Lake Kanyaboli over the years is consistent with most tropical lakes and reservoirs in developing countries due to overexploitation. This study highlights the need for biomonitoring in Lake Kanyaboli to protect its fish population, including the endangered O. esculentus and O. variabilis, and ensure the long‐term sustainability of the ecosystem.

Impacts of the Victoria Nile Bujagali hydropower dam on fisheries and livelihoods

AbstractThe Upper Victoria Nile (UVN) flows from Lake Victoria into Lake Kyoga, spanning 117 km and supports a diverse aquatic fauna that sustains livelihoods of riverside communities. The UVN is habitat to critically endangered (Labeo victorianus), near threatened (Oreochromis variabilis and Oreochromis esculentus), and rare species (Neochromis simotes). Four hydropower dams were built on the UVN to provide energy for industrial and socioeconomic development, notwithstanding consequential environmental and socioeconomic impacts. The impacts of Bujagali hydropower (BHP) dam on fisheries and livelihoods were assessed biannually (April and September) from 2006 to 2019, using fishing effort, species abundance, catch composition, and economic beach revenue at upstream, mid‐reservoir, and downstream transects. The fishing boats increased from 31 in 2009 to 293 in 2019 and fishers from 83 to 500 over the monitoring period. Maximum annual catch of 461.4 t was recorded in 2014 and lowest of 54.2 t in 2009. In all transects, Protopterus aethiopicus and Labeo victorianus were least in the commercial catches and the use of Mormyrus kannume juveniles as bait for Nile perch fishery corresponded with increased catches from 3.3 t in 2009 to 148.2 t in 2019. Women were mostly engaged in post‐harvest activities such as fish drying, smoking, and food vending. These observations suggest coupling effects of the hydropower dam and ineffective fisheries management. The harvest of M. kannume wild stocks for bait should be banned and research in possible domestication of the species undertaken. There is a need to strengthen fisheries enforcement to curb illegal effort and overexploitation and to implement conservation actions to mitigate potential biodiversity impacts from the hydropower dam operations.

Effects of environmental temperature on the growth performance of a tropical Oreochromis esculentus (Graham, 1928). Implications for the species conservation

AbstractClimate variability and change are currently recognised global challenges. However, the impacts of rising temperatures may be greatest on tropical freshwater fish, which already exist in warm environments and may be living close to their critical thermal maximum. However, there is limited species‐specific information on tropical species’ adaptive capacity to increased temperature. This study examined various growth attributes in juvenile populations (farmed, L. Kacheera and L. Nabisojjo) of a critically endangered tropical Oreochromis esculentus to determine its adaptive potential. Fish were acclimated at 25, 28, 31 and 34°C for 28 days. Overall, survival rates were high. Growth coefficient b was not significantly different from three (b = 3) in all treatments, implying isometric growth. The specific growth rates (SGRs), body condition K, and hepato‐somatic indices (his) declined as the acclimation temperatures increased. Although SGR showed significant variations only among populations, the highest in L. Nabisojjo, K and HSI showed significant variations among treatments. Overall, the growth attributes declined at higher acclimation temperatures, but remained within acceptable ranges, implying that the species may attain normal growth at elevated temperatures experienced in outdoor aquaculture systems, despite its warm environmental background. Hence, we recommend selective breeding for temperature resilience and species introduction into aquaculture using broodstock from L. Nabisojjo.

Ecology of the Critically Endangered Singidia Tilapia (Teleostei: Cichlidae: Oreochromis esculentus) of lake Kayanja, Uganda and its conservation implications

Singidia Tilapia Oreochromis esculentus is a Critically Endangered native tilapia fish species endemic to lakes Victoria and Kyoga basins of East Africa, however, it disappeared from these main lakes due to overfishing, environmental degradation and predation by the introduced Nile Perch Lates niloticus. Remnant populations of this fish species is now restricted to satellite lakes including Lake Kayanja of the Victoria basin. This study provides updated information about the population abundance, critical habitat, threats and diet of Singidia Tilapia to inform conservation decisions to revive its populations in the wild. Fish data collection and mapping of nursery and breeding habitats of Singidia Tilapia on Lake Kayanja was conducted between February 2016 and October 2017. In all the areas mapped and sampled, Singidia Tilapia (with a size range of 11–27 cm TL) was the most abundant (43%) relative to exotic Nile Tilapia Oreochromis niloticus (21%) and Redbelly Tilapia Coptodon zillii (36%). The emergence of introduced (exotic) tilapias like Nile Tilapia recorded in this study could be attributed to cage fish farming being carried out in this Lake. The diet of Oreochromis esculentus consisted mainly of detritus (60.8 %), plant materials (27.7%) and blue-green algae (5.5%). Destruction of critical habitats and presence of introduced fish species were noted as the major threats to this fish and its habitats. The generated information could contribute to guiding stakeholders to undertake appropriate actions to conserve this threatened fish species and its habitats.

