Oreochromis Andersonii Research Articles

The evaluation of anaesthetic efficacy of Coriandrum sativum essential oils in three-spotted tilapia (Oreochromis andersonii): a study of fish size and water temperature

The present study aimed to determine the anaesthetic effectiveness, safety, and applicable concentrations of coriander (Coriandrum sativum) essential oils (CEO) for three-spotted tilapia (Oreochromis andersonii). For this purpose, CEO concentrations of 400, 500, and 600 µL/L were tested at two temperatures, 20.90 ± 1.19 °C and 30.60 ± 0.92 °C, on three different sizes (small, medium, and large) of the fish in duplicate glass aquaria with 200 mg/L of tricaine methanesulfonate (MS-222) served as the control. The sedative efficacy during simulated transportation experiments was also carried out. The results demonstrated that the induction and recovery time in O. andersonii anaesthetised with CEO were significantly influenced by concentration, water temperature, and fish size (P < 0.05). A concentration of 600 µL/L of CEO was found to be highly effective in inducing deep anesthesia and promoting rapid recovery in small and medium-sized fish at both water temperatures (20 and 30 °C). Based on this study, 100 µL/L of CEO is recommended for short-term transportation lasting up to 4 h, while 50 µL/L is advised for longer-distance transport. Although coriander essential oil shows anaesthetic effectiveness in three-spotted tilapia, further research is needed to fully understand the efficacy and safety of CEO across different fish species.

Anaesthetic effect of sodium bicarbonate (NaHCO3, baking soda) and its effect on Hematological parameters of three spotted tilapia (Oreochromis andersonii, Castelnau 1861) broodstock

ABSTRACT This study aimed to determine the optimal concentration of sodium bicarbonate (NaHCO3) and its effect on the Hematological parameters of three-spotted tilapia (Oreochromis andersonii) broodstock. Using a Completely Randomized Design, 150 broodstock (weight: 180 ± 10 g) were immersed in nine NaHCO3 concentrations (0–50 g/L) in triplicate. Induction and recovery times were recorded, and Red Blood Cells (RBC), White Blood Cells (WBC), Packed Cell Volume (PCV), and Hemoglobin (Hb) were measured. Mean Corpuscular Volume (MCV), Mean Corpuscular Hemoglobin (MCH), and Mean Corpuscular Hemoglobin Concentration (MCHC) were calculated. Induction time decreased and recovery time increased with higher NaHCO3 concentrations, without any mortality. A 10 g/L concentration only significantly affected RBC and PCV. At 45 and 50 g/L there were significant changes to MCH, while significant increase was observed at 10 and 15 g/L in Hb and WBC, respectively. The optimal concentrations established for routine handling in hatcheries were 30 and 35 g/L.

Metal bioaccumulation and histological alterations in Oreochromis andersonii at Itezhi-tezhi Dam downstream of a mining area in the Central Province of Zambia

The Itezhi-tezhi Dam on the Kafue River in Zambia is a major capture fishery. However, the upstream reaches of the Kafue River receive effluents from copper mines. It was unclear whether fish health in the dam is adversely affected due to the mining effluents. We investigated the health status of fish in Itezhi-tezh Dam using a histology-based fish health assessment protocol with Oreochromis andersonii as a bioindicator. Fish were sampled in the Itezhi-tezh Dam and at a reference site further upstream on the Kafue River before it enters the mining region. Metal bioaccumulation, biometric indices and histological alterations in the gills, gonads, hearts, kidneys and livers were assessed. The findings revealed significantly higher copper and selenium sediment concentrations (p = 0.02843 and p = 0.02107 respectively), bioaccumulation of copper and selenium, and increased histological alterations in the gills, kidneys and livers of fish in the Itezhi-tezhi Dam.

