Male Nile Tilapia Research Articles

Lipid utilization, protein sparing effects and protein requirement of all male Nile tilapia (Oreochromis niloticus Linnaeus, 1758) in underground brackish water

Nile tilapia Oreochromis niloticus as a species with high growth rate and high tolerate to environmental conditions and stocking density, have been culturing in the world more and more. Because of feed expenses necessity and better growth rate and feed conversion efficiency in brackish water, despite of existing many studies on protein requirements, lipid utilization and protein sparing effect of Nile tilapia, they were studied on all males in brackish water, again. A 4 × 3 factorial design with three replicates was planned based on the proximate and amino acids analysis of protein sources to contain four crude protein (15, 22, 29 and 36 %) and three crude lipid (5, 9 and 13%) levels. Growth indices including weight gain and specific growth rate improved by protein accretion and reduced by lipid increasing. The best growth performance was obtained in 5% crude lipid. Feed conversion ratio reduced by protein increasing, and the best one observed in the diet containing 36% crude protein. Protein efficiency ratio and Protein conversion efficiency increased by protein accretion up to 29% crude protein. Protein efficiency, except 15% crude protein, increased up to 9% crude lipid and then reduced but not significantly; so, protein-sparing effect appeared, unclearly. In this study, at 25.6% crude protein with 265 mg protein per gram body weight, fish can keep their carcass protein percentage. In economic point of view, based on growth, feed and protein performances, the levels of 29% crude protein and 5% crude lipid are sufficient for convincing growth rate. The results show that isotonic environment cannot Change protein and lipid requirements compared to the other studies in the fresh water.

Single-Cell Gene Profiling Reveals Social Status-Dependent Modulation of Nuclear Hormone Receptors in GnRH Neurons in a Male Cichlid Fish.

Gonadotropin-releasing hormone (GnRH) is essential for the initiation and maintenance of reproductive functions in vertebrates. To date, three distinct paralogue lineages, GnRH1, GnRH2, and GnRH3, have been identified with different functions and regulatory mechanisms. Among them, hypothalamic GnRH1 neurons are classically known as the hypophysiotropic form that is regulated by estrogen feedback. However, the mechanism of action underlying the estrogen-dependent regulation of GnRH1 has been debated, mainly due to the coexpression of low levels of estrogen receptor (ER) genes. In addition, the role of sex steroids in the modulation of GnRH2 and GnRH3 neurons has not been fully elucidated. Using single-cell real-time PCR, we revealed the expression of genes for estrogen, androgen, glucocorticoid, thyroid, and xenobiotic receptors in GnRH1, GnRH2, and GnRH3 neurons in the male Nile tilapia Oreochromis niloticus. We further quantified expression levels of estrogen receptor genes (ERα, ERβ, and ERγ) in three GnRH neuron types in male tilapia of two different social statuses (dominant and subordinate) at the single cell level. In dominant males, GnRH1 mRNA levels were positively proportional to ERγ mRNA levels, while in subordinate males, GnRH2 mRNA levels were positively proportional to ERβ mRNA levels. These results indicate that variations in the expression of nuclear receptors (and possibly steroid sensitivities) among individual GnRH cells may facilitate different physiological processes, such as the promotion of reproductive activities through GnRH1 neurons, and the inhibition of feeding and sexual behaviors through GnRH2 neurons.

Use of essential oils for Nile tipalia breeders during breeding season

The present study aimed to evaluate the dietary supplementation of a commercial Saluto® compound containing essential oils extract of cinnamon (Cinnamomum zeylanicum), oregano (Origanum vulgare), rosemary (Rosmarinus officinalis) and pepper (Capsicum sp.) on breeding performance of female and male Nile tilapia (Oreochromis niloticus). The experiment had a completely randomized design with five treatments (0, 50, 100, 150, and 200 ppm of essential oil supplementation) and four repetitions during the breeding season of Nile tilapia. The apparent feed conversion, daily weight gain, condition factor, hepatosomatic index, gonadosomatic index, egg diameter, relative fecundity, egg weight, larvae yield, hatching rate, blood calcium and blood glucose levels were analyzed. In the correlations of variables at the female reproductive peak no statistical differences were found for the reproductive parameters and quality of the larvae. The nutritional additive inclusion did not impair reproductive parameters during the 120 days of evaluation, and inclusions above 50 ppm promoted an increase in the hepatosomatic index of Nile tilapia females during the reproductive season.

