1 Introduction
Culture of monosex Oreochromis species, preferably males, has been recognized as the most effective way of avoiding early maturation and uncontrolled reproduction. This is because Tilapia fish exhibits sexually related dimorphic growth in which males grow and reach a larger ultimate size faster than the females (Guerrero, 1976; Manosroi et al., 2004; Carandang, 2007). This can be achieved through manual sexing, hormonal administration and hybridization. Of these hormonal administration has been found to be easy and most effective (Green and Teichert Coddington, 2000). Of these hormones, the synthetic methyltesto- sterone (MT) has been widely used (Green and Teichert Coddington, 2000; Khalil et al., 2011) probably due to its simplicity and reliability to produce all male tilapia stocks, which consistently grow to a larger and more uniform size than mixed sex or all female tilapias (Mensah et al., 2013). Synthetic androgens are used in fish culture as sex controlling agents and as growth promoters if energy is shut away from developing ovaries towards growth of somatic tissues (Rizkallah et al., 2004). Hormonal sex reversal has been particularly effective in cichlids because the gonadal differentiation takes place early in the life history (Mensah et al., 2013). Tilapia species that have been successfully sex reversed are mouth brooding species where hormone treatment begins within a few days after hatching (Phelps and Popma, 2000). 

Nile-tilapia (Oreochromis niloticus) has long been known to aquaculturists as specie that can adapt to many environments and culture systems. It has also become well known to fish consumers across the world (Chowdhury et al., 2007). Recent statistics show that tilapia is cultured worldwide in over 100 countries (FAO, 2004). They can be raised in a wide range of production systems from small-scale, low-input, rural ponds to large-scale, intensive, and commercial operations (Chowdhury et al., 2006). Tilapia has the potential to be the tropical fish of choice to meet the future demand for animal protein for marginal populations (Little et al., 1994), and to improve the livelihood of resource-poor farmers and rural women (Chowdhury and Rahman, 1998; Brugere et al., 2001). Tilapia Oreochromis spp. are usually cultured in static-water ponds at levels of intensity ranging from dependence on natural fertility to the use of complete dry diets. The expansion of tilapia culture across the world, together with the shortage of fresh water and competition for it with agriculture and with urban activities, has gradually shifted tilapia culture from traditional semi-intensive systems to more intensive production systems (El-Sayed, 2006).

Intensive flow-through culture system using fibre glass tank offers several advantages over pond culture. Pond of mixed sex population breed so much that parent and offspring compete for food and become stunted. Flow-through culture system (using fibre glass tank) allow the fish culturist to easily manage stocks and to exert a relatively high degree of environmental control over parameters such as water temperature, dissolved oxygen (DO), pH, waste, that can be adjusted to maximum production with flow-through culture system. This may translate to better growth and fish yield for O. niloticus noted to have excessive reproduction as a limitation. Moreover, Chakraborty and Banerjee (2009) reported that flow-through culture of tilapia is also done on a very limited scale, for producing marketable fish. Although there have been studies on the effectiveness of the MT on masculinisation of the O. niloticus, there is a dearth of published information on the effectiveness of the MT and semi flow-through culture system as growth promoters and reproduction inhibitors in O. niloticus. The aim of this study is to ascertain the effectiveness of MT and intensive tank culture of the tilapias in a semi flow-through culture system, using fibre glass tanks, on the growth, survival and control of excessive reproduction of O. niloticus fed two feed type, which is yet to be well tested.