Nutrient composition of common fish species in the Okavango Delta: potential contribution to nutrition security

Fish can be a key strategy in addressing the global food and nutrition insecurity challenges. Fish nutrient data are critical for policy development targeting to reduce these problems. Despite this, much of the published studies on nutritional characteristics of fish have focused on marine ecology and there is little information on natural freshwater fish habitats like inland delta areas. This study investigated the nutrient composition of Okavango Delta fishes and their potential contribution to the recommended nutrient intake. Fish species sampled were Clarias ngamensis, Oreochromis andersonii, Schilbe intermedius, Marcusenius altisambesi, Brycinus lateralis, Barbus poechii and Tilapia sparrmanii. The nutrients analyzed were moisture, ash, proteins, Na, K, Ca, Mg, P and Zn. The small fishes (B. lateralis and B. poechii) had higher mineral content; Ca (643–968 mg/100 g), Mg (47–49 mg/100 g) and Zn (2.8–3.6 mg/100 g) compared to the larger species with average mineral content; Ca (73–460 mg/100 g), Mg (23–35 mg/100 g), Zn (1.2–2.9 mg/100 g). The most nutrient-dense species recognized were B. poechii and B. lateralis followed by the large species, M. altisambesi. These species illustrated the potential to contribute 80–121% Ca, 32–47% Mg and 70–150% Zn of the recommended nutrient intake to children (1–9 years), pregnant women and the elderly (65 + years). The study recommends prioritizing small and some large species (M. altisambesi and T. sparrmanii) for polyculture systems, primarily for securing food and nutrition security for local communities.

The Effect of Sausage Tree Fruit (Kigelia africana) on Gonadal Development and Growth Performance of Oreochromis andersonii

In Zambia fish farms, Oreochromis andersonii is an important common indigenous fish species. Naturally, safe phytochemicals can effectively improve fish reproduction performance and their production potential. Therefore, this study was conducted to determine the effect of Kigelia africana on the gonadal development and the performance of Oreochromis andersonii. A total of 96 male fingerlings were randomly assigned to four dietary treatments (D1-D4), and each treatment group had three replicates. The D1, D2, D3, and D4 groups were formulated to receive 0, 50, 100, and 150 g of powdered Kigelia africana/kg, respectively. The fish were fed the diets for 9 weeks, followed by the study parameter measurements at the end of the experiment. The highest mean body weight and gonadal weight were (29.8 ± 0.63 and 0.09 ± 0.010 g, respectively) for fish in the D2 group. There was no significant difference between the mean body weight of fish in different groups, but their mean gonadal weights differed significantly. The gonadal somatic index of fish differed significantly among treatment groups, with those in D2 having the highest mean value (0.36 ± 0.060). The highest mean standard length (103.3 ± 0.63 mm) and total length (126.0 ± 0.11 mm) of fish were observed for D1 and D2 groups, respectively. Additionally, the mean values for those parameters decreased with increasing Kigelia africana in the diet. The physicochemical parameters of water, including temperature and dissolved oxygen, ranged 16.8-23.1℃ and 0.6-2.2 mg/L, respectively; these were generally at low levels considering the optimum requirements for this fish species. In conclusion, Kigelia africana improved gonadal growth and development but did not promote overall fish growth. The best gonadal growth/development results of Kigelia africana powder were observed at a level of 50 g/kg, which can be used as a performance booster in the aquaculture production of Oreochromis andersonii.

A comparative analysis of the anaesthetic effect of sodium bicarbonate (NaHCO3) on male and female three spotted tilapia (Oreochromis andersonii)

ABSTRACT The anaesthetic effect of sodium bicarbonate (NaHCO3) on male and female three spotted tilapia (Oreochromis andersonii) was investigated. A total of 210 broodstock O. andersonii (weight: 160 ± 10 g) of different sexes were immersed in NaHCO3 at concentrations of 0, 5, 10, 15, 20, 25 and 30.0 g/L in triplicates. The induction and recovery times were monitored using a stopwatch. The induction and recovery times at all stages of anaesthesia for both sexes were significantly higher at concentrations ≥10 g/L. The induction times were significantly longer in the male fish than in the female at concentrations of 15 g/L for stages 1, 20 g/L for stages 1 and 2, 25 g/L for all the stages and 30 g/L for stage 2. The recovery time for male fish was significantly longer than that of the female fish at concentrations of 10 g/L for stages 2 and 3, 15 g/L for stage 1 and 20–30 g/L for stages 1 and 2. Overall, the induction and recovery times of the three spotted tilapia broodstock exposed to different concentrations of NaHCO3 could be affected by sex. Therefore, 30 and 25 g/L of NaHCO3 concentrations are recommended for use in male and female broodstock, respectively.