A review on the status of some major fish species in Lake Victoria and possible conservation strategies

AbstractLake Victoria, the second largest freshwater lake in the world, supports an enormous flora and fauna biomass, with a large human population around the Lake. The lake is a source of food (fish), water for domestic use and crop and livestock production, transportation and tourism, among other uses. As a result of these varied uses, human intervention within the lake and its catchment has resulted in several ecological changes in the lake in recent years, with profound effects on its fish resources. One of the most notable changes is the reduction and risk of extinction of some fish species, including the endemic tilapiine species (Oreochromis esculentus and Oreochromis variabilis), catfishes (Xenoclarias eupogon), haplochromines and cyprinids (Labeo victorianus and Barbus altinialis) in the lake. The reduced biodiversity, as well as extinctions within the lake, has been attributed to the introduction of alien fish species (e.g. Nile perch (Lates niloticus) and Nile tilapia (Oreochromis niloticus) into the lake, habitat loss and cultural eutrophication. If left unchecked, these changes will have devastating effects on the lake's resources, as well as to people living around the lake and beyond who depend on the lake fisheries for their livelihood. Thus, there is an urgent need for management measures based on sound scientific research to be implemented in order to curb the loss of ichthyodiversity within the lake, as well as to save the livelihoods of those stakeholders who are either directly or indirectly dependent on the lake.

Heavy metals bio-accumulation in tilapia and catfish species in Lake Rukwa ecosystem Tanzania

Investigation on accumulation of selected heavy metals of Zinc, Mercury, Copper, Lead, Chromium and Nickel in sediment, water and muscle tissues of Clarias gariepinus (African catfish) and Oreochromis esculentus (Singida tilapia) fish was done in Lake Rukwa, Tanzania. Samples were obtained from transects of 100 m long extending from Luika and Songwe River mouths to offshore. Water and sediment samples were collected directly from the study sites while fish were obtained from fisherfolk operating in the Lake. Sampling was done in dry and wet seasons. Heavy metals analysis was done using the Atomic Absorption Spectrophotometry. Concentration of heavy metals was higher in catfish than in tilapia (p < 0.05). There were no significant differences in metal concentration between seasons except for Zn (p < 0.05). In this study only Zn was above standard WHO concentrations in fish muscles. Likewise, the concentrations of heavy metals were within recommended limits in water except Pb. The detected metals in sediment were above recommended limits. Other heavy metals in particular Hg, Ni and Cr were not detected in all samples. Therefore, studied fish from Lake Rukwa may threaten human health upon consumption. The detected heavy metals in water were within the maximum residual levels (MRLs) permitted by WHO. Sustainable Lake Rukwa's fish, ecosystem management and conservation are recommended to discourage heavy metals discharge from elevating beyond permissible limits and thus prevent harmful health effects to fish consumers and water users.

Hydroalumination of Oligoalkynylgermanes and ‐digermanes – Reactions with Heterocumulenes by Al–C or Ge–C Bond Activation and Formation of a Hexazenedialuminum Complex

Hydroalumination of the dialkynylgermane Ph2Ge(C≡CtBu)2 (1) and the digermanes Phn(tBuC≡C)3–nGe–Ge(C≡CtBu)3–nPhn (2a: n = 2; 2b: n = 1) with two equivalents of H–AltBu2 or H–AlEt2 yielded the mixed Al/Ge compounds Ph2Ge[C(AltBu2)=CH‐tBu]2 (3), [Ph2GeC(AltBu2)=CH‐tBu]2 (4a), and [Ph2GeC(AlEt2)=CH‐tBu]2 (4b). Reactions of 2b with both aluminum hydrides afforded inseparable mixtures of products. 3 reacted with heterocumulenes by retrohydroalumination. Phenyl isocyanate gave insertion of the C=N group into the resulting Ge–C(≡C–tBu) bond (5), whereas the NCS and NCN groups of phenyl isothiocyanate and a carbodiimide inserted into Al–Cvinyl bonds (6 and 7) with unaffected terminal Ge–C≡C–tBu moieties. The generation of 5 represents the first example of the insertion of a heterocumulene into a Ge–C bond, which may be favored by the activation of the isocyanate group by the Lewis acidic Al atom and the increased polarity of the N=C=O fragment as determined by NBO calculations. The reactions of the digermanes 4 with heterocumulenes were unselective and afforded inseparable mixtures. Treatment of 4a with the azide (4‐tBu)C6H4CH2–N3 led interestingly to reductive coupling of two azide molecules, and the hexazene complex (tBuC6H4CH2N3–N3C6H4tBu)(AltBu2)2 (8) was isolated in moderate yield. Six nitrogen atoms form a dianionic chain, which coordinated two AltBu2 fragments by formation of two joint AlN4 heterocycles.

