1 Introduction

Aquaculture industry in the last five decades has grown at an unprecedented rate with an average growth of 3.2 percent per year (FAO, 2012). This growth has been stimulated by the increased demand for fish and fishery products as global awareness of fish as healthy food increased and the production from capture fisheries becomes almost static (Ataguba and Okomoda, 2011). Aquaculture has traditionally played a role in ensuring food security, and it is a very important component of rural development programs to alleviate poverty (FAO, 2006). There are many aquaculture species and among of these important species indigenous to Africa is the Tilapia (Oreochromis niloticus).

Tilapia is the common name applied to three genera of family Cichlidae (Sarotherodon, Oreochromis and Tilapia) including about 70 species (Meyer, 2002). Tilapia is an important food fish in many tropical and sub-tropical countries as it provides one of the most important sources of animal protein in the world (FAO, 2012). They are considered suitable for culture because of their high tolerance to adverse environmental conditions, relatively high growth rate, good taste, ability to efficiently convert organic and domestic wastes into high quality protein and the ease with which they can be bred (Yi et al., 1996).

Tilapia is among the most studied groups of fish in Africa waterbodies and they are increasingly prominent in freshwater aquaculture in many regions of the world (Admassu, 1996; Coward and Bromage, 1998). Hence, they have become one of the most commercially important groups of fish sold around the world (Coward and Bromage, 1998).

The culture of tilapia is still beset with problems of their prolific breeding habit which subsequently lead to their stunted growth. Tilapia starts breeding early enough when they are about 8 cm in size. Within a few months of culture, a tilapia pond becomes full with small fish, resulting to slower growth as a result of overpopulation. Subsequently the fish farmer gets very little or no profit as a result. Various strategies have been devised to control breeding of tilapia and increase its body size, among which is all male tilapia production which eliminates reproduction and result to faster growth (Guerrero, 1985).

The Scientist report has shown that male tilapia had better growth than the female. The superiority of the performance of male tilapia over the female goes beyond just genetic capability (Muhammad et al., 2008). Also, the breeding pattern of females which involve incubation of eggs in the mouth during spawning greatly prevent active feeding, hence lead to reduced growth (Muhammad et al., 2008). The propagation of all male tilapia however is fast becoming popular in most part of the world as it removes the prolific breeding characteristics associated with tilapia production and enhances faster growth to table size. This study was designed to investigate the performance of all male and mixed sex tilapia reared in an outdoor system.

2 Material and Methods

This study was conducted at the Department of Fisheries and Aquaculture, University of Agriculture Makurdi, Benue State, Nigeria. Hatchlings of Nile tilapia were collected from the mouth of females maintained in outdoor earthen pond at the Fisheries Research Farm. Half of the hatchlings were move indoor for hormonal administration following the methods described by Olufeagba and Okomoda (2015). Fish for sex reversal were maintained in a well aerated 60 x 30 x 30 cm³ glass aquaria tanks. One hundred grammes (100 g) of coppens^® powdered feed were mixed with 30 μg of 17 α-methyltestosterone (Sigma E-4876) (already dissolved in 95% ethanol). This was mixed in a plastic bowl and oven-dried at 65°C for one hour. The prepared feed was fed for twenty-eight days before the commencement of this study. Triplicate batches each of fifty hormonal sex reversed and fifty mixed sex tilapia fingerlings were transferred into six outdoor rearing tanks where they were fed with coppens^® commercial diet (45% CP) for twenty-four weeks. Water quality parameters such as temperature, conductivity, total dissolved solids and pH were monitored fortnightly using a digital multi-parameter water checker kit (Hanna water tester, Model HL 98126, Made in Romenia). Fish were fed twice daily (08:00 am and 06:00 pm) at 5% body weight. The fish were weighed weekly to determine new weight gain and adjust rations based on the new body weight gain. Growth performance and nutrient utilization were assessed at the end of the experiment using the equations below.

The different variables measured across the treatment were obtained using Minitab 14 for windows software. Result was then subjected to Analysis of variance and where significant differences occurred; means were separated using Fisher’s least significant difference.