Sodium Bicarbonate and Acetic Acid: An Effective Anaesthetic for Three Spotted Tilapia, Oreochromis andersonii Fingerlings

Abstract This study aimed to study the effectiveness of different concentrations of sodium bicarbonate (baking soda) combined with acetic acid (vinegar) in three spotted tilapia (Oreochromis andersonii) fingerlings. Fingerlings (body weight 0.82 ± 0.00 g, and body length of 3.91 ± 0.03 cm) were subjected to three concentrations of sodium bicarbonate (30 g/L, 40 g/L and 60 g/L) combined with three concentrations of acetic acid (30 ml/L, 45 ml/L and 60 ml/L), each replicated five times to assess the anaesthesia induction and recovery time. Sodium bicarbonate and acetic acid induced anaesthesia in three spotted tilapia fingerlings regardless of the concentration used. However, high concentrations induced anaesthesia within a short period of time. Meanwhile, the full recovery time increased with concentration combinations. In essence, low concentrations of sodium bicarbonate and acetic could be recommended for anaesthetizing three spotted tilapia fingerlings. However, more studies on sodium bicarbonate and acetic acids as anaesthetics in aquaculture and fisheries is deemed necessary.

Effect of water level and water quality on small‐sized and juvenile fish assemblages in the littoral zones of the Zambezi/Chobe floodplain

AbstractThe effect of flooding phase and water quality on the littoral fish species of the Zambezi/Chobe floodplain was assessed between March 2017 and February 2018. Using the similarity percentage analyses (SIMPER), a 61.9% dissimilarity was detected in fish assemblages among the four flooding phases of the Zambezi/Chobe floodplain (rising, peak, recession and low flooding phase). Cichlids (63.1%) were the most dominant taxa during the peak flooding phase while cyprinids were dominant during the rising (40.7%), recession (45.3%) and low flooding phases (40.9%), respectively. Juveniles of the two commercially important species, Oreochromis andersonii and Coptodon rendalli, were among the four abundant species across the four flooding phases of the Zambezi/Chobe floodplain. Higher densities of these two species were in synchrony with the peak flooding phase, characterised by high levels of dissolved oxygen (5.7 ± 1.0 mg/L), neutral pH (7.9) and moderate water temperature (27.6 ± 1.1°C). During the flood recession phase, a severe decline in juvenile of the commercially important species (O. andersonii and C. rendalli) coincided with fish migration off the littoral zone into the main river at a mean size between 70 and 80 mm SL. Generally, the observed variations in the fish species composition during the different stages of flooding might reflect the influence of water quality and water volume to sustain large fish assemblages on the littoral zone of the Zambezi/Chobe floodplain. Any change in the natural flow and reduction in water volume of the Zambezi/Chobe floodplain might negatively affect the recruitment of juvenile fish into adult populations.

Cadmium and lead levels in three freshwater fish species from the Zambezi region, Namibia

Muscle tissue from three-spotted tilapia (Oreochromis andersonii) (n = 11), sharp tooth catfish (Clarias gariepinus) (n = 14) and tigerfish (Hydrocynus vittatus) (n = 12) were sampled from a market in the Zambezi region, Namibia in June 2018 and analysed for cadmium and lead. Cadmium was not detected in the three fish species. Mean lead levels in all three fish species (0.09–0.19 mg kg−1) were below the recommended safe level of 0.3 mg kg−1. Catfish had the highest mean lead concentrations, followed by tigerfish and tilapia in descending order. However, the differences in mean lead concentrations between the three fish species were not statistically significant (ANOVA, p = 0.18). These results show that the fish tested were safe for human consumption with respect to the metals tested. It is recommended that more metals and other contaminants, such as pesticides, be monitored robustly in water, sediment and fish, using the recommended protocols, in order to generate data that can be reliably used for human health risk assessments in the future.

