Tilapia Stocks Research Articles

Unlocking Nile tilapia (Oreochromis niloticus Linn., 1758) selective breeding programmes in Uganda through geographical genetic structure mapping

AbstractNile tilapia (Oreochromis niloticus), native to Africa and the Levant, is an important species for both aquaculture and capture fisheries. Despite these attributes, Nile tilapia has been negatively impacted by anthropogenic activities ranging from overfishing and habitat destruction to translocations. These human‐mediated activities have threatened the genetic evolutionary integrity of native populations of Nile tilapia and congeneric species through admixture, demographic bottlenecks and introgressive hybridization. In this context, the genetic contrast between farmed/bred strains and wild Nile tilapia populations in the major lakes of Uganda remains understudied. Understanding the genetic structure of Nile tilapia populations in the major water bodies of Uganda (lakes Victoria, Kyoga, Edward, George, Albert and Nile River) is a key resource to guide selective breeding programmes, thus minimizing the effects of maladaptation under both aquaculture and natural stocks. Although the population genetics of Nile tilapia in Uganda's major lakes have been studied, this was based on limited sampling scope, especially in the wild, and the key information on the species could have been overlooked. We fill this knowledge gap by genotyping 756 Nile tilapia individuals from multiple populations for each major water body of Uganda using a panel of 34 microsatellite loci based on the microsatellite genotyping‐by‐sequencing (SSR‐GBS) technique. The results indicate two discrete gene pools/stocks: the Edward‐George system and the Albert‐Kyoga system‐Victoria. Evidence of loss of genetic diversity and admixture of some Nile tilapia stocks was found, most likely resulting from anthropogenic perturbations. This study contributes useful information key for understanding the potential Nile tilapia broodstock sources for selective breeding programmes, aimed at improving aquaculture production in Uganda.

Production of fertile YY pseudofemales in Nile tilapia with LG23 as the sex chromosome

Nile tilapia display conspicuous differences in growth rates between genders, as males tend to outpace and outgrow females. The production of all male Nile tilapia populations using genetically male tilapia (GMT) technology represents the most efficient and scientifically grounded approach. Most of the Nile tilapia stocks in aquaculture use LG23 as sex chromosome. The crux of GMT lies in the acquisition of YY supermales and YY pseudofemales (pF). Nonetheless, the utilization of GMT is impeded by the lower fertility and even infertility of feminized YY-pF. In this study, we found that YY-pF exhibited male-like genital papillae, and exhibited significantly higher androgen levels and lower E2 levels compared with that of fertile XY-pF. YY-pF carries two copies of Amhy but lacks Amh, and XY-pF possesses one Amhy and one Amh. In XY-pF, the expression of Amh/Amhy was only observed in granulosa cells of phase I and II follicles, while in YY-pF, Amhy expression persisted from phase I to IV. On the contrary, Cyp19a1a was found to be highly expressed in granulosa cells of phase III and IV follicles in XY-pF, but significantly reduced in granulosa cells of phase III and IV follicles in YY-pF, exhibiting a reciprocal expression pattern with Amhy. Consistently, YY-pF with one copy of Amhy mutation (YY-Amhy+/−-pF) were fertile. These results indicated that prolonged expression of Amhy led to a decrease in the expression of Cyp19a1a, followed by a decrease in serum E2 levels and an increase in androgen levels. Based on these data, we treated YY-pF with both E2 and trilostane (androgen synthetase inhibitors) simultaneously at the vitellogenic stage to make it fertile. The acquisition of fertile YY-pF laid the foundation for the widespread application of GMT.

Morphometric Variations of Nile Tilapia (Oreochromis niloticus) (Linnaeus, 1758) Local Strains Collected from Different Fish Farms in South Western Highland Agro-Ecological Zone (SWHAEZ), Uganda: Screening Strains for Aquaculture

