Background

Nile tilapia (Oreochromis niloticus L., 1758) is one of the most dominant and popular indigenous fish species in sub-Saharan water bodies. Several studies reported that tilapia is one of the widely farmed fish species in the tropics and sub-tropical areas (Liti et al., 2006; El-Sayed, 2006; Kassahun et al., 2012; Zenebe et al., 2012, Abdelhamid et al., 2017; Katya et al., 2017). It has been reported that tilapia has got different merits to be considered as one of the candidate culture fish species with good adaptive ability, fast growth rate, able to growth in adverse conditions, diverse feeding habit and preferred by the consumers.

Ethiopia has high potential for developing fish culture both in terms of land/water and suitable agro-ecology with rich biological diversity of native fish species (Shibru, 2016) including O. niloticus. Using geographic information system, Eshete and Zemenu (2012) reported that quite a large area of the country is suitable for tilapia farming (e.g from moderately to highly suitable about 886,889 km²). Despite the country’s conducive environment for aquaculture development, fish farming is more of theoretical than actual practice. Limited skilled human power and lack of aquaculture inputs suppliers particularly fish seed and fish feed are the most important lacking well adapted practice of fish farming in the country.

Recently researchers from the research institutions and universities are conducting research to evaluate the performance of candidate aquaculture fish species, to evaluate and formulate fish feeds from locally available feed ingredients and to adopt some aquaculture practices like mono-sex culture, integration of fish farming with other agriculture practices etc. (Zenebe et al., 2012; Kassahun et al., 2012; Workagegn and Gjoen, 2012; Adamneh et al., 2013; Zelalem, 2013; Abelneh et al., 2015). However, the issue of fish seed and fish feed are still major problems for the development of the sector in the country. Nile tilapia one of the favorite fish species used for aquaculture practices in Ethiopia has different growth performances in the natural system. Gashaw and Zenebe (2008) compiled research results on length at first maturity of O. niloticus from Ethiopia lakes and found out that it ranges from 19.4 to 42 cm total length for males and 18.1 to 42 cm total length for females. Thus, the objective of this paper was to evaluate the growth performances for O. niloticus from geographically isolated lakes in pond system at the National Fishery and Aquatic Life Research Center (NFALRC).

1 Results and Discussion

1.1 Physico-chemical parameters

Physico-chemical parameters measured during the study period are presented on Table 1. The minimum and maximum temperatures of the experimental units during the beginning of the experiment in November/December were 16.8°C in the morning and 20.4°C in late afternoon. However, there was an increasing trend in the water temperature after December where the minimum and maximum were 20.4°C and 24°C. On the other hand, dissolved oxygen concentration (DO) ranged between 5.1 mg/L (at 9:00 am) and 14.7 mg/L (at one measurement) in late afternoon (5:30 pm) which is expected as the water temperature increases the photosynthetic activity in the water also increases.

Water temperature is key for the metabolic activity and physiological functions of aquatic animals (e.g. feed utilization, feed conversion, growth rates) (Halver and Hardy, 2002; Azaza et al., 2008; Workagegn and Gjoen, 2012; Zenebe et al., 2012). Temperature has indirect effect on the survival and growth of fish. The temperature effect could be also one of the factors for the generally low performance of O. niloticus populations in the current experiment compared to the results of similar studies. The DO concentration was relatively higher compared to similar studies (Liti et al., 2005; Workagegn and Gjoen, 2012). This could be due to algal growth which had low grazing pressure from fish as they mainly fed on formulated feeds. Relative abundance of phytoplankton species (Table 2) also contributed for the elevated DO concentration in ponds.

1.2 Plankton species composition

A total of 17 phytoplankton species: six green algae, four diatoms, four blue greens, two euglenas and one dinophyta were identified during the study period (Table 2). The most frequently observed algal species were Pediastrum, Haematococcus and Euglena species which can be utilized by the fish. On the other hand, zooplanktons were represented by 10 rotifers, 2 cladocerans and 1 copepod species. The cladocerans were smaller sized species like Moina and Diaphanosoma which could favor for the dominance of rotifers.

1.3 Growth performances

Data on the growth performances of O. niloticus from the three lakes are presented in Table 3. The maximum body weight attained by O. niloticus from Lake Chamo, Lake Tana and Lake Hashengie were 67.2, 42.1 and 49.9 g, respectively. The minimum body weight (48.9 g) of O. niloticus for Lake Chamo was even higher than the maximum recorded of O. niloticus from Lake Tana (42.1 g) but was similar to that of Lake Hashengie (49.9 g).