Low oxygen: A (tough) way of life for Okavango fishes.

Botswana’s Okavango Delta is a World Heritage Site and biodiverse wilderness. In 2016–2018, following arrival of the annual flood of rainwater from Angola’s highlands, and using continuous oxygen logging, we documented profound aquatic hypoxia that persisted for 3.5 to 5 months in the river channel. Within these periods, dissolved oxygen rarely exceeded 3 mg/L and dropped below 0.5 mg/L for up to two weeks at a time. Although these dissolved oxygen levels are low enough to qualify parts of the Delta as a dead zone, the region is a biodiversity hotspot, raising the question of how fish survive. In association with the hypoxia, histological samples, collected from native Oreochromis andersonii (threespot tilapia), Coptodon rendalli (redbreast tilapia), and Oreochromis macrochir (greenhead tilapia), exhibited widespread hepatic and splenic inflammation with marked granulocyte infiltration, melanomacrophage aggregates, and ceroid and hemosiderin accumulations. It is likely that direct tissue hypoxia and polycythemia-related iron deposition caused this pathology. We propose that Okavango cichlids respond to extended natural hypoxia by increasing erythrocyte production, but with significant health costs. Our findings highlight seasonal hypoxia as an important recurring stressor, which may limit fishery resilience in the Okavango as concurrent human impacts rise. Moreover, they illustrate how fish might respond to hypoxia elsewhere in the world, where dead zones are becoming more common.

Genetic diversity of two Southern African cichlids (Oreochromis andersoniiandO. macrochir) in the Zambezi and Congo River basins

Oreochromis andersonii and O. macrochir are two important cichlid species native to Southern Africa. We describe in this paper their genetic population structure in the Upper Zambezi River, Kafue River, and Lake Bangweulu representing part of the Congo and Zambezi River Basins. Microsatellite genetic markers were employed to analyse the genetic population structure of the two species using 177 tissue samples. The average allele richness of O. andersonii was higher in the Zambezi River (10.500) than in the Kafue River (9.583) though not statistically different. For O. macrochir, it was highest in the Zambezi River (11.170) followed by the Kafue River (9.781) and least in Lake Bangweulu (7.067) and their differences were significant. The gene diversity indices; gene diversity (hs), observed heterozygosity (HO), expected heterozygosity (HE) were high and similar in O. andersonii populations. However, in O. macrochir HE was significantly lower in Lake Bangweulu (0.678) compared to the Kafue River (0.799) and Zambezi River (0.802) populations. Population differentiation estimated by RST and DEST revealed high differentiation in both species (RST = 0.598, DEST = 0.777 for O. andersonii; RST = 0.379, DEST = 0.710 for O. macrochir). The highest source of variation was among populations (84.71%) for O. andersonii and within populations (67.09%) for O. macrochir. Comparisons of population pairs revealed a close genetic similarity between the Zambezi River and Lake Bangweulu populations of O. macrochir. Bottlenecks were observed in both species using the Two-Phase Model (T.P.M.) indicative of a recent genetic loss or reduction in effective population size. Though our results indicate that the populations of both species still maintain sufficiently high levels of genetic diversity in the sampled areas, the bottlenecks observed are a source of concern. We recommend a more robust study of genetic diversity of these species in all sections of these river systems and that some key conservation sites should be identified to protect the gene pool of these native species.