Despite the global contribution of aquaculture as an important pillar for food and nutritional security, its rapid growth has been hampered by inadequate and poor-quality seed due to inbreeding, hybridization of related stocks, and poor-quality broodstock. Phenotypic and genotypic characterization of Nile tilapia in natural water bodies of Uganda has been studied but research gaps remain on the morphometric characterization of farmed O. niloticus in Uganda, which hinders selective breeding initiatives. This study characterized O. niloticus from various farmers in Uganda’s Southwestern Highland Agro-Ecological Zone using their morphometric traits. In this study, 8 morphometric traits were taken on each of the 258 fish samples collected with at least 30 samples sampled from each pond system in the zone. The morphometric data generated were analyzed using multivariate analysis. In order to delineate the populations from the different ponds, Discriminant Function analysis (DFA) was performed on the dataset and distribution plots generated to test on the purity of the strains. There were significant differences in the morphometric traits among the populations with the Victoria fish population having the highest standard length while the Kyoga population had the highest weight. Morphometric characterization grouped the fish into three clusters, indicative that fish farms in the zone possibly had different sources of fish seed. The condition factor, which is a measure of wellbeing of the fish, showed that all the populations displayed an isometric growth which is indicative of the fish growing in length as it is in weight. The results of this study provide evidence that local O. niloticus from different farms display differences in body shape as a result of using fish seed from different sources. This aspect can be applied in selective breeding programs after establishing the genetic structure of the farmed Nile tilapia stock.

Population dynamics of the Nile tilapia (Oreochromis niloticus L. 1758) stock in Lake Langeno, Ethiopia

Nile tilapia, Oreochromis niloticus, is a widely distributed and economically most import fish species in Ethiopia. Effective management is essential to sustain their fisheries and the benefits for the local communities. However, little is known about the population dynamics of the Nile tilapia stock in Lake Langeno. Vital parameters of its population were determined using length frequency data collected from 5,949 specimens ranging from 8.5 to 35.7 cm total length (tl). These specimens were collected from August 2017to July 2018. The population parameters were determined using the elefan I routine in FiSAT software. Estimated von Bertalanffy growth parameters were asymptotic length (L∞) = 35.70 cm, growth curvature (K) = 0.32 yr-1, age at length zero (to) = -0.49, and growth performance index (Փ') = 2.61. Instantaneous total mortality (Z), natural mortality (M), and fishing mortality (F) rates were determined from length-converted catch curve and empirical models, respectively, and their values obtained were Z = 2.31 yr-1, M = 0.82 and F =1.56 yr-1, respectively. The exploitation rate (E) of Nile tilapia computed from its mortality rates was 0.67 yr-1, suggesting state of overfishing. The size at first capture (Lc) was estimated at 14.0 cm which is much lower than the size at first maturity (Lm=16.62 cm), which further substantiated the state of overfishing. The results of the study are very useful for fishery managers and scientists who wish to manage and further explore the Nile tilapia stock in Lake Langeno.

Mixed-sex Nile tilapia (Oreochromis niloticus) can perform competitively with mono-sex stocks in cage production

All-male tilapia stocks are widely used by farmers to supply both domestic and international markets with homogenous, large sized fish (500 g+). While a number of strategies are possible, hormonal treatment of fry with 17α-methyltestosterone (MT) is the most common method used by commercial farmers due to its low cost and ease of application. However, contrasting to its current widespread use, the implications of MT in tilapia farming have raised concerns especially from public and environmental perspectives. Therefore, in this study we tested the impact of stocking a mixed-sex fast growing strain of Nile tilapia (Oreochromis niloticus) fry at high density and then grading out females at 4 or 8 weeks intervals during grow-out and compared final production with a standard MT-treated mono-sex system. From a production perspective, the strategies to remove females at 4 or 8 weeks were successful as no differences in harvest weight, survival and feed conversion rate were observed when compared to the MT-treated group. Similarly, no differences at harvest were obtained in terms of external appearance, Fulton's condition factor, gonadosomatic index, fillet yield, fat-somatic index and visceral-somatic index (%) between MT-monosex group and the groups where females were removed (4 or 8 weeks). However, a financial analysis of this approach showed that the additional costs (fry, feed and labour) involved in the mixed-sex strategy resulted in lower profits. This could be mitigated if a proportion of the removed females could be sold at a premium price as potential broodstock. In the model presented, sales of 13% of the removed females for broodfish at current Thai prices, or a premium of at least 8% for non-sex-reversed final product would be sufficient for the mixed-sex system to return a higher profit than the mono-sex system. The latter strategy could also enable further social licence through use of small fish in nutritional and outgrower initiatives.