3 Results

The result of water quality parameters in the culture unit used for the experiment (Table 1) revealed that no significant difference (P>0.05) were obtained between parameter measured for the rearing tanks for all male tilapia and that of the mixed tilapia. The pH varied from 7.2 in all male to 7.3 in mixed sex. Temperature and total dissolved solid were 27.4°C and 64.82 mg/L respectively in all male tilapia and 27.7°C and 65 mg/L respectively in mixed-sex tilapia. Conductivity recorded was 93.05 and 92.99 in all male and mixed sex tilapia respectively.

The result for growth parameter as summarized in Table 2 revealed final weight of all male tilapia to be significantly higher (5.94 ± 0.13 g) than mixed sex (2.20 ± 0.09 g) (P ≤ 0.05). Similarly, weight gain was higher in all male fish (5.93 ± 0.13 g) as against (2.19 ± 0.09 g) recorded for mixed-sex. Also, Figure 1 shows the biweekly growth pattern of all male and mixed-sex O. niloticus during the study period. The same trend was observed for growth rate, specific growth rate, feed fed, feed conversion efficiency and protein efficiency ratio with higher values recorded for all male tilapia fish (0.035 ± 0.001, 3.94 ± 0.02, 16.24 ± 0.03, 36.50 ± 0.75 and 0.198 ± 0.01 respectively) compared to the mixed sex tilapia (0.013 ± 0.001, 3.29 ± 0.001, 6.45 ± 0.15, 34.13 ± 2.22 and 0.073 ± 0.01 respectively). However, feed conversion ratio was higher in the mixed-sexes (2.74 ± 0.06) than the all male O. niloticus (2.94 ± 0.19). The study observed no significant variation in survival of all male and mixed-sex fish (P ≥ 0.05). No breeding activity was noticed in both groups under study.

4 Discussion

The temperature, pH, conductivity and total dissolved solids measured during this study were closely related and were within recommended range for the culture of tropical fishes as described by APHA (1998). Hence, application of 17 α-methyltestosterone in the diet O. niloticus did not alter the water quality. This study shows that weight gain for all-male O. niloticus were significantly higher than the value recorded for mixed sex tilapia (P≤0.05). This report is similar to the work of Little et al. (2003), Dan and Little (2000), Mair et al. (1995) and Abella et al. (1990). Tran-Duy et al. (2008), Dan and Little (2000) had earlier reported that better growth of all male tilapia is related to the lower energy expenditure on courtship and spawning. However, since no spawning was observed in the mixed-sex fish during this study (as evident in the number obtained as survival and fairly uniform size of the experimental fish) other factor may have been responsible for observed performance. It may be right to infer that the better performance of all male compared to mixed sex tilapia in this study may not be due to reduced growth as a result of spawning rather its justified by the hypothesis 17 α-methyltestosterone has growth-promoting actions on tilapia (Shepherd et al., 1997). This was linked to elevations in growth hormone (GH) metabolism and consequently higher insulin-like growth factors (IGFs). Also, Bhasin et al. (2001) reported that that testosterone produces muscle hypertrophy by increasing muscle protein synthesis.

The decrease in food conversion ratio (FCR) and an increase in protein efficiency ratio (PER) for all male fish compared to the mixed-sex fish observed in this study was in accord with the report of Pechsiri and Yakupitiyage (2005) who reported that FCR decreases while PER increases with increased feeding rate. The underlining consequence of this is that it would require less feeding to get all male tilapia fish to table size compared to a mixed sex tilapia. Feed conversion ratio is used to assess feed utilization and absorption. The result of the present stud shows that all male tilapia had better feed utilization and absorption when compared to mixed sex tilapia and there was significantly difference (p<0.05) between the treatment. This study negate the report of Sule (2004) and Guerrero (1985) who reported that there were no significant difference between the food conversion ratio of the all-male and those of the all-female and the mixed sex population (P≥0.05). The discrepancies in the findings of these studies may be due to differences in feeding response of the different species, culture system used and feed type in the experiment. The result of this study shows similar survival rates were observed in both mixed-sex and all male fish indicating that hormone treatment has no adverse effect on general fish health. This is in agreement with the report of Cruz and Mair (1994) that 17 α-methyltestosterone treatment have no effect on survival of tilapia.

5 Conclusion

It may be rightly inferred that 17 α-methyltestosterone treatment of tilapia achieved masculation, higher weight gain, and it is not toxic to fish at the dosage used in this study. However, the mechanism of better performance as a result of hormonal administration needs to be clearly understood, hence the need for further research.

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