English

The study was conducted in the Copperbelt and North-Western Provinces of Zambia which represented the urban and rural strata respectively. A total of two hundred and forty farmers were randomly sampled from the farmer registers kept by the Department of Fisheries (DOF) in the Ministry of Fisheries and Livestock (MFL), Zambia. The main objective of the survey was to determine the productivity of the exotic fish species Oreochromis niloticus in comparison to closely related indigenous fish species (Oreochromis andersonii, O. machrochir and Tilapia rendalli) at farm level. Correlation analysis showed that stocking density was positively, though weakly, correlated (r = 0.31, N = 292, P&lt; 0.05) with yield, with a higher stocking density associated with higher yield. Since the correlation coefficient was lower than 0.5, regression analysis was not performed. Sex reversed fingerlings gave almost double the yield (5.5 tons/ha/year) compared to mixed sex fish (3.8 tons/ha/year). Productivity was highest in those farmers using commercial feed (6.4 tons/ha/year) compared to those using single ingredients or fertilization or a combination of the two. There was no evidence to show that farmers who were using O. niloticus were performing better than those farmers using O. andersonii since the productivity of the farmers culturing the non-indigenous O. niloticus (4.7 tons/ha/year) did not differ significantly from those farmers using O. andersonii (4.6 tons/ha/farmer). There should be deliberate efforts to promote O. andersonii both at production and consumption levels in order to create demand for the fish species. Genetic studies should be conducted to establish the genetic variability and strains of the indigenous fish species in Zambia. Furthermore, there is an urgent need to establish gene banks for most indigenous fish species to conserve the genetic resources for aquaculture and capture fisheries. Key words: Exotic, indigenous, production, productivity, genetics. &nbsp

Emergence of infection with Aphanomyces invadans in fish in some main aquatic ecosystems in Zimbabwe: A threat to national fisheries production.

The first outbreak on the African continent of infection with Aphanomyces invadans (the causative agent of epizootic ulcerative syndrome) in fish was confirmed in the Chobe-Zambezi rivers in 2007. The emergence of massive outbreaks of infection with A.invadans in multiple fish species exposed serious aquatic biosecurity challenges in the Southern African region. This study investigated the incursion of infection with A.invadans in fish from the main aquatic ecosystems of Zimbabwe from 2012 to 2015 using data obtained from the Department of Livestock and Veterinary Services, Zimbabwe. In some outbreaks, fish samples were collected and tested at the University of Zambia, for confirmation by histopathology and species-specific PCR. The infection was first confirmed at Darwendale water impoundment (Mashonaland West Province) in 2012, followed by Matabeleland South Province at Mtshabezi water impoundment and Nkankezi River (both 2013). An apparent southward spread continued in 2014, with virgin outbreaks at Ntalale water impoundment (Matabeleland South Province) and Mwenezi River in Midlands Province. In 2015, inland incursion was confirmed at Dutchman's Pool in Midlands Province and further north-west at the Sanyati River Basin in Lake Kariba (Mashonaland West Province). In all outbreaks, infection with A.invadans was confirmed in seven fish species, namely the African sharptooth catfish (Clarias gariepinus, Burchell, 1822), blunt-toothed African catfish (Clarias ngamensis Castelnau, 1861), yellow belly bream (Serranochromis robustus Gunther, 1864), straight fin barb (Enteromius paludinosus Peters, 1852), dashtail barb (Enteromius poechii Steindachner, 1911), large-mouth bass (Micropterus salmoides Lac'epe'de, 1802) and the three-spot tilapia (Oreochromis andersonii Castelnau, 1861). Cases were most common in the African sharptooth catfish, with mortalities more pronounced in young fish of all species. The results suggested a gradual emergence of an intractable infection with A.invadans in fish in the main aquatic ecosystems of Zimbabwe, which may have negative impact on biodiversity conservation and aquaculture.

Parasites of fishes in the recently inundated ephemeral Lake Liambezi, Namibia

Lake Liambezi forms the periodic connection between the upper Zambezi, Kwando and Okavango rivers. A full parasitological assessment was conducted on 86 fish, representing 14 species in six families sampled in August 2011. Parasite diversity was low and dominated by species with complex life cycles involving intermediate hosts. Most prevalent were larval nematodes (Contracaecum sp.) infecting 12 and Trypanasoma sp. infecting nine of the 14 host species. The intra-erythrocytic parasite Babesiosoma mariae was found in the blood of Coptodon rendalli and Oreochromis andersonii with prevalence of 50% and 60%, respectively. The host-specific monogenean Annulotrema hepseti was recorded only from H. cuvieri with a prevalence of 100%. Notable absences were the copepod and branchiuran parasites that have direct lifecycles and usually occur in high prevalence and abundance in the region. Because parasites with direct life cycles can only be transported into the lake on the host fish, their absence suggests limited immigration of infected fishes into the lake. This suggests that internal recruitment dominates over immigration in the fish population dynamics in Lake Liambezi.

