Tilapia Populations Research Articles

Morphometric Variations of Nile Tilapia (Oreochromis niloticus) (Linnaeus, 1758) (Perciformes, Cichlidae) Collected from Three Rift Valley Lakes in Ethiopia

The pattern of morphometric differentiation among three populations of Tilapia (Oreochromis niloticus) sited in Koka, Ziway, and Langano lakes in the Ethiopian rift valley was examined. Morphometric differentiation was examined, compared and quantified by the use of twelve different body measurements in 391 specimens to generate baseline information for conservation and product improvement plans. The result revealed that comparisons of mean values of morphometric measurements in the three studied tilapia populations statistically significant differences (P<0.05). The Fulton’s condition factor (K) of Oreochromi niloticus samples collected from Lakes Koka, Ziway, and Langano was 1.48, 1.2, and 0.66, respectively. The Discriminant Function Analysis performed on seven variables led to a correct classification in 90.5 % of cases. Differences observed between samples in this study indicated that there is important morphometric variation between the studied tilapia populations. Plotting the two significant canonical discriminant functions separated the Lake Langano population from the other two.

Efficacy of Lactobacillus plantarum and Saccharomyces cerevisiae on growth improvement of hybrid Nile and Rufiji tilapia populations

An investigation of the role of Lactobacillus plantarum and Saccharomyces cerevisiae on growth performance of hybrids from Oreochromis niloticus and Oreochromis urolepis urolepis was carried out in plastic tanks at 10 fish/m3 density and a salinity of 25 units. The probiotic treatments were 2, 4 and 6 g/kg feed while the control feed did not include any probiotics. Growth parameters were measured fortnightly and water quality parameters monitored every day. Survival rate and water quality factors revealed non-significant variations (p > 0.05). Final weight differed significantly among the treatments (p < 0.05). Furthermore, Food Conversion Ratio (FCR), Specific Growth Rates (SGR), and weight gain did not differ significantly (p > 0.05) between the control and 2 g/kg treatments. However, the 4 and 6 g/ kg feed treatments showed significant differences (p < 0.05) from the control diet and 2 g/kg for both L. plantarum and S. cerevisiae. The condition factor did not show any significant difference (p > 0.05) among the treatments. Results of this study indicate that increasing the quantity of commercial probiotics in feeds improved growth rates. Condition factor observed in the treatments could be due to the role of the tested probiotics in improving the water quality of tanks. Therefore, probiotics could potentially be used to enhance coastal aquaculture development.

Growth and status of Nile tilapia (Oreochromis niloticus L.) stock in Lake Chamo, Ethiopia

AbstractGlobal inland freshwater fisheries support livelihoods for several million people. These important resources, however, are suffering from excessive overfishing as a result of increasing fishing pressures attributable to an increased human population and subsequent demand for food and economic livelihoods. To this end, very little is known about the vital population parameters and stock status of Nile tilapia (Oreochromis niloticus) in Lake Chamo in Ethiopia. Thus, the present study focused on determining the Nile tilapia population parameters and assessing their stock status in Lake Chamo, using length‐frequency and catch‐effort data collected between February 2018 and January 2019 from commercial fish catches from 10 major fish landing sites. The TropFishR software package was used to determine von Bertalanffy growth parameters, and length‐converted catch curve and empirical models were used to calculate mortality parameters. The maximum sustainable yield (MSY) and its corresponding level of effort (f MSY) were determined using the length‐based Thompson & Bell yield prediction model. The growth parameters L ∞, K and Ф′ of the Nile tilapia stock were found to be 59.4‐cm, 0.41/year and 3.16, respectively. The annual rate of total (Z), natural (M) and fishing mortality (F) were estimated to be 2.442, 0.558 and 1.884/year, respectively. The calculated current yield (711 t/year) is lower than the predicted values of MSY (771 t/year). The present level of fishing effort (2564 nets/day) is more than twice higher than the optimum level of fishing effort (1026 nets/day), clearly indicating overfishing of the Nile tilapia stock in the lake. This finding is also substantiated by the high level of exploitation rate (E = 0.771). Thus, a recommendation based on the results of the present study is a 60% reduction in the fishing effort (1538 nets/day), which will also provide a higher fish yield. The results of the present study also are useful facilitate development of appropriate management strategies for the Nile tilapia fishery in Lake Chamo.