Mosaic or melting pot: The use of monogeneans as a biological tag and magnifying glass to discriminate introduced populations of Nile tilapia in sub-Saharan Africa

The origin of introduced Nile tilapia stocks in sub-Saharan Africa is largely unknown. In this study, the potential of monogeneans as a biological tag and magnifying glass is tested to reveal their hosts' stocking history. The monogenean gill community of different Nile tilapia populations in sub-Saharan Africa was explored, and a phylogeographic analysis was performed based on the mitogenomes of four dactylogyrid species (Cichlidogyrus halli, C. sclerosus, C. thurstonae, and Scutogyrus longicornis). Our results encourage the use of dactylogyrids as biological tags. The magnifying glass hypothesis is only confirmed for C. thurstonae, highlighting the importance of the absence of other potential hosts as prerequisites for a parasite to act as a magnifying glass. With the data generated here, we are the first to extract mitogenomes from individual monogeneans and to perform an upscaled survey of the comparative phylogeography of several monogenean species with unprecedented diagnostic resolution.

Identification of Nile tilapia (Oreochromis niloticus) and its hybrids in natural environments in Hawaii

The history of tilapia in Hawaii began in the 1950s with the introduction of five species for recreational, research, and commercial purposes (Hida et al., 1962; Szyper et al., 2000). In 2000, the existing tilapia stocks were evaluated for their utility in establishing a tilapia producing industry (Szyper et al., 2000). The study showed that the stocks were inferior in terms of growth when compared to Nile tilapia (Oreochromis niloticus), a species that could not be imported for private and commercial use (Szyper et al., 2000). Supporters of the species' entry into Hawaii argued that it was already established in natural bodies of water, and its introduction would have little effect on the environment. Tilapia were collected from 10 sites throughout the state of Hawaii to determine if O. niloticus is indeed established in the wild. Total genomic DNA was extracted from fin clips. Potential species identification was based on the sequencing of the mitochondrial DNA control region (mtDNA CR). Unique sequences were compared with those published in GenBank. Nile tilapia was found on the island of Hawaii, and its potential hybrids were identified on the islands of Molokai, Maui, as well as Hawaii.

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.

Stock status of Nile tilapia (Oreochromis niloticus) in Aneuk Laot Lake, Sabang District, Aceh Province, Indonesia

Abstract. Prianto E, Purwoko RM, Kasim K. 2021. Stock status of Nile tilapia (Oreochromis niloticus) in Aneuk Laot Lake, Sabang District, Aceh Province, Indonesia. Biodiversitas 22: 3364-3370. The Nile tilapia (Oreochromis niloticus) fishery in Aneuk Laot Lake plays a vital role in the local livelihoods. There is an indication of the massive exploitation of the species that potentially harm the stock and threaten its sustainability. This research aims to evaluate and assess the Nile Tilapia's stock status and use the results as baseline information to the management authorities. Surveys were conducted from May 2018 to April 2019 by collecting the length-weight data, sex, and gonadal maturity stage of 600 individuals. The Length-Based Spawning Potential Ratio (LB-SPR) was used to determine the Spawning Potential Ratio (SPR) and compared the results to the reference point. The results show that 50% of the females have matured at 15.1 cm, while the males at 17.8 cm. The individual fish length varies from 12.15 to 20.5 cm, whereas 50% of them were caught at size 17.8 cm (Lc50), meaning that predominantly of the fish had experienced first spawning before they were captured. The current SPR is about 5%, confirming the Nile tilapia's stock status is at an unhealthy state. Consequently, the precautionary approach, such as increasing the mesh size and limiting effort, should be considered to ensure the sustainability of the recruitment.

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.

Cold tolerance and performance of selected Nile tilapia for suboptimal temperatures

This study evaluated the performance and cold tolerance of different Nile tilapia stocks, selected at suboptimal temperature and identified alleles of microsatellite markers associated with cold tolerance. After 67 days of rearing at average temperature of 22.4°C, tilapia juveniles from different stocks were evaluated, tagged with a transponder and acclimated to 16°C. Subsequently, the temperature was reduced 1°C per day until 11°C and from that temperature decreases of 0.5°C per day were made until total mortality. A caudal fin sample was collected, and their genetic profile was characterized through the microsatellite markers UNH879, UNH916 and UNH999. Tilapias of stocks S3, S6 and S7 showed higher growth at suboptimal temperatures, 42 to 47% higher than base population. In the cold tolerance assay, tilapias from stocks S5 and S7 showed lower mean death temperature. Through molecular analysis, it is stated that allele 186 bp of UNH879 and 147 bp of UNH916 were more frequently found in animals cold-tolerant. Artificial selection for final weight during winter in southern Brazil can assist in the selection of tilapias improved to rearing in suboptimal temperatures. Finally, using microsatellite markers, it is possible to direct selective breeding to obtain tilapias more adapted to low temperatures.