Validation of annulus formation in sagittal otoliths of three tropical cichlids Oreochromis andersonii (Castelnau, 1861), Oreochromis macrochir (Boulenger, 1912), and Coptodon rendalli (Boulenger, 1896)

Summary The periodicity of growth zone formation in sagittal otoliths of Oreochromis andersonii, Oreochromis macrochir and Coptodon rendalli from the Upper Zambezi region, southern Africa, was validated to test the hypothesis that a single growth zone is deposited annually. Edge analysis was carried out on 754 O. andersonii (54–492 mm TL), 601 O. macrochir (83–438 mm TL) and 622 C. rendalli (45–379 mm TL) caught between August 2010 and August 2011 in Lake Liambezi and the Kavango, Kwando and Upper Zambezi rivers. This was corroborated for O. andersonii (n = 17) and C. rendalli (n = 16) using a fluorochrome marking experiment conducted in a large earthen pond (625 m2, 1.5 m deep) adjacent to the Kavango River. Both methods demonstrated that growth zone deposition occurred annually in juvenile and adult fishes. For edge analysis, the highest proportion of otoliths with an opaque margin was observed in summer from October to December. The fluorochrome marking experiment, however, revealed that opaque zone deposition took place in winter between June and September when water temperatures were lowest (17°C), indicating a lag between opaque zone deposition and detection on the otolith edge.

A growth comparison among three commercial tilapia species in a biofloc system

Despite a worldwide boom in tilapia aquaculture, South Africa has failed to follow suit, resulting in a small, very much undeveloped local industry. Much of the justification for this reality lies on the inadequacy of the species used and the stringent legislation preventing the use of the Nile tilapia, Oreochromis niloticus. With momentum gained in recent years, the local industry lobbying for the legalization of the O. niloticus has made great progress and its use in certain systems has recently been facilitated. This study was conducted to compare the growth performance between the indigenous Mozambique tilapia, Oreochromis mossambicus, another African alternative gaining momentum in Zambia, the Three Spot tilapia, Oreochromis andersonii, and the globally reputable O. niloticus. The study was conducted in a biofloc technology (BFT) system, showing great potential as an environmentally friendly technology. The trial was run for 10 weeks with 60 fish/tank stocked into four repeats tanks per species. Fish were sampled on six occasions with a regression fitted to the mass data. Oreochromis niloticus showed a significantly higher growth rate with an average daily gain of 0.693 ± 0.018 g/day and the lowest feed conversion ratio (FCR) of 1.01 ± 0.05, followed by O. mossambicus with average daily gain of 0.405 ± 0.025 g/day and FCR of 2.24 ± 0.16, and then O. andersonii with an average daily gain of 0.185 ± 0.025 g/day and an FCR of 2.53 ± 0.28, respectively. From this study, O. niloticus therefore appears to be the most adequate species for use in BFT systems in South Africa.

Ecology, fish and fishery of Lake Liambezi, a recently refilled floodplain lake in the Zambezi Region, Namibia