Comparative study of the breeding performances of five populations of Nile tilapia (Oreochromis niloticus) (F1) in an experimental ongrowing system in Benin (West Africa)

ABSTRACT In order to investigate the breeding performances of five F1 populations of Oreochromis niloticus originated from wild broodstock of Nangbéto, Sohoumè and Togbadji in the Mono basin, Gobé in the Ouémé basin and Gbassa in the Niger basin, breeding tests were conducted for 10 weeks in an ongrowing experimental system. All tested populations were checked daily in order to collect spawn and to compare the reproductive performances. Compared with others, Sohoumè, Togbadji, and Gobé populations significantly (P < .05) displayed the best breeding performances in terms of rate of multi-spawning females (89.3 ± 15.2–92.3 ± 10.8%); mean relative fecundity (4 ± 2–6 ± 3 eggs/g of female), mean gonadosomatic index (3.1 ± 2–4.8% ± 2.6); mean absolute fecundity rates (865 ± 339–989 ± 547 eggs); mean fertilization rates (98 ± 4.3–99.3 ± 1.1%); and mean hatching rates (83.7 ± 10–88.9 ± 9.1%).

Cyto-histopathological and protein polymorphism alterations in five populations of Nile tilapia (Oreochromis niloticus) as biomonitor for water heavy metal pollution.

Cytological, histopathological and sodium dodecyl sulphate polyacrylamide gel analyses were carried out on five populations of common Nile tilapia fish (Oreochromis niloticus) occurring in five sites - River Nile (reference site), Bahr Yusef canal, Ibrahimia canal, Irrigation drain and El Moheet drain of El Minia Province, Egypt - to evaluate the usability of Nile tilapia as a biomonitor for water heavy metal contaminants. Water surface samples were collected from the five sites, and lead (Pb) concentration was shown to surpass the limits defined by WHO. Ni and Cd levels were shown to be elevated in Ibrahimia canal samples. Moreover, the concentration of heavy metals in fish muscles collected from Bahr Yusef canal and El Moheet drain was the highest in comparison with those of the other water sites. Cytological examinations of blood smears showed not only a significant percentage of micronuclei in Irrigation drain population but also a significant percentage of binucleated cells in Ibrahimia canal and El Moheet drain populations. In addition, pathological alteration was observed in blood cells, especially in samples collected from Irrigation drain and El Moheet drain. Histopathological changes were strongly observed in the liver and the kidneys of El Moheet and Irrigation drain population. Moreover, total protein band pattern profiles showed extra bands in both Ibrahimia canal and Irrigation drain more than that recorded for the River Nile population. In conclusion, cyto-histopathological and total protein band pattern results confirmed that O. niloticus responds sensitively to the excess of heavy metals present in the water.

Relationship Between the Productivity Losses of Tilapia and Aeromonas veronii Infection

Nile tilapia (Oreochormis niloticus) is considered one of the top cultured fish in Egypt and the second globally. The high demand for animal protein was associated with intensive fish culturing, which resulted in the emergence of disease outbreaks causing significant losses in tilapia aquaculture among the recent years. With special reference to bacterial outbreaks, Aeromonas veronii (A. veronii) is a recently reported bacterial pathogen affecting farmed tilapia and other fish species worldwide. A. veronii has been notified as a significant threat on Nile tilapia populations resulting in heavy mortality and was recorded to have a wide range of antibiotic resistance. Subsequently, recent publications focused on immunostimulant alternatives. The previous science data on summer tilapia mortality phenomena were outlined in this review article with special reference to A. veronii as an important pathogen. Clinical and postmortem symptoms, histopathological lesions, genetic diversity, and associations with others submitted to GenBank, diagnosis, control steps, and significance to public health were highlighted.