Growth performance and flesh quality of tilapia (Oreochromis niloticus) fed low concentrations of Rubrivivax gelatinosus, Saccharomyces cerevisiae and Spirulina platensis

Saccharomyces cerevisiae biomass generated as a by-product in industrial processes and other biomasses grown in industry waste waters like Spirulina platensis and Rubrivivax gelatinous can be used in aquafeed, thus reducing the costs of discard and minimizing environmental damage. In this study, 840 male Nile tilapias (Oreochromis niloticus; 26.8 ± 1.03 g average weight) were distributed among 21 tanks (40 fish/tank, 1.07 kg/m3). Fish were fed to satiation three times a day throughout the trial (72 days). A completely randomized design experiment with 7 treatments (control group and 6 different diets, 3 replicates per treatment) was conducted to investigate the effects of these three microbial biomasses (added at 0.25 and 0.5% into an isoproteic/isoenergetic tilapia diet) on the growth and visceral indexes of the fish and on the composition and texture of the fillets. Feed conversion ratio was significantly lower in fish fed 0.5% R. gelatinosus than that in the control group, but no difference was detected among the treatments either for the other growth indicators or for the organs’ indexes. Including microbial biomass in the diet did not significantly affect either fillet pH or texture parameters. However, the use of microbial biomass increased the protein content and decreased the n-6/n-3 ratio of the fillets compared with that of the control group. So, it was concluded that the dietary intake of low concentrations of S. cerevisiae, S. platensis and R. gelatinosus biomasses did not affect the growth performance of tilapias, increased the nutritional quality and preserved the texture features of the fillets.

Effect of exposure to palm oil mill effluent on reproductive impairment of male Nile Tilapia (Oreochromis niloticus, Linnaeus 1758)

One of the negative impacts arising from the existence of palm oil mill industries is the increase of pollution from Palm Oil Mill Effluent (POME), particularly for the aquatic environment. This study was conducted to investigate the reproductive impairment of male Nile tilapia (Oreochromis niloticus Linnaeus 1758) after exposed to POME. An experiment of a chronic test was carried out using a completely randomized design consisting four treatments with five replication: control (0 mg/L POME), treatment A (1.565 mg/L POME), treatment B (2.347 mg/L POME), and treatment C (3.130 mg/L POME). The exposure of POME was performed for 45 days. Reproductive hormone concentration, gonadosomatic index and spermatocrite value in each treatment statistically analyzed by using one-way analysis of variance (ANOVA). Results showed that the POME exposure had no significant impact on estradiol and testosterone concentrations (p>0.05), but, it triggered a decline in the progesterone concentration (p<0.05). Progesterone concentration decreased significantly from 0.57 ± 0.24 ng/mL in control to 0.28 ± 0.04 ng/mL in treatment C. The hormonal decline resulted in a significant decrease in gonadosomatic index and spermatocrite of male Nile Tilapi (p<0.05). In conclusion, exposure to POME induced reproductive impairment in male Nile Tilapia.