Widespread colonisation of Tanzanian catchments by introduced Oreochromis tilapia fishes: the legacy from decades of deliberate introduction

From the 1950s onwards, programmes to promote aquaculture and improve capture fisheries in East Africa have relied heavily on the promise held by introduced species. In Tanzania these introductions have been poorly documented. Here we report the findings of surveys of inland water bodies across Tanzania between 2011 and 2017 that clarify distributions of tilapiine cichlids of the genus Oreochromis. We identified Oreochromis from 123 sampling locations, including 14 taxa restricted to their native range and three species that have established populations beyond their native range. Of these three species, the only exotic species found was blue-spotted tilapia (Oreochromis leucostictus), while Nile tilapia (Oreochromis niloticus) and Singida tilapia (Oreochromis esculentus), which are both naturally found within the country of Tanzania, have been translocated beyond their native range. Using our records, we developed models of suitable habitat for the introduced species based on recent (1960–1990) and projected (2050, 2070) East African climate. These models indicated that presence of suitable habitat for these introduced species will persist and potentially expand across the region. The clarification of distributions provided here can help inform the monitoring and management of biodiversity, and inform policy related to the future role of introduced species in fisheries and aquaculture.

The mitochondrial genome of an endangered native Singidia tilapia, Oreochromis esculentus: genome organization and control region polymorphism

Singidia tilapia (Oreochromis esculentus) is a native Cichlid fish of important commercial value, distributed in Lake Victoria, East Africa. Due to its declining population levels in its natural habitat, this species has now been classified as a Critically Endangerd by the International Union for the Conservation of nature (IUCN). In the present study the complete nucleotide sequence of the mitochondrial genome (mtDNA) of O. esculentus was determined. In addition, polymorphism analysis based on the mtDNA’s control region sequence was investigated on two of its remaining populations of Yala and Borabu as well as a phylogenetic consideration using 16S rRNA mtDNA genes to explore its position and relationship within Cichlidae fish. The length of the complete mitogenome of O. esculentus is 16 622 bp, containing the same order and an identical number of genes and regions with the other reported Cichlid fishes, which consists of 13 protein-coding genes, 22 transfer RNA (tRNA) genes, 2 ribosomal RNA (rRNA) genes, and a putative non-coding region. The phylogenetic analysis shows O. esculentus being clustered within the Oreochromini sub-tribe of the Cichlidae. The high genetic diversity and low genetic differentiation between the two populations indicated the need for conservation of both the refuge ecosystems and the fish species under study.

Mercury and Lead Contamination in Three Fish Species and Sediments from Lake Rukwa and Catchment Areas in Tanzania.

Mining activity in the catchment area of Tanzania's Lake Rukwa is suspected of adding to the lake and connected rivers' heavy metal load. There has been no study done, however, on the levels of mercury (Hg) and lead (Pb) in lake sediment and fish muscle, and what the results could mean for human health. This study investigated the concentration of Hg and Pb in lake sediment and in the muscles of African sharptooth catfish (Clarias gariepinus), Lake Rukwa tilapia (Oreochromis rukwaensis) and Singida tilapia (Oreochromis esculentus) from Tanzania's Lake Rukwa and connected rivers. Concentrations of Hg and Pb in fish muscle and lake sediment were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mercury analyzers, respectively. Levels of Pb and Hg from C. gariepinus ranged between 0.01 to 1.9 μg/g and 0.03 to 0.33 μg/g, respectively. Pb and Hg in O. esculentus varied between 0.02 to 1.4 μg/g and <0.01 to 0.29 μg/g, respectively. Pb and Hg levels in O. rukwaensis ranged from 0.12 to 0.88 μg/g and 0.12 to 0.88 μg/g, respectively. On the other hand, concentrations of Pb and Hg in the sediment samples ranged between 0.02 to 16.23 μg/g and from 0.01 to 1.43 μg/g, respectively. Concentrations of Hg in the muscles of C. gariepinus and O. esculentus were above World Health Organization (WHO) permissible limits, indicating that they are not safe for human consumption. Concentrations of Pb in fish muscle samples were below WHO permissible limits and United States Environmental Protection Agency (USAEPA) provisional tolerable weekly intake (PTWI) standards. Furthermore, Hg and Pb in sediment were below the threshold value of Environment Canada and Florida's 'No effect level'. Although levels of Pb in fish samples and Hg and Pb levels in sediment were below international standards, it is important to consider that fish forms an important source of animal protein for local inhabitants, who are likely to consume more fish than considered by these standards. The study recommends further research on the levels of mercury and lead in humans, especially children and pregnant women.