The effects of single nucleotide polymorphisms in Neuropeptide Y and prepro-orexin on growth in Nile tilapia (Oreochromis niloticus)

In teleosts, starvation induces an increase in appetite-related gene expression and peptide secretion in the hypothalamus. This reduces autonomic nervous activity and promotes food intake and body weight gain. We have previously demonstrated that single nucleotide polymorphisms (SNPs) in the 5′ flanking region of ghrelin affect gene regulation and are associated with superior growth traits in Nile tilapia (Oreochromis niloticus). Recent findings have also suggested that Ghrelin, Neuropeptide Y (NPY), and Prepro-orexin combine in the hypothalamus to regulate appetite in organisms, thus we hypothesized that SNPs in NPY and prepro-orexin may also indirectly regulate the growth of Nile tilapia. Accordingly, we examined the effects of SNPs in NPY and prepro-orexin in two populations of Nile tilapia—the GIFT (GF) and Egyptian (EG) populations—by cloning the 5′ flanking and coding regions of each gene and screening the SNPs in these regions to identify associations with superior growth traits. After preliminary screening and secondary prediction of function, 6 SNP loci (1 from NPY and 5 from prepro-orexin) were identified in the GF population, from which 9 diplotypes were constructed. Of these, four loci from prepro-orexin (g. −1108C > G, g. −1107C > G, g. −1063C > T, and g. −883C > T) and one diplotype were associated with superior growth traits. In the EG population, 4 SNP loci (1 from NPY and 3 from prepro-orexin) were identified and 6 diplotypes were constructed; however, none of these were associated with superior growth traits. Considering these findings, we discuss the necessity for functional prediction of SNP loci, and consider the differential conservativeness of the 5′ flanking and coding regions of appetite-related genes.

Morphological divergence within the largest genetically consistent group of wild Tilapia

Morphometric and meristic data were used in this study to establish morphological variations within the largest genetically consistent groups of five wild populations of tilapia from Kyushu and Okinawa ecological regions of Japan. The genetic analysis based on mitochondrial DNA relatedness was used to determine the data range used for the morphological analyses. During the morphological analyses, the morphometric and meristic data were correlated by two multivariate statistical methods, such as PCA and LDA. Both statistical methods observed morphological differences of higher divergence in Okinawa contrary to Kyushu. Additionally, the statistical relationship between the estimated non-parametric test of ANOSIM (R = 0.21; p ˂ 0.05) and PERMANOVA (F = 4.20; p ˂ 0.05) indicated a higher magnitude of morphological separation, especially in Okinawa; nevertheless, water temperature, conductivity, and salinity levels vary widely in Kyushu than in Okinawa. All observed differences could be attributed to either genetic and/or environmental factors, which are major variables in influencing phenotypic variability. However, since tilapia in Japan experienced multiple introductions from different countries and environmental backgrounds, we recommend further studies under a controlled environment to ascertain the environmental responsiveness during morphological differentiation within a genetically consistent group of tilapia.

Identifying geographically differentiated features of Ethopian Nile tilapia (Oreochromis niloticus) morphology with machine learning.

Visual characteristics are among the most important features for characterizing the phenotype of biological organisms. Color and geometric properties define population phenotype and allow assessing diversity and adaptation to environmental conditions. To analyze geometric properties classical morphometrics relies on biologically relevant landmarks which are manually assigned to digital images. Assigning landmarks is tedious and error prone. Predefined landmarks may in addition miss out on information which is not obvious to the human eye. The machine learning (ML) community has recently proposed new data analysis methods which by uncovering subtle features in images obtain excellent predictive accuracy. Scientific credibility demands however that results are interpretable and hence to mitigate the black-box nature of ML methods. To overcome the black-box nature of ML we apply complementary methods and investigate internal representations with saliency maps to reliably identify location specific characteristics in images of Nile tilapia populations. Analyzing fish images which were sampled from six Ethiopian lakes reveals that deep learning improves on a conventional morphometric analysis in predictive performance. A critical assessment of established saliency maps with a novel significance test reveals however that the improvement is aided by artifacts which have no biological interpretation. More interpretable results are obtained by a Bayesian approach which allows us to identify genuine Nile tilapia body features which differ in dependence of the animals habitat. We find that automatically inferred Nile tilapia body features corroborate and expand the results of a landmark based analysis that the anterior dorsum, the fish belly, the posterior dorsal region and the caudal fin show signs of adaptation to the fish habitat. We may thus conclude that Nile tilapia show habitat specific morphotypes and that a ML analysis allows inferring novel biological knowledge in a reproducible manner.