Effect of the digestible protein-energy relationship on the growth performance of Nile tilapia (Oreochromis niloticus) fed fishmeal-free diets

This study aimed to determine the best digestible protein: digestible energy (DP:DE) ratio for adult Nile tilapia (Oreochromis niloticus) fed diets formulated with vegetable ingredients (soybean and corn), and without fishmeal. Four hundred and twenty male Nile tilapia (GIFT strain) (initial mean weight 193 ± 12.6 g) were allocated to 60 plastic tanks (1000 L) connected to a water recirculation system, and there were 7 fish in each tank. The experimental design was a 5 × 4 factorial with three replicates for each diet treatment. The fish were fed one of 20 diets containing a combination of five digestible protein levels (140, 190, 240, 290 and 340 g/kg DP) and four digestible energy levels (10.89, 11.72, 12.56 and 13.40 MJ/kg DE) for 96 days. The fish were given three meals each day (8 AM, 1 PM and 6 PM) and feeding was to apparent satiation. At the end of the experimental period, diets presented an interactive effect (P < 0.05) in relation to the productive performance. Final weight (FW = 448.82 g), fillet yield (FY = 32.60 %) and specific growth ratio (SGR = 2.65 %) were increased and a decrease in feed conversion ratio (FCR = 1.20) was observed in diet containing 240 g/kg DP and 12.56 MJ/kg DE. Otherwise, the highest protein efficiency ratio (P < 0.05) was found in diet containing 140 g/kg DP and 12.56 MJ/kg DE. FW, WG, FY and SGR were increased significantly (P < 0.05) up to the 12.56 MJ/kg DE level, independent of digestible protein levels. A quadratic effect was observed for FW, WG and SGR, independent of energy levels. An increase in these variables was observed up to 250.78, 240.70 and 216.70 g/kg DP respectively, and the lowest FCR was 260.58 g/kg DP. A linear increase was observed for FY (P < 0.05) in diets containing higher DP levels. No interaction (P > 0.05) was observed in any treatment for fillet centesimal composition and whole body. However, an increase (P < 0.05) in crude protein of fillet and body moisture was observed in diets containing levels above 240 g/kg DP, independent of DE. The present findings indicate that the optimal DP:DE level was 240 g/kg DP and 12.56 MJ/kg DE in the fishmeal-free diets for males Nile tilapia, promoting adequate growth performance without affecting the proximal tissue composition.

Effect of different levels of citric acid as supplementation on growth performance, feed utilization, body composition, water quality, and blood profile of Monosex Male Nile Tilapia (Oreocheromis niloticus) fingerlings

Effect of different levels of citric acid as supplementation on growth performance, feed utilization, body composition, water quality, and blood profile of Monosex Male Nile Tilapia (Oreocheromis niloticus) fingerlings

A Preliminary Survey of Estrogenic Effects in Cultured Adult Male Nile Tilapia (Oreochromis niloticus) in the Seven Lakes of San Pablo City, Philippines

This study aimed to investigate the impact of estrogenic endocrine disruptors in adult male Nile Tilapia (Oreochromis niloticus) from the Seven Lakes (Bunot, Calibato, Mohicap, Palakpakin, Pandin, Sampaloc and Yambo) of San Pablo City in Laguna, Philippines. Fish were collected from aquaculture operations in each lake from July to September 2017 and examined for plasma vitellogenin (VTG) levels, hepatosomatic index (HSI) and gonadosomatic index (GSI). In comparison to the reference group (Pandin Lake), fish from Mohicap, Sampaloc and Yambo had higher levels of plasma VTG. HSI was increased in fish from Bunot Lake while it was markedly reduced in fish from Sampaloc Lake. Comparable GSI were obtained in all samples. While most examined fish are in active spermatogenesis, fish from Sampaloc Lake had immature gonads with no spermatozoa, and with minimal to moderate Leydig cell vacuolation and hyperplasia. The results suggest that cultured Nile tilapia from Sampaloc Lake is most adversely affected by endocrine-active compounds. The identification and quantification of these chemicals are recommended in future studies.