Mean growth performance values of O. niloticus from Lake Chamo (final mean weight, mean weight gain per fish, mean daily growth rate, SGR and FCR) were significantly higher (p<0.05) than O. niloticus populations from Lake Tana and Lake Hashengie. On the other hand, no significant differences (p<0.05) in percent survival of O. niloticus were observed between Lake Chamo and Lake Hashengie. Similarly there was no significant difference (p<0.05) in SGR and FCR between O. niloticus from Lake Tana and Lake Hashengie. Relatively lower percent survival of O. niloticus was recorded from Lake Tana than from those in Lake Chamo and Lake Hashengie (Table 3).

O. niloticus from the three lakes showed similar growth curve trend (Figure 1). Slow linear growth in the first 60 days of culturing periods and then exponential growth after wards were observed in all the treatments. Growth curves of O. niloticus from Lake Chamo separated from that of Lake Tana after 15 days of stocking and get widened after 60 days till the final harvest. But growth curves of O. niloticus from Lake Chamo and Lake Hashengie start widened after 45 days of stocking and the maximum (11.4 g) in the final harvest. The curve for O. niloticus from Lake Chamo demonstrated higher fish growth than O. niloticus from Lake Tana and Lake Hashengie.

Higher growth rate and similar growth curve trend for O. niloticus from Lake Chamo compared with growth rates of O. niloticus from Lakes: Ziway, Tana and Hashengie were also reported (Zelalem, 2013). Data from similar experiment on O. niloticus from geographically isolated lakes: Lake Chamo, Lake Ziway, Lake Tana and Lake Hashengie conducted between similar period with this study also revealed that O. niloticus from Lake Chamo showed higher growth rate over the others (Source: Adamneh unpublished data). However, the daily growth rate obtained in this study is much lower than the results reported for O. niloticus. According to Liti et al. (2005), O. niloticus fed with and without premix attained a daily growth rate ranging between 1.3 and 1.5 g/day. Worekagegn and Gjoen (2012) also reported a daily growth rate ranging between 0.68 and 0.86 g/day for different O. niloticus strains.

In the present study the daily growth rate (0.4 g/day) attained by O. niloticus from Lake Chamo is close to the one reported for O. niloticus by Liti and his colleagues a growth rate of 5 g/day (Liti et al., 2006). Lower daily growth rate (0.16-0.23 g/day) is also reported by Ogunji et al. (2008). In the present study low pond water temperature (16.8-24°C) which was not in the optimum rage for tilapia (25-32°C) and the feed quality particularly digestibility and palatability which were not addressed in the present study could be the major reasons for the lower growth performance of O. niloticus.

The Specific Growth Rates of O. niloticus populations (0.8-1.1% per day) are comparable with the reports of Abdel-Tawwab (2004) that ranged between 0.97-1.23% per day and Ashagrie et al. (2008) from 0.79-1.03% per day. But it was lower compared to the previous reports for O. niloticus from other lakes in Ethiopia, 2.59-2.73% per day (Workagegn and Gjoen, 2012). The result on Food conversion ratio that ranged between 2.5 and 4.2 in the present study is higher than the recommended value of 1.5 for aquaculture (Stickney, 1979). Such variation might probably be due to genetic variation, different culture environment and feed quality as it has been reported by Guimaraes et al. (2008). It could also be the digestibility of the feed given since there is no information on the digestibility of the locally formulated feed.

The gross fish yield per hectare per year of Lake Chamo O. niloticus was 1.3 and 1.7 times greater than to that of Lake Hashengie and Lake Tana, respectively. Compared to the results from other study conducted in Kenya by Liti et al. (2006) that ranged between 4909-5867 kg ha^-1yr^-1 is far less than the result in the current study (198.6-3433 kg/ha/yr) but is similar with the result of another study in Kenya (1579-2328 kg/ha/yr) which was conducted in fertilized ponds supplemented by either maize, wheat or rice bran (Liti et al., 2005). However, our result is twice greater than the result (131-160 g/m³) from similar experiment at NFALRC but supplemented with wheat bran (Workagegn and Gjoen, 2012). The reason for such low gross yield could be due to the feed quality in terms of digestibility which was not examined in the present study. We also used mixed sex-tilapia fingerling for the experiment because there was no sex reversed fingerling which all together contributed for the lower performance in growth. Slow growth in females than their male counterpart as they did not feed during oral incubation of eggs and fry is a general fact which was also visualized in this experiment while weighing the fish in monthly bases and also in the final harvest.