Improving genetic quality of cultivated aquatic species under a breeding program: Case study of first generation (G1) population of red tilapia (Oreochromis spp)

A body weight based selection protocol is the most common practice in the genetic improvement programs for aquaculture species. It might be one of the criteria to be considered in the aquatic resources management scheme. This experiment aimed to evaluate the genetic enhancement of selected first-generation (G1) of red tilapia. Fish were bred under fullsib mating design at hapa of 2×2 m2. The communal growing out of fingerlings was carried out in an earthen pond for 120 days. The stocking density of fingerlings was 7 fish per m2. Fish were fed with commercial floating fish feed at a rate of 5-10% of biomass twice a day. The results showed that the population of males and females of red tilapia in the first generation (G1) grew faster than the previous generation (G0). The selection response values were 19.52 % and 12.05 % for male and female populations, respectively. The heritability values in male and female red tilapia populations were in the medium category with a value of 0.33 and 0.25, respectively. The results showed that the breeding program increased the genetic quality of fish in terms of body weight based growth in the first generation of red tilapia.

Evaluación del periodo de masculinización en la tilapia del Nilo var spring empleando 17α-metiltestosterona

Una alternativa para reducir el uso de hormonas con aplicación inmediata en el cultivo comercial de la tilapia del Nilo, es la evaluación precisa de los tiempos de masculinización en variedades domesticadas. El objetivo del trabajo fue evaluar el porcentaje de masculinización y el crecimiento obtenido en la tilapia del Nilo alimentada con alimento comercial adicionado con 17α- metiltestosterona a diferentes tiempos. Para ello se evaluaron cinco tiempos (0, 10, 15, 20 y 25 días) de administración de hormona a una población mixta de tilapia del Nilo variedad Spring. Para cada tratamiento se utilizaron tres réplicas. Los alevines fueron sembrados a densidad de 0.5 alevines L−1 en un sistema de recirculación cerrado. El cultivo consistió de 30 días en acuarios de acrílico de 85 L y 20 días en tanques exteriores de 1000 L. La determinación del sexo se realizó con la técnica de squash. El crecimiento se determinó por medio del peso húmedo y la longitud total obtenida de las biometrías. Los resultados indican una masculinización del 100% desde los 10 días de aplicación del esteroide, pero los mayores valores de crecimiento se observaron en los grupos que recibieron el esteroide por mayor tiempo. Los resultados muestran que no es necesario administrar el esteroide durante 30 días para lograr una masculinización del 100% en variedades domesticadas y que el esteroide tiene efecto positivo en la tasa de crecimiento de los alevínes, persistiendo por algunas semanas después de terminado el tratamiento.

Assessment of morphological variations in wild populations of Nile tilapia from South West Nigeria

Abstract. The pattern of morphological variations among Oreochromis niloticus (Linnaeus, 1758) (Cichlidae; Teleostei) from three populations from South West Nigeria (Asejire reservoir, Ureje reservoir and Ogbese River were examined (n=10 speciments from each water body) with a view to investigate the genetic diversity using morphometrics and meristics indices. In general, 77.22% of the total variation in the sampled populations is from first principal component (PC1) and the characters that contributed to this variation are with pre-dorsal length, pre-pectoral length, pre-anal length, dorsal spine length and anal spine length. Canonical variate analysis (CVA) showed that O. niloticus populations from the three water bodies formed a separate cluster from each other and that there was an intersection between the Asejire and Ureje reservoirs samples and the Asejire reservoir with Ogbese River samples. However, the samples from Ureje reservoir and Ogbese River had no intersection. Cluster analysis revealed two major clusters with Ureje reservoir samples belong to one and Ogbese River and Asejire reservoir fall in the other cluster. Principal Components for meristic (PC1 and PC2) accounted for 46.49% and 24.17% revealed that transverse scales and scales surrounding the caudal peduncle, pre-dorsal scales accounted for the variation. The CVA revealed that of all the populations sampled, samples from Ureje reservoir are more diverse that the other two populations. The research disclosed that there is variation in O. niloticus populations from the three water bodies, which could be a result of genetic structure and environmental conditions, being major factors affecting morphological variability among different geographical populations. Further study on genetic differentiation of individuals from different species is necessary to confirm findings of the present study.