Temperature-induced testicular germ cell loss and recovery in Nile tilapia Oreochromis niloticus

Water temperature is a critical external factor influencing gonadal development in fish. This research aimed to study the impact of elevated temperature on testicular germ cell survival and reproductive capacity of Nile tilapia. Male Nile tilapia were exposed to high temperatures of either 36 (HT1) or 37 °C (HT2) for 3000 degree-days (DD) and thereafter returned to the control temperature of 27 °C (CT) for 2200 DD. The deleterious effects on testicular germ and somatic cells were observed histologically, characterised by vacuolisation, atrophy and the loss of spermatogenic cells in testes with a more severe impact of HT2 compared to HT1. Interestingly, serum 11-ketotestosterone (11-KT) and testosterone (T) levels tended to be higher during the heat treatments than CT. Expression levels of germline-specific genes piwil1, piwil2 and nanos2 and Bcl-2 family genes, bcl-xLb and baxa were significantly reduced during the heat treatment compared to CT, more so in the HT2, while the levels of nanos3 and gfra1 transcripts were only significantly reduced in HT2, implying a significant loss of spermatogonial stem cell (SSC) and spermatogonia in HT2. The effect of HT2 is further evidenced by the significantly reduced sperm density and fertilisation rate compared to CT and HT1 at the end of the recovery period but complete sterility was not induced by HT2. Overall, the present study showed significant effects of HT2 on germ cell survival with histological changes in testes, reduced milt quality, increased 11-KT, and decreased expression of germline-specific genes, SSC marker genes and Bcl-2 family genes in testes which could therefore be potential target genes for sterilisation by genome editing.

All male nile tilapia Oreochromis niloticus fry nursing in net cages at haor of Sunamganj, Bangladesh

Nile tilapia (Oreochromisniloticus) of average 2.08 ± 0.06 to 2.14 ± 0.19 g fry nursed in nine-floating net-cages of 3 × 3 × 1.5 m in highstocking densitiesof 600, 800 and 1000 fry m–3 densities in triplicate. The fish were hand-fed a commercial tilapia floating feed at a rate of 5 to 30% of their body weight thrice daily. The final mean weight of tilapia ranged 15.13 ± 0.28 to 24.10 ± 0.35 g with the mean daily weight gain of 0.21 ± 0.11 to 0.38 ± 0.05 g/day. The low stocking density of 600 fry m–3 had significant higher mean weight, daily growth rate and specific growth rate than did the higher density treatment of 1000 fry m–3.The food conversion ratio were (1.46 to 2.30) significantly affected by stocking density (p&lt;0.05). Moreover, the net profit (BDT 1262 ± 85 - 2152 ± 55) showeda significant difference with increasing stocking density (p &lt; 0.05). The growth of nursing fry was found to be density dependent. However, survival rates were found high in all treatments and were not affected by density. But, higher gross yield was observed at the highest density. The results of this study has clearly demonstrated that the higher density (800and1000 fry m–3) could be more profitable for tilapia cages than the lower density (600fry m–3) in terms of cost effectiveness and to meet the demand for large scale fish production.&#x0D; Bangladesh J. Zool. 47(1): 149-158, 2019

Nonylphenol ethoxylate (NPE) influence on reproductive performance of male Nile tilapia, Oreochromis niloticus

Nonylphenol ethoxylate (NPE) influence on reproductive performance of male Nile tilapia, Oreochromis niloticus

Effects of methylphenidate on the aggressive behavior, serotonin and dopamine levels, and dopamine-related gene transcription in brain of male Nile tilapia (Oreochromis niloticus).

The occurrence of pharmaceuticals in the aquatic environment has increased considerably in the last decades, causing negative biochemical, physiological, and behavioral effects in aquatic organisms. In this study, we evaluated the effects of methylphenidate (MPH) on the aggressive behavior, dopamine-related gene transcript levels, monoamine levels, and carboxylesterase transcript levels and activity in the brain of male Nile tilapia (Oreochromis niloticus). Carboxylesterase activity was also measured in the liver and gills. Fish were exposed for 5days to MPH at 20 and 100ngL-1. Fish exposed to 100ngL-1 of MPH showed increased aggressiveness and decreased dopamine (DA) and serotonin (5-HT) levels. No changes were observed in plasma testosterone levels and in the transcript levels of D1 and D2 dopamine receptors, dopamine transporter (DAT), and carboxylesterase 2 (CES2). Exposure to 100ngL-1 of MPH caused a decrease in the transcript levels of carboxylesterase 3 (CES3) and an increase in tyrosine hydroxylase (TH), while exposure to 20ngL-1 of MPH increased the transcript levels of D5 dopamine receptor. Carboxylesterase activity was unchanged in the brain and liver and increased in the gills of fish exposed to 20ngL-1. These results indicate that MPH at 100ngL-1 increases aggressiveness in Nile tilapia, possibly due to a decrease in 5-HT levels in the brain and alterations in dopamine levels and dopamine-related genes.