2 Methodology
The experiment was conducted at the Nigerian Institute for Oceanography and Marine Research (NIOMR) Sapele station between July 2013 and December 2013 (24 weeks). Twelve circular fibre glass tanks (Plate 1) were used, each with capacity of 3.08 m³ of water at a nominal flow rate of 2 L/min. The tank was mounted indoor in a flow through and arranged in a row. The floor of each tank was drained to the centre while the drainage of each tank was on the outside via 100 mm PVC pipes with gate valves. The tank used in this study was continuously flow-through with water source from a bore hole connected to a water treatment plant. Each tank was washed, cleaned, and disinfected with sodium chloride (NaCl) after which the tanks were filled with water to a depth of 60.5 cm and allowed to settle for a day before introducing the fish. Each tank received continuous flow-through with water 6-8 hours per day from a bore hole passing through a water treatment plant to correct the pH at 4.0 which is acidic. The gate valve was open to allow the metabolic wastes in the water to be flushed out while at the same time there was influx of water from the inlet pipe for aeration of the water. Fish were acclimated in indoor tank for 2 weeks to laboratory conditions. The experimental fish (mixed sex and the sex reversed tilapia) from NIOMR hatchery were randomly allocated at a stocking density of 53 fish m³ to the twelve fibre glass tanks receiving two feed types: Coppens and Farm Produced Feed of 56% and 25% crude protein respectively and fed twice daily at 0800 hour and 1600 hour. The feeding rates were readjusted The daily feeding rate for the first ten weeks was 5% of their total body weight while the feeding rate for the remaining fourteen weeks of their production cycle was 3% of their total body weight. The experiment was designed as 4 treatments x 24 weeks factorial replicated thrice. Twenty fish from each tank were randomly sampled fortnightly to measure their weight and determine their water quality parameters. Growth parameters such as average body weight gain (BWG), daily weight gain (DWG), specific growth rate (SGR) and survival rate were calculated as following:
(a) Initial mean weight = (g/fish)
(b) Final Weight of fish= (g/fish)
(c) Average body weight gain (g) = Total bi-weekly weight gain (g) / Total surviving fish weight (g)
(d) Daily weight gain (g^-1 fish^-1 day) = Final Weight – Initial Weight (g) / Culture period  (days)
(e) Specific growth rate = Log e Final weight – Log e Initial weight / Culture period (days) x 100
Where e is the base of natural logarithm (Brown, 1957) and t, the culture period (days).
(f) Fish survival (%) = No of survival after culture / No of fish stocked x 100 or % survival = (100 – mortality) %

Plate 1 Experimental Fibre Glass Tanks in a semi flow-through culture system

Total fish production per tank at the end of the experiment was also calculated. The water quality was monitored using the method described by (APHA, 1985; Lind, 1979; and Boyd, 1990) to ensure adequate and appropriate water quality for fish growth. Table 1 shows the water quality parameters monitored and the methods of determination. The physical qualities of the water i.e. temperature and some chemical properties such as pH, dissolved oxygen, unionized ammonia and total ammonia were measured bi-weekly in situ using the LaMotte Freshwater Aquaculture test kit (Model AQ-2). The Genstat Statistical Package (version 8.1) was used for the analysis of data.

Table 1 Water quality parameters monitored and method of determination

3 Results
3.1 Growth Parameters
The steroid hormone, 17α-methyltestosterone (MT), significantly affected the growth of O. niloticus (P < 0.05). All the treatments which received MT (Treatments II and IV), showed more average final body weight, gain in body weight and daily weight gain of O. niloticus than the mixed sex fishes without MT in Treatments I and III (Table 2). Fish in all treatments gradually grew with fortnights, and the highest final average weight was obtained in the last fortnights (Table 2), and there was also a steady increase in the average gain in weight from the first fortnights to the last fortnights. Treatment II (sex reversed tilapia fed Coppens) showed 48.65 g gain in weight followed by Treatment I (mixed sex tilapia fed Coppens) 46.34 g, Treatment IV (sex reversed tilapia fed Farm Produced Feed) 28.20 g and Treatment III (Mixed sex tilapia fed Farm Produced Fed) 23.42 g at the end of the experiment (Table 2). Statistical analysis on gain in body weight, showed a highly significant difference among the treatments and fortnights (P < 0.05). The daily weight gain followed the same trend with the average final weight and gain in body weight as presented in Table 2. In terms of total fish production, the sex reversed fishes in Treatments II, having the highest final average weight, average gain in weight and daily weight gain showed the highest total fish production 4.87 kg/3.08 m³tank/168 days as presented in Table 3. The overall fish production showed that the sex reversed fishes had the highest total fish production compared to the mixed sex fishes. Hundred percentage survivals were recorded in all the treatments.

Table 2 Fortnightly observations on increase in body weight of O. niloticus under different treatments Means in same row with different superscripts are significantly different (P < 0.05). Key:  I = Mixed sex Coppens II = Sex reversed Coppens III = Mixed sex Farm Produced Feed IV = Sex reversed Farm Produced Feed

  

Table 3 Total fish production of O. niloticus observed under mixed sex and sex reversed treatments Means in same row with different superscripts are significantly different (P < 0.05) Key:  I = Mixed sex Coppens II = Sex reversed Coppens III = Mixed sex Farm Produced Feed IV = Sex reversed Farm Produced Feed

3.2 Water Quality Parameters
Physical and chemical water quality parameters are presented in Table 4. In general, there was no significant difference (P > 0.05) in the water quality parameters recorded among all the treatments as recorded in Table 4. Results revealed that all tested physical and chemical parameters were within the permissible levels required for tilapia growth. This was due to the occasional flushing of the water through semi flow-through culture system.  