Conditions for initiating Lake Victoria haplochromine (Oreochromis esculentus) primary cell cultures from caudal fin biopsies.

The global decline of freshwater fishes has created a need to cryopreserve biological materials from endangered species in an effort to conserve the biodiversity within this taxon. Since maternal gametes and embryos from fish are difficult to cryopreserve, somatic cells obtained from caudal fins have become an increasingly popular resource as they contain both maternal and paternal DNA ensuring valuable traits are not lost from the population. Somatic cells stored in cryobanks can be used to supplement endangered populations with genetically valuable offspring with the use of assisted reproductive technologies. However, initiating primary cell cultures from caudal fin biopsies of endangered species can be challenging as standardized protocols have not yet been developed. The objective of this study was to identify culture conditions, including antibiotic supplementation, biopsy size, and culture temperature, suitable for establishing primary cell cultures of ngege (Oreochromis esculentus), a critically endangered African cichlid. Six-millimeter caudal fin biopsies provided sufficient material to develop a primary cell culture when incubated at 25°C using standard fish cell culture medium containing 1× Primocin. Further investigation and application of these culture conditions for other endangered freshwater fishes is necessary.

Particle retention in suspension-feeding fish after removal of filtration structures

The suspension-feeding cichlids Oreochromis aureus (blue tilapia) and Oreochromis esculentus (ngege tilapia) are able to selectively retain small food particles. The gill rakers and microbranchiospines of these species have been assumed to function as filters. However, surgical removal of these oral structures, which also removed associated mucus, did not significantly affect the total number of 11–200μm particles ingested by the fish. This result supports the hypothesis that the branchial arch surfaces themselves play an important role in crossflow filtration. Both species selectively retained microspheres greater than 50μm with gill rakers and microbranchiospines intact as well as removed, demonstrating that neither these structures nor mucus are necessary for size selectivity to occur during biological crossflow filtration. After removal of the gill rakers and microbranchiospines, O. esculentus retained significantly more microspheres 51–70μm in diameter and fewer 91–130μm microspheres compared to retention with intact structures, but the particle size selectivity of O. aureus was not affected significantly. These results support conclusions from previous computational fluid dynamics simulations indicating that particle size can have marked effects on particle trajectory and retention inside the fish oropharyngeal cavity during crossflow filtration. The substantial inter-individual variability in particle retention by suspension-feeding fish is an unexplored area of research with the potential to increase our understanding of the factors influencing particle retention during biological filtration.

Comparison of the genetic and ecological diversity of the native to the introduced tilapiines (Pisces: Cichlidae), and their population structures in the Lake Victoria region, East Africa