The role of fish-poultry integration on fish growth performance, yields and economic benefits among smallholder farmers in sub-Saharan Africa, Tanzania

Aquaculture practices from sub-Saharan Africa are characterised by low production, owing to improper technology. Production can be increased through integrating fish farming with other existing on-farm activities, particularly livestock husbandry. We assessed the role of fish-poultry integration on all male Nile tilapia, Oreochromis niloticus growth performance, yields and economic benefits among smallholder farmers in sub-Saharan Africa, Tanzania. The study also compared phytoplankton species composition, abundance and biomass between the fish-poultry integration and non-integrated system. After 180 days of the experiment, all male O. niloticus cultured under fish-poultry integration exhibited significantly higher growth rates than those in the non-integrated system (p < 0.05). Gross fish yield (GFY), net fish yield (NFY) and net annual yields (NAY) obtained from fish-poultry integration were significantly higher than those from non-integrated system (p < 0.05). Partial enterprise budget analysis revealed that fish-poultry integration was more profitable than the non-integrated system. Moreover, fish-poultry integrated system produced significantly higher phytoplankton abundance and biomass than those from the non-integrated system. Results demonstrate that rural smallholder farmers can achieve higher growth rate, farm net yields and income by integrating all male O. niloticus with other on-farm activities than practising a stand-alone fish culture system.

AN ASSESSMENT STUDY OF GROWTH PERFORMANCE AND GONADS DEVELOPMENT OF MONO SEX NILE TILAPIA IN DIFFERENT AGE STAGES DURING THE PRODUCTION PERIOD

The objective of this study was to follow up the growth performance and gonads development of sex reversed male Nile tilapia. Fish fed commercial diet contained 30% crude protein at a rate of 3% of the biomass and 10 fish were randomly taken biweekly during the experimental period (16 weeks). Growth performance, feed utilization and survival rate were calculated. Histological examination of the gonads was done every two weeks to follow the growth and development of the gonads. Results showed positive interactions between growth parameters and reproductive development during different periods of fish productive cycle. Histological examination illustrated that during early ages, testicular sections of mono sex male’s tilapia had abnormal architecture with deterioration of germinative tissue. Despite abnormal testis texture, fish started spermatogenesis, a step toward puberty when their body weight reached 29.16g. As fish reached 4-5 months, male start to recover their testes normal structures and at age of 5.5-6 months, testicular sections appeared normally as pointed out by firmed testicular lobules, existence of all germ cell types and the intensively stored spermatozoa in testicular lumen and testicular ducts, confirming the full maturity of males. Based on the obtained results, it can clearly conclude that there is a relationship between age, body weight and development of sexual glands. It has also been shown that the hormonal treatment of tilapia seeds in the early stages for production of mono sex (all males) leads to a marked deterioration in the testes structure, continues near the fish enters the sexual maturity. At a later age, males can overcome this deterioration, restore the histological structure of the testes and achieve full sexual maturity at the age around from 5.5 to 6 months.

Social Defeat Stress Decreases mRNA for Monoamine Oxidase A and Increases 5-HT Turnover in the Brain of Male Nile Tilapia (Oreochromis niloticus).