Genomic selection for resistance to Francisellosis in commercial Nile tilapia population: Genetic and genomic parameters, correlation with growth rate and predictive ability

Nile tilapia has become the most important species within freshwater aquaculture worldwide, the global tilapia market being expected to reach US$ 25 billion before the end of 2028. Since on an average 50% of the biomass is being lost during outbreaks of Francisellosis in tilapia farms, this study was undertaken to estimate the genetic parameters for resistance to this economically important disease of Nile tilapia, and calculate the genetic correlation with body weights at different ages. A further aim was to characterize the potential of, and to select the best genomic model for, the implementation of genomic selection for resistance to Francisellosis in the commercial Nile tilapia breeding program. The study revealed moderate to high heritabilities (0.37 ± 0.05 to 0.51 ± 0.06) for resistance to Francisellosis using both pedigree and genomic-based estimates. The genetic correlation between resistance to Francisellosis and five different growth traits at different ages ranged from −0.09 to −0.29. Compared to pedigree-based models, GBLUP was found to increase the predictive ability for resistance to Francisellosis by 57%, and the use of Bayesian models would increase this predictive ability by up to 217% in the practical breeding scenario. Thus, the study shows the potential to increase resistance to Francisellosis by genetic selection in commercial Nile tilapia breeding programs. The genetic correlation between resistance to Francisellosis and growth traits showed negative trends, although they were significantly not different from zero. A very large increase in predictive ability for resistance to Francisellosis can be achieved by using Bayesian models for genomic selection.

Genetic structure of wild and farmed Nile tilapia (Oreochromis niloticus) populations in Benin based on genome wide SNP technology

For the last three decades, Nile tilapia Oreochromis niloticus (Linnaeus, 1758) is considered as one of the most productive and internationally traded food fish. Although economically important, our knowledge on the genetic structure of natural and farmed populations is scarce, especially in Africa. Sustainable aquaculture however requires integrating genetic information to elaborate appropriate management practices. The aims of the present study were: (i) to characterize the genetic structure and diversity of several O. niloticus wild populations collected in four drainage basins: Mono, Niger, Ouémé and Volta in Benin; (ii) to compare the identified genetic profiles of these wild populations with domesticated strains bred in two Beninese fish farms and one Belgian aquaculture research center; (iii) and finally to use the data as a tool to improve management of wild genetic resources and domesticated farmed strains.In order to characterize the genetic structure of the thirteen sampled populations of O. niloticus 2.950 SNPs were used to perform a clustering analysis and investigate the genetic diversities and population differentiations. Our results showed that, populations of O. niloticus collected in different basin and farms in Benin showed low to moderate genetic differentiation (Fst from 0.018 to 0.143) with the exception of the Nangbéto population of Mono basin, which is genetically more differentiated (Fst from 0.091 to 0.278). Compared to wild populations, there is a greater genetic proximity between the breeding populations of CRIAB and the Pendjari river population (Fst = 0.0 47), and between the Yohan-Esteve farm populations and the Gbassa population (Fst from 0.045 to 0.055). In view of the low level of inbreeding and the good growth and reproductive performance of the Togbadji population in the Mono basin, it would be a potential candidate for the development of a local strain of O. niloticus for aquaculture in Benin.

Differences in Phenotypic Characters of Nile Tilapia (Oreochromis niloticus L) in Three Ethiopian Rift Valley lakes; Screening Strains for Aquaculture

Nile tilapia (Oreochromis niloticus L.) is indigenous species to Ethiopia and constitutes major proportion in the country’s fish production. In an attempt to select better performing strains for aquaculture development, tilapia populations from different Ethiopian rift valley lakes showed different growth performances in pond culture. Investigation of desired culture characteristics of target tilapia populations is required to improve their productivity in aquaculture system. The current study was made to investigate phenotypic characters of the tilapia populations in three geographically isolated Ethiopian rift valley lakes (Chamo, Koka and Ziway). A total of 450 adult tilapias of commercial catches were sampled from the three lakes and their phenotypic characters were analyzed during May 2018 to March 2019. Twenty six morphometric character indices, eight meristic counts, total length, standard length, total weight, length-weight relationship and Fulton’s condition factor were considered in the analysis. The results revealed significant differences (p ≤ 0.05) in most of the morphometric character indices, meristic counts, mean length and weight and Fulton’s condition factor among the three tilapia populations. Chamo tilapia population were found to have highest mean values of total weight, total length and standard length while Koka population have highest mean value of Fulton’s condition factor and positive allometric growth as characters desired in aquaculture. Further research is required to investigate whether the fish populations could maintain those characters in pond cultures.