Table 4 Mean values of water parameters monitored during the 24 weeks culture period Key:  I= Mixed sex Coppens @ 53 fish/m3 II= Sex reversed Coppens @ 53 fish/m3 III = Mixed sex Farm Produced Feed @ 53 fish/m3 IV= Sex reversed Farm Produced Feed @ 53 fish/m3

Water temperature
The water temperature ranged from 25.89 to 26.11℃ among the treatments as presented in Table 4.However, there was no significant difference (P > 0.05) in the water temperature among the treatments. The results of the study revealed that fish fed Coppens (Treatments I and II) had a higher mean water temperature compared to those fed Farm Produced Feed (Treatments III and IV) which were not significantly different from each other. Based on sex type, it was observed that the sex reversed group (Treatments II and IV) recorded higher mean water temperature compared to the mixed sex group (Treatments I and III) which was not significantly different from each other.

pH
The results of the pH of the water showed no significant difference among the treatments. The pH values ranged from 6.80 in Treatment IV to 6.82 in Treatment II as presented in Table 4. Based on the sex type, sex reversed group (Treatments II and IV) recorded lower mean pH compared to the mixed sex group (Treatments I and III) which were not significantly different from each other. Also, the result indicated that tilapias fed Farm Produced Feed (Treatments III and IV) recorded lower mean pH compared to those fed Coppens (Treatments I and II) which were not significantly different from each other. 

Dissolved oxygen (DO)
The highest mean Dissolved oxygen (DO) of 10.96 mg/L was recorded for Treatment III followed by Treatment II with mean DO of 10.95 mg/L which were not significantly different from each other while the least mean DO of 10.88 mg/L was recorded in Treatment I as indicated in Table 4. The mixed sex group had a lower mean DO compared to the sex reversed group. The result also indicated that tilapias fed Coppens had a lower mean DO compared to those fed the Farm Produced Feed.

Total ammonia nitrogen (NH₃N)
The total ammonia nitrogen values ranged from 1.59 mg/L in Treatment IV to 1.63 mg/L in Treatment I (Table 4). The highest total ammonia nitrogen (NH₃N) of 1.63 mg/L was recorded for Treatment I while the least total ammonia nitrogen (NH₃N) of 1.59 mg/L was recorded in Treatment IV. The sex reversed group had lower total ammonia nitrogen compared to the mixed sex group. Based on the feed type, the result showed that tilapias fed Farm Produced Feed had lower total ammonia nitrogen compared to those fed Coppens which were not significantly different from each other.

Unionized ammonia (NH₃)
The value for the unionized ammonia ranged from 0.01 mg/L in Treatments III and IV to 0.02 mg/L in Treatment II as shown in Table 4. The highest mean unionized ammonia (NH₃) of 0.02 mg/L was recorded for Treatment II while the least mean unionized ammonia (NH₃) of 0.01 mg/L was recorded in Treatments III and IV. Based on the sex type, the mixed sex group had lower mean unionized ammonia compared to the sex reversed group which was not significantly different from each other. Based on the feed type, tilapias fed Farm Produced Feed had lower mean unionized ammonia compared to those fed Coppens.

4 Discussion
The highly significant difference (P < 0.05) on gain in body weight recorded in the sex reversed fishes could be as a result of the anabolic and androgenic effects of MT in the sex reversed fishes treated with the steroid hormone. These results are in line with the findings regarding anabolic effect of MT in fish and all male culture of tilapia by different authors (Ahmad et al., 2002; El-Saidy, 2005; El-Greisy and El-Gamal, 2012; Mensah et al., 2013). It is well known that anabolic steroids may produce fish with increased weight gains and muscle deposition (Ahmad et al., 2002).  Similarly, El-Saidy (2005) revealed that the growth in weight and length was higher significantly in monosex male compared with monosex female and normal mixed sex Nile tilapia. The increase in fish growth may be because of that MT induce the feed digestion and absorption rate causing increase in body weight (El-Greisy and El-Gamal, 2012), or may be MT administration increased the proteolytic activity of the gut as the case in mirror carp leading to increase in growth rate (Lone and Matty, 1981). In addition, methyletestosterone has been reported to enhance growth of various fish species such as Nile tilapia, O. niloticus (Tayamen and Shelton, 1978). Hormonal sex reversal has been particularly effective in cichlids because the gonadal differentiation takes place early in the life history (Mensah et al., 2013). Growth increase in androgen treated fish was also reported in O. mossambicus (Kuwaye et al., 1993), Oncorhynchus kisutch and Cyprinus carpio (Pandian and Sheela, 1995). Tilapia species that have been successfully sex reversed are mouth brooding species where hormone treatment begins within a few days after hatching (Phelps and Popma, 2000). (Shepherd et al., 1997) suggested that the growth-promoting actions of the androgen, 17α-methyltestosterone in tilapia were linked to elevations in growth hormone (GH) metabolism and consequently to insulin-like growth factors (IGFs).