Diet overlap of non‐native common carp and native Nile tilapia: A potential cause for population reduction of Nile tilapia stock in Lake Ziway, Ethiopia

AbstractLake Ziway harbours indigenous and exotic fish species including Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio). Nile tilapia was the dominant and preferred fish. However, its contribution to total catch has dramatically declined from 89.3% in 1994 to 27% in 2014 while the introduced common carp has increased from 0% before 2012 to 25% in 2014. Common carp potentially compete with Nile tilapia for available resources and could be a cause for the decline. Thus, the study explored the dietary overlap of the two species from April to August 2017. Schoener's overlap index (α) revealed significant dietary overlap between the two species (α = 0.84, between juveniles and α = 0.63, between adults). Juveniles fed mainly on animal origin (zooplankton and insects), while adult Nile tilapia have consumed plant origin (macrophyte and phytoplankton), and adult common carp fed on macrophytes and detritus. The presence of significant dietary overlap between the two species, particularly due to intense competition among juveniles, might cause the reduction of Nile tilapia stock because the native species has shown a competitive disadvantage for food in the presence of common carp. This study provides baseline information to researchers and decision makers working towards the sustainable resource utilisation of the system.

Evidence of two XX/XY sex-determining loci in the Stirling stock of Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) has been the subject of much research on sex determination due to the problems caused by maturation and reproduction in culture before harvest, and because of the emerging complexity of sex determination in this species. Analysis of the stock held at the University of Stirling, derived directly from Lake Manzala in Egypt about 41 years ago, detected an XX/XY locus in linkage group (LG) 1, and then a locus in LG20 that affected sex-ratio in some families, and was associated with the effects of elevated temperature on sex-ratio. Another XX/XY locus in LG23 with the candidate male-specific Amh gene was found recently to control sex determination in a Nile tilapia stock held at the Agricultural Research Organization (ARO) in Israel which was derived from a stock held at the University of Swansea, which was in turn derived from the Stirling stock. Extensive screening of the Stirling Nile tilapia stock for a marker in this male-specific Amh variant (Y-linked deletion in Amh exon VII) showed that this was present at a low frequency (7 out of 51 male broodstock, 13.73%) and associated with male phenotypic sex. It seems most likely that the dominance of the LG23 locus in sex determination in the ARO stock came about through founder effects associated with serial transfer. The presence of the previously undetected LG23 sex-determining locus may have led to some confusion in earlier attempts to develop YY males in the Stirling stock. Increased knowledge of sex determination in this species should aid marker-assisted selection (MAS) to produce all male fish to prevent unwanted reproduction in tilapia aquaculture, although the presence of multiple sex-determining loci in single stocks is likely to complicate such efforts.

Genetic characterization of selected Nile tilapia in Santa Catarina

Different Nile tilapia stocks belonging to the fish breeding program of the Epagri (Empresa de Pesquisa Agropecuaria e Extensao Rural de Santa Catarina) were characterized by microsatellite markers. A total of nine stocks (S1 to S9) were evaluated, and for each stock the caudal fin of 30 individuals were sampled. A total of 75 alleles were found at the 11 microsatellite loci used (UNH104, UNH108, UNH160, UNH208, UNH222, UNH848, UNH879, UNH898, UNH952, UNH998). Among the loci used, only UHN160 showed significance for null alleles in stocks S1, S2, S3 and S5. The average number of alleles per loci was 6.8, while the average number of alleles per tilapia stock was 4.4. Five unique alleles were identified between the stock S1 and S5. The observed heterozygosity values (Ho) exceeded the expected heterozygosity (He), resulting in a negative inbreeding coefficient (FIS = -0.092). FST for the total population was 0.109, demonstrating moderate genetic differentiation between the stocks. According to the Euclidean distance, three groups were formed as follows: I - S6, S7 and S9; II - S2, S3 and S4; and III - S1, S5 and S8. However, the existence of two groups can be observed from the PCoA representation: I - S6, S7, S8 and S9; and II - S1, S2, S3, S4 and S5. The formation of these two genetic groups is consistent with the genealogy of stocks. The formation of group III (S1, S5 and S8) in the dendrogram can be explained by the higher average observed heterozygosity values of these stocks. Bayesian analysis revealed the formation of 16 groups with an FST value of 0.2107. This result reinforces the existence of variability existing in the Epagri breeding program, from which it is possible to form heterotic groups to enable the direction of potential crosses to obtain genetic gain. The study enabled genotypic characterization of the tilapia brood stock used in the Epagri breeding program, determining the genetic distance between the stocks, which will enable more accurate selection of individuals for mating for the next generation. It was possible to verify that there is high heterozygosity within the stocks, and moderate genetic differentiation between the stocks. Furthermore, all evaluated markers were polymorphic for this brood stock and will be used to characterize the next generations.