The impact of introduced tilapiines in Lake Victoria Region (LVR) was studied using experimental fishing survey methods and microsatellite DNA marker analysis. The experiments were carried out between 1993 and 2000. Oreochromis esculentus and O. variabilis, the two native forms of the LVR and original principals of the fisheries of these waters, were found to be completely displaced from the major lakes and were only occasionally found in a few minor satellite lakes surrounding Lakes Kyoga and Victoria. Nile tilapia, O. niloticus, is now the dominant tilapiine, and one of the three key fisheries species of LVR along with Nile perch, Lates niloticus, and the minnow-like Rastrineobola argentea. Oreochromis leucostictus, also introduced to the LVR from the nilotic system, was largely restricted to swampy fringes of LVR water bodies. Tilapia zillii, also introduced from nilotic waters and earlier recorded to have been the most prevalent introduced tilapiine in LVR, occurred only sparsely and was very restricted in distribution. Genetic studies showed Nile tilapia having the highest genetic diversity, and being the least subdivided among LVR tilapiines. O. leucostictus was next in variability, but had the greatest gene flow, probably due to its ability to traverse low oxygen waters such as swamps which separate most LVR water bodies. The native forms were genetically the least variable, except where they coexist with introduced congeners. ‘Uncontaminated’ native forms were observed only in satellite Lakes of Kayanja, Kayugi, Manywa, Kawi and Kanyaboli. The population structure of native forms of LVR suggest they are being genetically swamped, especially for O. variabilis by Nile tilapia alleles, due to genetic exchange with introduced forms, resulting in highly localized differentiation of native species. T. zillii was equally highly subdivided, but has higher genetic variation within subpopulations, a situation linked to monogamous breeding behaviour which allows most individuals of its population to participate in breeding. Hybridization with T. rendalli, which was only occasionally encountered and previously mistaken for dwarf Nile tilapia, may also contribute to high within population variation in small T. zillii populations. On a macro-evolutionary level, the sister relationship between O. niloticus and O. esculentus is questionable and in need of further phylogenetic analysis. Overall the study showed a changed fishery, and the need to manage satellite lakes in a manner that will preserve the previous premier elements of the region's fishery (O. esculentus and O. variabilis) within natural refugia.

Some possible factors leading to decline in fish species in Lake Victoria

The decline in fish species in Lake Victoria is one of the largest documented losses of biodiversity in an ecosystem. The reduction in species in the lake was attributed to overexploitation through increased fishing capacity, use of illegal fishing gears and poor enforcement of regulations. Introduction of the predatory Nile perch is blamed for the decline of the native species, especially the haplochromine cichlids. The native tilapiines, Oreochromis esculentus and Oreochromis variabilis, declined due to hybridisation and competition with the introduced Oreochromis niloticus. Diversity loss in haplochromine cichlids has also been attributed to hybridisation caused by increased water turbidity, which reduces visibility in recognising conspecifics during breeding. Degradation of the environment through poor farming patterns and waste disposal has led to increased nutrients into the lake, in turn leading to changes in water quality, increased algal blooms and subsequent anoxia which led to frequent fish kills in the 1990s. However, recent resurgence of haplochromines thought to be extinct, disputes the fact that extinction of several species occurred. Though not denying that a drastic reduction in the number of native species occurred, the much hyped extinction could be a result of a lack of adequate information on taxonomy and ecology of the haplochromines as well.

Biological diversity of the Yala Swamp lakes, with special emphasis on fish species composition, in relation to changes in the Lake Victoria Basin (Kenya): threats and conservation measures

During the second half of the last century, the Lake Victoria ecosystem has undergone drastic ecological changes. Most notable has been the decline in the populations of many endemic cichlid fishes. The lake has lost nearly 200 haplochromines and one tilapiine, Oreochromis esculentus. The above changes have been attributed to effects of species stocking and, in particular, from predation pressure by the introduced Nile perch, Lates niloticus. Other factors that have led to the decline of the endemic species include intensive non-selective fishing, extreme changes in the drainage basin, increased eutrophication, and the invasion of the lake by the water hyacinth, Eichhornia crassipes. However, the remnants of some species that had disappeared from Lake Victoria occur abundantly in the Yala Swamp lakes (Kanyaboli, Sare and Namboyo). This paper discusses the biodiversity of the swamp and the three lakes and gives suggestions for their conservation.

The tilapiine fish stock of Lake Victoria before and after the Nile perch upsurge

Since the beginning of fisheries in Lake Victoria, two native tilapiine species, Oreochromis esculentus and Oreochromis variabilis, were the main target of the local fishermen. A continuous increase in fishing pressure led initially to a declining catch per unit of effort, and a smaller average fish size; eventually, there was a reduced landing of tilapiines. To boost the fisheries, three alien tilapiine species and the Nile perch Lates niloticus were introduced. Thirty years after its introduction, Oreochromis niloticus appeared to be the most successful tilapiine species. It replaced the indigenous tilapiines almost completely before the Nile perch came to dominate the ecosystem of Lake Victoria. Reduced fishing pressure on the tilapiines in the 1980s, due to the shift of the local fishery towards the Nile perch, resulted in an increase in the stock of O. niloticus and an increase in average fish size. Subsequently, the total mass of O. niloticus landed increased. The stocks of the indigenous tilapiines did not recover but declined to extremely low levels, or vanished from the main lake. Currently, these species still occur in satellite lakes of Lake Victoria, from which O. niloticus is absent. Nile perch feed on O. niloticus; however, the limited overlap in distribution between piscivorous Nile perch and O. niloticus of consumable sizes is probably an important factor in explaining the coexistence of the two species. The main cause of the disappearance of the native tilapiine species is presumed to be competitive dominance by O. niloticus.

