Abbreviations
NFALRC: National Fisheries and Other Aquatic Life Research Center; EIAR: Ethiopian Institute of Agricultural Research; DO: Dissolved Oxygen concentration; TW: Total weight of the fish; TL: Total length of the fish; FCR: Food conversion Ratio; DWG: Daily weight gain; SGR: specific growth rate; Eth. Birr = Ethiopian currency.

Introduction
Tilapia have been successfully farmed under a wide range of environmental conditions and are important group of cultured fish species in many parts of the world, particularly in developing countries (Bentsen et al., 1998; Gjedrem, 2005; Pillay and Kutty, 2005; El-Sayed, 2006) . They are considered to be hardy, rapid in growth with high food conversion ratio, easy breeding under captivity, high fecundity and generally resistant to many diseases. These characteristics make tilapia suitable for culture in most developing countries (El-Sayed, 2006). Among the tilapia, Oreochromis niloticus was found to be suitable for semi-intensive culture system because of its ability to utilize a wide range of feed stuff originating from plants and/or animals (Liti et al, 2005). Moreover, tilapia grows and matures at younger age under captivity, contrasting to the natural system (Balarin and Hatton, 1979; Lowe-McConnel, 1982) and reproduces in a wide range of environmental conditions and tolerates stress induced by handling (Tsadik and Bart, 2007; Chakraborty et al., 2011). However, their breeding habit has undesirable consequences. Some of the major problems associated with the reproductive efficiency of Oreochromis niloticus are prolific reproduction and stunted growth in pond culture system (Phelps and Popma, 2000). Within a few months of culture period, the pond gets packed with various sizes of fishes and later due to overpopulation, the growth of fish gets slow and the fish farmers get virtually no revenue. There exist a number of methods to control reproduction in a mixed-sex population of Oreochromis niloticus. One of these methods is the rearing of male mono-sex tilapia (Phelps and Popma, 2000). There is significant sex-specific difference in the growth of fish where males usually grow faster and more uniform in size than females (Bwanika et al., 2007). This is mainly attributed to reproduction which drains energy primarily for the production of eggs and offspring’s (Eyualem and Getachew, 1992; Tadesse, 1997). Moreover, in a mouth brooder fish species like O. niloticus females fast during the early stages and probably throughout the brooding period which causes inconsistent feeding and subsequently affects the body condition (Tadesse, 1988; Demeke, 1994). Elsewhere, the culture of all-male tilapia is well established for increased production potential and low management requirements in semi-intensive pond culture system, however this has not been tested under Ethiopian condition. This study was therefore, conducted to compare the growth performance of male mono-sex and mixed-sex Nile tilapia (Oreochromis niloticus) in pond culture system under Ethiopian culture condition at Sebeta.

1 Results
The mean value of key water quality parameters of the experimental ponds are given in Table 1. With the exception of conductivity, other water quality parameters measured were not significantly different (P>0.05) among the treatments. The mean DO concentrations were (8.76±2.03) mg/L and (9.18±1.97) mg/L (SD) for mixed sex and mono-sex groups, respectively. pH ranged between 8.00 and 9.00, whilst the specific conductivity ranged from 190 μS/cm to 230 μS/cm in both experimental groups.

Table 1 The mean value of some water quality parameters recorded from the experimental ponds

Data on the growth performance, feed conversion ratio, yield, Fulton’s condition factor and survival rate of male and mixed-sex Oreochromis niloticus are presented in Table 2. Significant variations were observed in growth performance among male and mixed-sex Oreochromis niloticus reared under the same culture condition (P<0.05, Figure 1). After 240 days the fish attained an average weight of (176.20±18.01) g and (108.20±15.40) g for male and mixed-sex group respectively. The net weight and daily growth rate per fish were (132.65±5.30) g and 0.55 g/d for male and (68.85±3.18) g and 0.29 g/d for mixed sex group respectively. Mortality rate of fish in both treatments was around 33% and showed no significant difference (P>0.05) among the experimental groups throughout the culture period (Table 2). The gross and net fish production were 4 087 kg•ha^-1•Y^-1 and 2 384 kg•ha^-1•Y^-1 for all male group, whereas 2 437 kg•ha^-1•Y^-1 and 1 421 kg•ha^-1•Y^-1 for mixed-sex group, respectively. The maximum harvest weight of a fish in the male mono-sex (at 6th month) and mixed sex (at 7th month) were 324 g and 197 g, respectively.

Table 2 Comparisons of growth performance, feed conversion ratio, yield, Fulton condition factor and survival rate of mixed and mono-sex tilapia in pond culture system (Mean ± SD)

 

Figure 1 Growth curve of live body weight for male mono-sex and mixed-sexd Oreochromis niloticus during the study period (mean ± SD)

 
Data on economic performance of Oreochromis niloticus for both treatments are depicted in Figure 2. Net revenues over variable costs were lower in the mixed-sex than in the male mono-sex group. However, both treatments showed positive net return for 6, 7 and 8 culturing months. Apparently, there was a difference in attaining optimum net return between male mono-sex and mixed-sex group (Figure 2).
 

Figure 2 Net returns from culturing Oreochromis niloticus as male mono-sex and mixed-sex at different culture periods, 6th, 7th and 8th months

2 Discussions
2.1 Water quality
Performance of fish in aquaculture is extremely dependent on the water quality. For fish to effectively feed, grow, excrete wastes and reproduce in a water body, its aquatic environment has to be optimum and conducive.Both physical and chemical aspects of the aquatic environment must be suitable to the requirements of the cultured fish species. Based on the data presented herein, the key water quality parameters: temperature, dissolved oxygen, pH and conductivity measured during the study period were all within the optimum range for rearing tilapia (Boyd and Tucker, 1992; Xu et al., 2005; Azaza et al., 2008). The percentage dissolved oxygen level barely fell below 100%, with slight super-saturation occurring in both experimental ponds. Specific conductivity varied strongly among the experimental ponds, however the values were within the range recommended for fish ponds (Boyd, 1997; Hargreaves and Heussel, 2000). As referenced in table 3, Hanley (2005) gave environmental requirements of Oreochromis niloticus.