The sex reversed fishes in this study recorded the highest total fish production at the end of the experiment which could be attributed to the anabolic effect of the MT. Steroid treatment may produce fish of more robust size containing more muscle per unit length than untreated fish without changing the anabolic efficiency or proportion of muscle to whole body weight (Ostrowski and Gariing, 1988). (Little et al., 2003) showed more or less to the same conclusion, where sex-reversed tilapia grew better and economic than the non sex-reversed fish.

In the present study, no mortality was recorded throughout the experimental period. This could be as a result of the occasional flushing of the water of the flow-through culture system. The flow-through culture system adopted in the present study may have attributed to the high survival rates of the fish among all the treatments. Similarly, (Chakraborty et al., 2011) reported that continuous exchange of water in flow-through system might have sustained a better general environment for fish growth than in cistern culture system (Chakraborty et al., 2011). However, further research on impact of water flow rate during flow-through culture system is to be carried out since it has been observed that at a very high water flow rate, fish spend a substantial amount of dietary energy for continuous swimming, leading to reduced growth and increased mortality (El-Sayed, 2006).

Surprisingly, no spawning was observed in mixed sex fish under semi flow-through culture system in the present study. This could be attributed to the constant water flow in all the treatments. This is in agreement with (Delong et al., 2009) and (Yakubu et al., 2012) who reported that intensive tank culture offers several advantages over the use of pond in the sense that it disrupts breeding behavior and allows male and female tilapia to be grown together to marketable size as observed in this present study. In addition, the inability of the tilapias to spawn in the experimental fibre glass tanks could also be attributed to the sex reversal method adopted in this study, to alter the sex of the genotypic female tilapias to phenotypic males. This is in agreement with research findings of Ajiboye and Yakubu (2010) who reported that mono-sex culture method has been widely used to control the precocious maturity and uncontrolled reproduction in tilapias. Similarly, faster growth of monosex tilapia has been related to the lack of energy expenditure in egg production and mouth brooding by females and lower energy expenditure on courtship by males (Dan and Little, 2000; Tran-Duy et al., 2008). 

The water quality parameters are shown in Table 4. The values ranges were 25.89-26.11℃, 6.80-6.82, 10.88-10.96 mg/L, 1.59-1.63 mg/L, and 0.01-0.02 mg/L for water temperature, water pH, dissolved oxygen, total ammonia nitrogen and unionized ammonia respectively. The physico-chemical parameters monitored in the experimental fibre glass tanks, as indicated in Table 4 showed that water temperature, DO, pH, TAN and unionized ammonia value varied among all the treatments and this is in accordance to (Frank, 2000; Marjani et al., 2009) findings, who stated that DO should be above 3 mg/L, pH of 6.0-9.5, temperature (28 ± 1℃). No significant differences in water temperature, pH, dissolved oxygen (DO), total ammonia nitrogen (TAN) or unionized ammonia was observed among the treatment tanks (Table 4). The semi water flow-through system used in this experiment appeared to be a successful way to control nutrient addition, as there were no differences in most chemical and biological variables among the treatments. This was in agreement with (Diana et al., 1995) who conducted a similar experiment on effects of stocking density and supplemental feeding in Nile tilapia and similar results were reported. All the water quality parameters were within the acceptable ranges as recommended for tropical aquaculture (Boyd, 1982; Beveridge, 1996). Continuous exchange of water in flow-through system might have sustained a better general environment for fish growth when compared to other culture systems. 

There was an increased trend towards the growth with alterations of reproduction in the fibre glass tanks. Use of such growth promoters (MT) adopted in this study coupled with the semi flow-through culture system adopted throughout the culture period of O. niloticus would help tilapia farmers to meet the goal of controlling tilapia reproduction with fast growth. Data generated from this study asserted that intensive culture of O. niloticus in a semi flow-through system is a successful culture system of controlling excessive reproduction of O. niloticus and thus could help in guiding the fish farmers in intensive culture of Nile tilapia (O. niloticus). The study also indicated that sex reversed treatments (II, IV) which are able to grow faster than typical mixed sex populations is ideal for the rapid growth rates of tilapia.

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