Stress induces various neurobiological responses and causes psychiatric disorders, including depression. Monoamine oxidase A (MAO-A) plays an important role in various functions of the brain, such as regulation of mood, anxiety and aggression, and dysregulation of MAO-A is observed in stress-related psychiatric disorders. This study addressed the question whether acute social stress induces changes to transcriptional and/or post-transcriptional regulation of MAO-A expression in the brain. Using male Nile tilapia (Oreochromis niloticus), we investigated whether acute social stress, induced by the presence of a dominant male fish, changes the expression of MAO-A. We measured gene expression of MAO-A by quantitative PCR, enzymatic activity of MAO-A by the luminescent method, and 5-HT and 5-HIAA levels by liquid chromatography–mass spectrometry in the brain of socially stressed and control fish. Socially stressed males showed decreased MAO-A mRNA levels, consistent MAO-A enzymatic activity, increased 5-HT turnover in the brain, and elevated plasma cortisol levels, compared to controls. Our results suggest that acute social stress suppresses the transcription of MAO-A gene, enhances 5-HT metabolism but does not affect the production of MAO-A protein.

Orange peel fragment improves antioxidant capacity and haematological profile of Nile tilapia subjected to heat/dissolved oxygen‐induced stress

This study evaluated the ability of orange peel fragment (OPF) to act as a functional feedstuff, influencing growth, haematological profile, and antioxidant enzyme activity of Nile tilapia subjected heat/dissolved oxygen-induced stress (HDOIS). A group of 440 male Nile tilapia (31.7 g ± 0.34) was randomly distributed in 40 250-L aquaria (11 fish/tank) and fed five practical diets with graded levels of OPF at 0%, 0.2%, 0.4%, 0.6%, and 0.8% for 70 days. The diets were formulated to contain 30% crude protein and 18 MJ/kg crude energy. After the feeding period, growth performance was evaluated and six fish per treatment were sampled for haematological profile and antioxidant enzyme activity, before and after HDOIS. Then, fish were subjected to HDOIS (32°C/2.3 mg/L dissolved oxygen) for three days and the same haematological profile and antioxidant enzyme activity were determined. There was no effect of OPF on the haematological profile, either before or after HDOIS. The polynomial regression model was used to express the relationship between antioxidant enzymes activity and OPF supplementation level. The maximum activity of superoxide dismutase, catalase, and glutathione peroxidase was reached at 0.66%, 0.63%, and 0.68% of OPF respectively. Results of the present study suggest that a dietary supplementation level of 0.63%–0.68% of orange peel fragment was appropriate to maintain Nile tilapia haematological profile and improve its antioxidant capacity under HDOIS.

Comparison of organic and inorganic microminerals in all plant diets for Nile tilapia Oreochromis niloticus

An 8-week feeding trial was conducted to evaluate the production performance of all male Nile Tilapia (Oreochromis niloticus) fed diets supplemented with increasing levels of trace mineral premixes (copper, selenium, zinc, iron, manganese) from inorganic (I) and organic (O) sources. A practical trace mineral basal diet was developed using only plant ingredients. Nine experimental diets were prepared from the basal formula by adding graded levels of inorganic or organic trace mineral premixes to deliver 0 (control diet), 0.5, 1.0, 2.0 or 4.0 times the micromineral requirement for tilapia as stated in the NRC (2011). Juvenile Nile tilapia (initial weight 7.13 ± 0.24 g) were randomly stocked into 50-L aquaria at 25 fish per aquarium. Each of the nine treatments was replicated five times except treatment 8 (I-4) with four replicates. According to the results of our study, there were no significant effects of trace mineral premix levels and sources on the growth performance, survival rate and whole body proximate composition of Nile tilapia (P > .05). However, the trace mineral concentrations in the whole body and fillet were significantly influenced by the dietary levels of premixes. With the exception of selenium, there were no major differences in the micromineral contents in the fillet between fish fed the organic or inorganic sources of microminerals. Fish fed diets with organic selenium (Selplex, Alltech®) had significantly higher selenium levels in the fillet with the adjusted mean of 0.0191 mg/100 g fillet compared to 0.0161 mg/100 g in fish fed inorganic selenium (Na-Selenite).