Sex determination in the GIFT strain of tilapia is controlled by a locus in linkage group 23

BackgroundTilapias (Family Cichlidae) are the second most important group of aquaculture species in the world. They have been the subject of much research on sex determination due to problems caused by early maturation in culture and their complex sex-determining systems. Different sex-determining loci (linkage group 1, 20 and 23) have been detected in various tilapia stocks. The ‘genetically improved farmed tilapia’ (GIFT) stock, founded from multiple Nile tilapia (Oreochromis niloticus) populations, with some likely to have been introgressed with O. mossambicus, is a key resource for tilapia aquaculture. The sex-determining mechanism in the GIFT stock was unknown, but potentially complicated due to its multiple origins.ResultsA bulk segregant analysis (BSA) version of double-digest restriction-site associated DNA sequencing (BSA-ddRADseq) was developed and used to detect and position sex-linked single nucleotide polymorphism (SNP) markers in 19 families from the GIFT strain breeding nucleus and two Stirling families as controls (a single XY locus had been previously mapped to LG1 in the latter). About 1500 SNPs per family were detected across the genome. Phenotypic sex in Stirling families showed strong association with LG1, whereas only SNPs located in LG23 showed clear association with sex in the majority of the GIFT families. No other genomic regions linked to sex determination were apparent. This region was validated using a series of LG23-specific DNA markers (five SNPs with highest association to sex from this study, the LG23 sex-associated microsatellite UNH898 and ARO172, and the recently isolated amhy marker for individual fish (n = 284).ConclusionsPerhaps surprisingly given its multiple origins, sex determination in the GIFT strain breeding nucleus was associated only with a locus in LG23. BSA-ddRADseq allowed cost-effective analysis of multiple families, strengthening this conclusion. This technique has potential to be applied to other complex traits. The sex-linked SNP markers identified will be useful for potential marker-assisted selection (MAS) to control sex-ratio in GIFT tilapia to suppress unwanted reproduction during growout.

Genome-wide association study and genomic prediction for resistance against Streptococcus agalactiae in hybrid red tilapia (Oreochromis spp.)

Streptococcosis is a major disease that causes severe mortality in tilapia aquaculture worldwide. Although the conventional BLUP family selection to enhance disease resistance in a commercial red tilapia stock was successful, the response was low due to the low heritability of the traits. An alternative strategy is the utilization of genomic information to identify the best performing candidates within families. In this study, we performed genome-wide association studies for red tilapia resistance to Streptococcus agalactiae using 11,480 SNPs within 110 families represented by 1020 fish. Nineteen SNP markers were found to explain ~10% of the genetic variation. We compared the accuracies of genomic prediction using the pedigree-based (PBLUP), marker-based (GBLUP), and Bayesian models. The prediction accuracy was assessed by performing ten replicates of five-fold cross-validation. In each replicate, approximately 80% of the data (n~816) were sampled for the training set and the remaining data (n~204) were used for the validation. The BayesB model yielded the highest accuracies (0.31 and 0.20) followed by GBLUP (0.25 and 0.15) and PBLUP (0.15 and 0.06) for days to death and binary trait.