The tilapiine fish stock of Lake Victoria before and after the Nile perch upsurge

Since the beginning of fisheries in Lake Victoria, two native tilapiine species, Oreochromis esculentus and Oreochromis variabilis, were the main target of the local fishermen. A continuous increase in fishing pressure led initially to a declining catch per unit of effort, and a smaller average fish size; eventually, there was a reduced landing of tilapiines. To boost the fisheries, three alien tilapiine species and the Nile perch Lates niloticus were introduced. Thirty years after its introduction, Oreochromis niloticus appeared to be the most successful tilapiine species. It replaced the indigenous tilapiines almost completely before the Nile perch came to dominate the ecosystem of Lake Victoria. Reduced fishing pressure on the tilapiines in the 1980s, due to the shift of the local fishery towards the Nile perch, resulted in an increase in the stock of O. niloticus and an increase in average fish size. Subsequently, the total mass of O. niloticus landed increased. The stocks of the indigenous tilapiines did not recover but declined to extremely low levels, or vanished from the main lake. Currently, these species still occur in satellite lakes of Lake Victoria, from which O. niloticus is absent. Nile perch feed on O. niloticus; however, the limited overlap in distribution between piscivorous Nile perch and O. niloticus of consumable sizes is probably an important factor in explaining the coexistence of the two species. The main cause of the disappearance of the native tilapiine species is presumed to be competitive dominance by O. niloticus.

Branchial arches of suspension–feeding Oreochromis esculentus: sieve or sticky filter?

Only 4% of 600 particles slid on or adhered to buccopharyngeal surfaces of Oreochromis esculentus. Only a minute quantity of mucus was visible during feeding (0·6% of 68 224 video frames; 30 frames s–1), and very few particles were retained in this mucus. Filtration by mucus entrapment on the branchial arches is rejected as a mechanism of particle retention in this species. Since particles contacted the branchial arches infrequently, and the inter–raker spaces are too large to retain particles less than approximately 50 μm, sieving by gill rakers as a particle retention mechanism is rejected also. Unlike the suspension–feeding congener O. niloticus, O. esculentus does not use branchial arches to retain food particles by sieving or by mucus entrapment.

Genetic characterization of a pure relict population of Oreochromis esculentus, an endangered tilapia

Genetic variation of the Oreochromis esculentus population from Lake Kanyaboli was studied using 24 allozyme loci and three microsatellite loci and compared with four populations of O. niloticus. Results strongly suggest that this population can be considered as pure.

Comparative feeding efficiency of indigenous and introduced phytoplanktivores from Lake Victoria : Experimental studies on Oreochromis esculentus and Oreochromis niloticus

The ngege, Oreochromis esculentus Graham (Perciformes;Cichlidae), was a numerically important phytoplanktivore inLakeVictoria, East Africa prior to its extirpation and subsequentreplacement following the introduction of its sister species,O. niloticus Boulenger (Cichlidae). The disappearance of O. esculentus has been attributed to predation, competition,andintrogression by introduced species and to overfishing. Anoverlooked factor is a shift in algal community dominance inLakeVictoria, from large filamentous diatoms (a preferred fooditem ofO. esculentus) to smaller colonial bluegreen and greenalgae.We performed a series of experiments to test the hypothesisthatfeeding performance of the two species differs sufficientlythatalgal shifts could have contributed to the speciesreplacement.Adults of the two tilapiine species were fed individually on amonospecific culture of the small green alga Scenedesmusacuminatus, which grows in a range of colony sizes. Clearingrateswere compared at 4 algal food densities and at the 4 colonysizes.The results indicate that O. niloticus has the abilitytofeed on either smaller or less bulky particles than O. esculentus such as prevail in the postdisturbance LakeVictoria. O. niloticus is also a more efficient filterfeederoverall, with a differential that is inversely proportional toparticle size. Additional data would be required to show thatfeeding performance and limnological shifts were primary tothereplacement of O. esculentus by O. niloticus.Researchers should however, be cautious about invokingcompetitionas the proximal cause of exotic impacts when simpler,extrinsic mechanisms are equally plausible.