Table 3 Some environmental physiology of Oreochromis niloticus

2.2 Fish growth
Although, the growth performance of Oreochromis niloticus is highly influenced by genetics, quality and quantity of food, stock management and environmental factors (Gjedrem, 1997; El-Sayed, 1999), sex-specific differences in the growth of O.niloticus is apparent (Green et al., 1997; Schreiber et al., 1998; Bwanika et al., 2007). The results of the present study revealed that the growth performance between all male and mixed-sex Oreochromis niloticus reared for 240 days under the same culture condition was significantly different, where the male mono-sex fish attained a larger final individual size which is nearly twice the mixed-sex group. Several investigators have studied the sex-specific growth difference of Oreochromis niloticus under semi-intensive pond culture system. For example, Chakraborty et al. (2011) documented the faster growth of all male tilapia than females and mixed-sex. This might be attributed to sex-specific growth ability, female mouth brooding behavior or the efficient feeding habits of males. In a mouth brooding fish like O. niloticus females fast during the early stages and probably throughout the brooding period which causes inconsistent feeding and subsequently affects the body condition (Tadesse, 1988; Demeke, 1994). Pandian and Sheela (1995) and Green et al. (1997) further reported similar result, where all male tilapia showed superior growth rate over the females and mixed-sex which is in agreement with the results of the present study. They attributed this to the fact that energy is not utilized for reproduction and there exist no competition with younger fish in all male tilapia culture.

3 Conclusions and Recommendations
The results of the present study revealed that the growth performance between male mono-sex and mixed-sex Oreochromis niloticus reared under the same culture condition was significantly different, where the male mono-sex fish grow faster and attained a larger size (2 times) than mixed-sex. Therefore, culturing all male tilapia will enable fish farmers to produce twice a year. However, it is very difficult to get 100% all male tilapia using manual sexing. Therefore, we strongly recommend action oriented on-farm training for fish farmers before implementing any stocking program to enhance the accuracy of manual sexing of small size fish (<40 g).

4 Materials and methods
4.1 Study area
National Fisheries and Other Living Aquatic Resources Research Center-Sebeta (08°54′N; 38°38′E) is located in south-west Ethiopia some 24 km from the capital Addis Ababa, at an altitude of 2 220 m a.s.l.

4.2 Sampling
Physico-chemical variables: Dissolved oxygen (DO), pH, specific conductivity and water temperature were measured on monthly basis using a multi-probe (Model HQ40d, HACH Instruments). Total length (TL) and total weight (TW) of 50% of the stocked fish were measured from each experimental pond every month throughout the study period. Fish were fed with wheat bran between 10:00 h and 16:00 h at a daily feeding rate of 5% of fish body weight, and the daily ration was then calculated and adjusted regularly according to the body weight gain of the fish every month. Dead fish were removed and recorded to evaluate the survival rate of the stocks. Subsequently, the total fish length and total weight were measured to the nearest 0.5 cm and 0.1 g, respectively. Fulton’s condition factor (k), an indicator of the “well-being” of an individual fish was calculated using the following formula (Lagler, 1956):

The daily growth rate, total weight gain and survival rate were calculated following the equations given below (Ridha, 2006):

DGR (g/day) = Final weight (g) – Initial weight (g)/culture period; Weight gain (g) = Final weight (g) – Initial weight (g); Survival rate (%) = (Number of fish harvested/Number of fish stocked)×100; FCF = TW/TL³×100.

where TW is total body weight in g, TL total length in cm.

4.3 Culture techniques
Growth performance of mixed-sex and male mono-sex were evaluated for eight months in earthen pond. Ponds were prepared by draining and sun drying for two weeks. Wire mesh enclosures were installed on the water inlets and outlets of each pond to prevent entry of unwanted fish and escape of experimental fish. Lime was spread over the pond bottoms at the rate of 250 kg/ha and the ponds were filled to 30 cm deep with water to increase the alkalinity of the ponds. After 5 days the levela of water in all ponds was raised to 1 m. Oreochromis niloticus fingerlings collected from Lake Babogaya were stocked and acclimatized at Sebeta earthen ponds for a period of two weeks. Prior to stocking sexing was done manually by visual inspection of the external urogenital pores with the aid of magnifying hand lens. Each pond was stocked at a stocking density of 2 fish/m².The average initial weight at stocking was 39.35 g and 43.60 g for mixed and male mono-sex, respectively. The relatively higher initial body weight of fish in the male mono-sex group was intentional to reduce percentage of errors in identifying sexes of Oreochromis niloticus at smaller size.

Acknowledgments
The authors thank all the technical staff members of National Fisheries and Other Aquatic Life Research Center (NFALRC) for their assistant during the data collection. Special thanks for Mrs. Abeba W/Gebriel for her technical assistance in identifying sexes of the fish and for her great effort during sampling. This study was financed by the EIAR-National Fisheries and Other Aquatic Life Research Centre, Sebeta, Ethiopia.

Author’s contributions
FD and AL contributed considerably during data collection, analysis of the result and write-up of the manuscript. FD’s contribution to organize the manuscript according to IJA-guideline was instrumental during the preparation of this paper.

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