Catch distribution and size structure of Nile tilapia (Oreochromis niloticus) in Lake Tana, Ethiopia: implications for fisheries management

Nile tilapia (Oreochromis niloticus) is the most important species for consumption and economic purposes in Lake Tana, yet, tilapia stocks have declined drastically over the past decades because of anthropogenic impacts. Thus, effective conservation management is required urgently. The aim of this study was to investigate the spatio-temporal distribution and size structure of O. niloticus populations in Lake Tana in order to evaluation conditions to promote sustainable exploitation. Data were collected monthly at three sites on the lake from April 2016 to March 2017. The size at first maturity was computed using logistic regression. The catch per unit effort (CPUE) of O. niloticus in Lake Tana has sharply declined from 75 kg trip−1 in 2001 to 24 kg trip−1 in 2016/2017. The mean CPUE values between sites and seasons were significantly different. The size at first maturity for O. niloticus was 21 cm in 2016/2017. Of the catches taken, more than 24% of those were motorised and reed boat fisheries and 46% of the chase-and-trap fishery catches were below the size at first maturity in 2016/2017. Additionally, large size specimens were rarely recorded. Widening the mesh size of the gillnets would give a chance for immature fish to spawn at least once. Considering the fish size distribution and social context of the fishers, limiting mesh size could be a more feasible management option than a closed season.

Aquaculture-Mediated Invasion of the Genetically Improved Farmed Tilapia (Gift) into the Lower Volta Basin of Ghana

The need for improved aquaculture productivity has led to widespread pressure to introduce the Genetically Improved Farmed Tilapia (GIFT) strains of Nile tilapia (Oreochromis niloticus) into Africa. However, the physical and regulatory infrastructures for preventing the escape of farmed stocks into wild populations and ecosystems are generally lacking. This study characterized the genetic background of O. niloticus being farmed in Ghana and assessed the genetic effects of aquaculture on wild populations. We characterized O. niloticus collected in 2017 using mitochondrial and microsatellite DNA markers from 140 farmed individuals sampled from five major aquaculture facilities on the Volta Lake, and from 72 individuals sampled from the wild in the Lower Volta River downstream of the lake and the Black Volta tributary upstream of the lake. Our results revealed that two farms were culturing non-native O. niloticus stocks, which were distinct from the native Akosombo strain. The non-native tilapia stocks were identical to several GIFT strains, some of which showed introgression of mitochondrial DNA from non-native Oreochromis mossambicus. We also found that the non-native cultured tilapias have escaped into the wild and interbred with local populations, and also observed potentially admixed individuals on some farms. Our results highlight aquaculture as a vector in the spread of invasive non-native species and strains, and underscore the importance of genetic baseline studies to guide conservation planning for wild populations.

Study on the Evaluation of a High Speed Tagging Device for Nile Tilapia Stock Assessment in the Vunania Dam in Kassena-Nankana District, Upper East Region

Study on the Evaluation of a High Speed Tagging Device for Nile Tilapia Stock Assessment in the Vunania Dam in Kassena-Nankana District, Upper East Region

Prediction of bio-economically sustainable yield and optimum fishing effort for the Nile <i>tilapia</i> (<i>Oreochromis niloticus</i> L.) fishery of Lake Hawassa, Ethiopia

The Nile tilapia ( Oreochromis niloticus , L. 1758) stock of Lake Hawassa, Ethiopia, was assessed to estimate bio-economically sustainable yield (MSY and MEY) and optimum fishing effort (f opt ) using length-based analytical models (Jone’s Cohort Analysis and Thompson and Bell). Pertinent data (length, weight, catch, effort, etc.) were collected on a daily basis between May 2012 and June 2013 from the two landing sites, where fishermen at the northern and the southern part of the lake land their catch for retail. The assessment revealed an estimated current yield of 240.23 t/year and 216.33 t/year for the southern and northern part of the fishery of the lake, respectively. The predicted biologically maximum sustainable yield (MSY) was 240.2 t/year and 218.7 t/year for the southern and northern fishery, respectively, which were obtained at an F-factor of 1 and 1.5, respectively. On the other hand, the maximum economic return was generated from the southern and northern fishery at an F- factor of 0.8 and 1.4, respectively. Therefore, the level of effort at the time of sampling (F-current) should be reduced by 20 % for the southern fishery while the current effort of the northern fishery can be expanded by 40% in order to obtain the maximum economically sustainable yield. Accordingly, the optimum level of fishing effort (f opt ) to be exerted at the lake level was estimated at about 650 nets per day in order to exploit O. niloticus stock of Lake Hawassa sustainably. Keywords: Analytical yield prediction models, Lake Hawassa, MEY, MSY, optimum fishing effort , O. niloticus , stock assessment