Monitoring of genetic variation in mono-sex tilapia stocks using randomly amplified polymorphic DNA markers in some private hatcheries of Bangladesh

Tilapia (Oreochromis niloticus) is one of the most promising aquaculture species in Bangladesh, the culture of which is mostly dependent on the supply of hatchery-produced seed. Published information on genetic monitoring and genetic variability of the hatchery-reared tilapia population is rare in Bangladesh. Therefore, intra- and inter-population genetic variations were estimated among the five mono-sex tilapia populations collected from the matchday-kanon, khan, nancy, city, and brac hatcheries located at different districts in Bangladesh. Random amplified polymorphic DNA (RAPD) markers were amplified from tilapia’s muscle DNA of each of the five populations using ten decamer random primers. The primers produced a total of 181 scorable bands of which 64.1% were found to be polymorphic ranged in molecular size from approximately 100bp to 2000bp. The brac population obtained the highest genetic variability (51%) of overall polymorphic loci and the lower index was estimated (55-62%) in that population with the four others in pair-wise similarity comparison. Moreover, a common pattern of distinctive genetic variation was observed in the brac hatchery population when the genetic differentiation (FST), gene flow (Nm) and genetic distance (D) had been analyzed. The distinctiveness was also supported from the unweighted pair group method of arithmetic mean (UPGMA) dendrogram analysis where the five populations aligned into two clusters- the brac alone belonged to one and the rest of the four grouped into another. Our overall findings reveal that a low level of genetic variation exists in the current tilapia populations of Bangladesh except in the brac hatchery population where broodstock is known to be well managed. This low level of genetic variation of the tilapia hatchery population will have a seriously detrimental effect on this species and its overall production if the genetic quality is not maintained.

Evaluation of Different Sex Reversal Treatments in Red Tilapia Hybrid

Effects of dietary inclusion of Tribulus terrestris and 17 α-methyltestosterone on sex reversal, growth performance, feed utilization, and survival rates of red tilapia were evaluated in this study. Red tilapia fry with an average weight of 0.02 ±0.003 g and an average length of 0.6 ±0.012 cm were subjected to six different sex reversal treatments. The experiment lasted for 112 days. Two T. terrestris extract treatments (1and 2 g/kg diet); three hormonal treatments with 17 α-methyltestosterone ( 30,60 and 100 mg/kg diet), were applied in this experiment plus a control group. The results of this study revealed that all plant and hormonal treatment showed significantly higher male percentages (p≤0.05) than the control. The highest male percentage of (90%) was achieved at the dose of 60 mg 17 α-methyltestosterone/kg followed by T. terrestris extract at 2 g/kg with a male percentage of (84.4%). Growth performance and feed utilization of red tilapia were better in the two treatments of T.terrestris extract, followed by the hormonal dose 60 mg/kg diet, than other treatments. A significantly higher survival rate of 85.44%, 83.33%, and 83.11% was achieved at 1 and 2 g T. terrestris extract/kg and the control respectively. The lowest survival (71.56%) was observed at the 100 mg 17 α -MT/ kg diet (p ≤ .05). Testes weight was significantly higher at 2 g T. terrestris extract/kg diet treatment compared with the other treatments and the control. The highest male gonado-somatic index records were found at the control and 2 g T. terrestris extract/ kg diet treatments. Conclusively, the addition of T. terrestris extract and 17 α –MT into red tilapia hybrid fry diets contributes in producing almost all-male tilapia population and enhancing growth performance and feed utilization of the fish.

Experimental validation of genetic selection for resistance against Streptococcus agalactiae via different routes of infection in the commercial Nile tilapia breeding programme.