Carbohydrate-to-lipid ratio in extruded diets for Nile tilapia farmed in net cages

The study evaluated productive and economic performance, physiological parameters and nutritional efficiency in net cage-reared Nile tilapia fed extruded diets containing four carbohydrate (CHO) and lipid (LIP) ratios. The 1280 fish in the trial were sex-reversed male Nile tilapia from the Gift strain (157.54 ± 3.50 g). Fish were randomly distributed into 16 net cages (1.0 m3), with 80 tilapias per cage. The four practical diets were formulated with the same ingredients at different proportions of starch and soybean oil contained similar digestible protein (251.40 g Kg−1), digestible energy (17.39 Mj Kg−1) and CHO:LIP ratios of: 3.89, 4.87, 6.14 and 7.83 (CHO35/LIP9, CHO39/LIP8, CHO43/LIP7 and CHO47/LIP6, respectively). The diets were provided four times a day, for 120 days. Fish fed a CHO43/LIP7 diet exhibited the best productive performance, with 252.01% percent weight gain (WG), 1.07 ± 0.028% daily specific growth rate (SGR) and 462.08 ± 5.06 g feed intake (FI). Fish fed the CHO43/LIP7 diet also showed the highest gross energy retention efficiency (57.17 ± 4.16%) and LIP use for weight gain (13.71 ± 1.47%). CHO39/LIP8 and CHO43/LIP7 diets produced the best economic performance, with partial net income of US$ 75.27 and US$ 73.40, respectively.

Studies on the biology of Nile tilapia (Oreochromis niloticus) in Lake Victoria, Kenya: in light of intense fishing pressure

Nile tilapia Oreochromis niloticus is now the most abundant and commercially important tilapiine in Lake Victoria. From the total of 1 512 fish sampled from commercial gill net fisheries during 2014 and 2015, 809 (54%) were males and 672 (44%) were females, giving an overall sex ratio of 1.20 males: 1.00 females. The mean (± SE) length and weight for all fish were 28.7 (±0.1) cm TL and 506.6 (±7.1) g, respectively. The slope b of the length-weight relationship was 2.98, 3.01 3.01, for males, females, and combined sexes, respectively. The relative condition factor was 1.02 for males and 1.04 for females with little variation across the months of sampling. The length at 50% maturity was estimated as 31.0 cm TL for male Nile tilapia and 26.0 cm TL for females. Sixty percent of the fish in the commercial catches surveyed were below 30 cm TL. Comparisons with earlier studies in this system suggest an overall decline in size at maturity over the past 30 years, which may reflect intense fishing pressure.

Effects of Estradiol and Testosterone on the Expression of Estrogen Receptor and Androgen Receptor Genes in Female and Male Nile Tilapia, Oreochromis niloticus

Recent studies strongly indicate a bidirectional relationship between sexual distinction and sex steroid hormones. In this study, Western blotting and immunohistochemistry were used to estimate the level of estrogen receptor (ER) and androgen receptor (AR) in the liver, gonads, kidney, and intestine of female and male Nile Tilapia, Oreochromis niloticus. The effects of a single injection of 17β-estradiol (E2), testosterone on the gene expression of ER-β and AR-α1 were determined. The expression of ER-β in females was higher than in the male liver and gonad (P<0.05). AR-α1 expression in in males was higher than females in the liver, gonad, and intestine (P<0.05). E2 increased the expression of ER-β in the livers of female fish (P<0.05). Testosterone (T) increased the expression of AR-α in the liver and intestine of male fish (P<0.05). Contrary to the result of AR, a Western blot analysis indicated that the level of ER protein in the female liver and gonad was significantly higher than that of males (P<0.05). Immunopositive results were the highest in the gonad followed by the intestine and kidney. Moreover, the immunolabeling signal was intensified in females but not in males, as shown via Western blot.