Lake Liambezi (300 km2) refilled in 2009 after a prolonged 22-year dry period. Its aquatic macrophyte populations, fish fauna and fishery shortly after refilling are described. The emergent aquatic macrophyte Phragmites australis formed dense stands covering large parts of the lake, while extensive beds of submerged Lagarosiphon ilicifolius and Najas horrida occurred in shallower areas. Twenty-nine fish species were recorded in experimental gillnets. Catches were dominated by Brycinus lateralis and Schilbe intermedius, contributing 39.5% and 38.5% by weight, respectively. Large-meshed (3–4.5 inch) monofilament and multifilament gillnets emulating those used in the commercial fishery caught mostly Oreochromis andersonii (42.7% by weight) and Serranochromis macrocephalus (28.3% by weight), respectively. Commercial gillnet catch per unit effort (CPUE) averaged 7.0 kg 100-m net−1 night−1, and the annual yield for 2011/2012 was estimated at 2 700 tonnes. Aquatic macrophytes and fishes quickly recolonised the lake and, just two years after refilling, the lake supported a thriving fishery that yielded 4.5 times more than the 600 tonnes recorded in 1974.

English

Developing countries remain overwhelmed by nutritional problems caused mainly by poverty, natural disasters and political instabilities. The aim of this study was to determine essential mineral content of some common fish species in the Okavango Delta, Botswana and assess their potential in enhancing mineral intake. Atomic Absorption Spectrometry was used for determination of Ca, Mg, Fe, Zn, Cu, Mn; flame photometry for Na and K; UV-VIS Spectrometry for P. Marcusenius altisambesi, Schilbe intermedius, Brycinus lateralis, Oreochromis andersonii, Barbus poechii. Only edible flesh was analysed for large fish (&gt;130mm) while small fish (&lt;130mm) were analysed whole. Single factor ANOVA analyses show that all minerals analysed except copper varied significantly between species, (p&le;0.05) and the small species, Barbus poechii has the highest mineral content. The concentration ranges of minerals were within FAO mean concentration ranges for fish and comparable to values obtained from other previous studies. The results show that fishes of Chanoga have a good supply of minerals and can be used for enhancing mineral intake and protecting the community from mineral deficiency diseases.&nbsp; &nbsp; Key words: Fish species, macro-nutrients, micro-nutrients, mineral content, Okavango Delta. &nbsp

Broad Niche Overlap between Invasive Nile Tilapia Oreochromis niloticus and Indigenous Congenerics in Southern Africa: Should We be Concerned?

This study developed niche models for the native ranges of Oreochromis andersonii, O. mortimeri, and O. mossambicus, and assessed how much of their range is climatically suitable for the establishment of O. niloticus, and then reviewed the conservation implications for indigenous congenerics as a result of overlap with O. niloticus based on documented congeneric interactions. The predicted potential geographical range of O. niloticus reveals a broad climatic suitability over most of southern Africa and overlaps with all the endemic congenerics. This is of major conservation concern because six of the eight river systems predicted to be suitable for O. niloticus have already been invaded and now support established populations. Oreochromis niloticus has been implicated in reducing the abundance of indigenous species through competitive exclusion and hybridisation. Despite these well-documented adverse ecological effects, O. niloticus remains one of the most widely cultured and propagated fish species in aquaculture and stock enhancements in the southern Africa sub-region. Aquaculture is perceived as a means of protein security, poverty alleviation, and economic development and, as such, any future decisions on its introduction will be based on the trade-off between socio-economic benefits and potential adverse ecological effects.

Predation on invasive redclaw crayfish Cherax quadricarinatus by native fishes in the Kafue River, Zambia

The stomach contents of eight species of predatory or omnivorous fish caught in gillnets in the Kafue River in May and June 2010 were examined to determine the relative importance, expressed as ‘prominence value’ (PV), of crayfish Cherax quadricarinatus in their diet. Four species, Clarias gariepinus, Schilbe intermedius, Serranochromis robustus and Serranochromis angusticeps, had fed on C. quadricarinatus. The PV of C. quadricarinatus was highest in C. gariepinus, in which the quantity and size of crayfish eaten was significantly correlated with standard length (SL) of fish >200 mm SL. Predated C. quadricarinatus were significantly smaller (mean carapace length [CL] 30 mm; SE 1.2) than those caught in gillnets (mean CL 76 mm; SE 0.9). No C. quadricarinatus remains were found in the stomachs of Oreochromis andersonii, Synodontis macrostigma, Serranochromis macrocephalus or Mormyrus lacerda.