The objective of this study was to validate the genetic selection for resistance to streptococcosis under experimental challenge conditions in a commercial population of Nile tilapia. Further, effects of using two different routes of infection of Streptococcus agalactiae; intraperitoneal injection (IP) and cohabitation with the shedder fish (cohab), on the genomic parameters, prediction accuracy and response to selection are compared. The comparison was made between two different lines of fish; one selected for S. agalactiae resistance for one generation and randomly mated for two generations (to mimic the multiplication activities occurring in distribution channels and hatcheries); and the other unselected. 1,500 fish, each from these two lines, were used for the experimental challenge test. Survival analysis using Kaplan-Meier estimators and Hazard's ratio was used to quantify differences in mortality between the two lines. Further genomic analysis was performed with 2,684 fish and 35,745 SNPs using both univariate and bivariate GBLUP models. Genetic selection for resistance to S. agalactiae led to the significant (p<.001) reduction in the risk of death by 65% in the selected line, compared to the unselected line. Similarly, the risk of death via cohabitation route of infection significantly (p<.01) decreased by 80%, compared to IP. The genetic correlation between these two routes of infection was ~0.9. Genetic selection changed the impact of the routes of infection, with the change in the distribution of estimated breeding values and the gain of 3.04±1.25days as selection response (p<.05).

Newly discovered cichlid fish biodiversity threatened by hybridization with non-native species.

Invasive freshwater fishes are known to readily hybridize with indigenous congeneric species, driving loss of unique and irreplaceable genetic resources. Here we reveal that newly discovered (2013-2016) evolutionarily significant populations of Korogwe tilapia (Oreochromis korogwe) from southern Tanzania are threatened by hybridization with the larger invasive Nile tilapia (Oreochromis niloticus). We use a combination of morphology, microsatellite allele frequencies and whole genome sequences to show that O. korogwe from southern lakes (Nambawala, Rutamba and Mitupa) are distinct from geographically disjunct populations in northern Tanzania (Zigi River and Mlingano Dam). We also provide genetic evidence of O. korogwe×niloticus hybrids in three southern lakes and demonstrate heterogeneity in the extent of admixture across the genome. Finally, using the least admixed genomic regions we estimate that the northern and southern O. korogwe populations most plausibly diverged ~140,000years ago, suggesting that the geographical separation of the northern and southern groups is not a result of a recent translocation, and instead these populations represent independent evolutionarily significant units. We conclude that these newly discovered and phenotypically unique cichlid populations are already threatened by hybridization with an invasive species, and propose that these irreplaceable genetic resources would benefit from conservation interventions.

Comparing Genomic Signatures of Selection Between the Abbassa Strain and Eight Wild Populations of Nile Tilapia (Oreochromis niloticus) in Egypt.

Domestication to captive rearing conditions, along with targeted selective breeding have genetic consequences that vary from those in wild environments. Nile tilapia (Oreochromis niloticus) is one of the most translocated and farmed aquaculture species globally, farmed throughout Asia, North and South America, and its African native range. In Egypt, a breeding program established the Abbassa Strain of Nile tilapia (AS) in 2002 based on local broodstock sourced from the Nile River. The AS has been intensively selected for growth and has gone through genetic bottlenecks which have likely shifted levels and composition of genetic diversity within the strain. Consequently, there are questions on the possible genetic impact AS escapees may have on endemic populations of Nile tilapia. However, to date there have been no genetic studies comparing genetic changes in the domesticated AS to local wild populations. This study used 9,827 genome-wide SNPs to investigate population genetic structure and signatures of selection in the AS (generations 9–11) and eight wild Nile tilapia populations from Egypt. SNP analyses identified two major genetic clusters (captive and wild populations), with wild populations showing evidence of isolation-by-distance among the Nile Delta and upstream riverine populations. Between genetic clusters, approximately 6.9% of SNPs were identified as outliers with outliers identified on all 22 O. niloticus chromosomes. A lack of localized outlier clustering on the genome suggests that no genes of major effect were presently detected. The AS has retained high levels of genetic diversity (Ho_All = 0.21 ± 0.01; He_All = 0.23 ± 0.01) when compared to wild populations (Ho_All = 0.18 ± 0.01; He_All = 0.17 ± 0.01) after 11 years of domestication and selective breeding. Additionally, 565 SNPs were unique within the AS line. While these private SNPs may be due to domestication signals or founder effects, it is suspected that introgression with blue tilapia (Oreochromis aureus) has occurred. This study highlights the importance of understanding the effects of domestication in addition to wild population structure to inform future management and dissemination decisions. Furthermore, by conducting a baseline genetic study of wild populations prior to the dissemination of a domestic line, the effects of aquaculture on these populations can be monitored over time.