2 Conclusions and Recommendations

The result of the present study revealed that O. niloticus populations from Lake Chamo showed better growth performance compared to those from Lake Tana and Lake Hashengie. However, the final mean weight, daily growth rate and Specific Growth Rate in the current experiment were lower compared with the results from similar studies. This raises a question on the quality of the feed used particularly on its digestibility and the use of mixed sex for the experiment that calls for further study. Based on our result O. niloticus from Lake Chamo can be used for tilapia stocking and for further strain selection programs.

3 Materials and Methods

The study was conducted at National Fisheries and Aquatic Life Research Center (NFALRC), Sebeta, Ethiopia (8°55.076’N; 38°38.161’E), located 24 km South-West of Addis Ababa at an altitude of 2220 m above sea level. The experiment was designed to evaluate growth variations among O. niloticus populations collected from geographically isolated lakes: Chamo, Tana and Hashengie and were reared separately in earthen ponds at NFALRC. O. niloticus fingerlings from the first generation of each parents hatched at NFALRC research ponds here after called CH O.n; TA O.n and HA O.n, respectively were used for the experiment. Concrete ponds (25 m² each) were dried and limed with quick lime and fertilized using manure. Uniform sized fingerlings obtained from the parents of each lake were stocked into experimental ponds with a stocking density of 2 fish/m². After two weeks of acclimatization, 50 fingerlings with mean total body weight of 16.7, 15 and 16.9 g for CH O.n, TA O.n and HA O.n, respectively were stocked randomly in three replicates. The experiment was conducted for 120 days and the experimental animals were fed formulated diets from locally available agro-industrial bi-products (Brewery waste, Wheat bran and Noug cake) supplemented with commercial (Deutsche Vilomix GmbH) vitamin and mineral premix at 2% inclusion level (Chapman, 1992). The formulated feeds were minced in pelleted form using meat grinder. We used two different mesh size sieves, 2 mm mesh size sieve used for first and second months and 3 mm mesh size sieve for the later months. Percent inclusion and proximate composition of the feed ingredients and vitamins and mineral premix are presented in Table 4 and Table 5. Proximate analysis of the feeds was carried out in triplicates as described in AOAC (1990). We used micro-Kjeldhal method for crude protein analysis and Ether extraction method for fat.

The fish were fed 5% of their body weight daily (half of the feed at 10:00 am and the remaining half at 4:00 pm). The feed was delivered at the same position on a round feeding tray made of mosquito net to minimize wastage through sinking. Water was flashed into the experimental ponds every 3^rd day.

Biological data (Length-weight) was measured every 15 days by taking 50% of the fish stocked using seine net. Total length and total weight were measured to the nearest one decimal centimeter and gram, respectively. The difference in the feed requirement in 15 days was not much and feed was adjusted based on the monthly weight gain. Mortality of the fish was monitored and recorded every day.

At the end of the experiment, all fish were harvested, counted and length-weight data measured. Based on the data collected during the experimental period, growth parameters such as mean weight gain per fish, mean daily weight gain, specific growth rate (SGR), feed conversion ratio (FCR) and survival rate were calculated using the following formulae as described by Ridha (2006).

Some physic-chemical parameters such as water temperature, dissolved oxygen (DO), pH and conductivity of the experimental pond water were measured three times a day (morning, noon and in the afternoon) using multi-line probe (model HQ 40 d Multi meter, HACH Company, USA). To get information on the natural food conditions, plankton samples were sampled using plankton nets (30 µm for phytoplankton and 50 µm for zooplanktons). Live samples were identified in the laboratory to the genes level.

4 Statistical Analysis

One-way ANOVA was used to test whether there are any statistically significant differences between the means of independent groups and Duncan's multiple range test was used to identify which means were significantly different from one other. Differences were considered significant at p<0.05. Data was analyzed using the SPSS version 18.

Authors’ contributions

The authors wish to thank Ethiopian Institute of Agricultural Research for financing this research. We would like to extend our gratitude for the assistances from technical assistants and fishermen at National Fishery and Aquatic Life Research Center for the entire experimental period. All authors read and approved the final manuscript.

Acknowledgements

AD’s contributions were in data analysis, manuscript write up and organized the manuscript according to IJA’s guidelines. AY contributed in data collection and partly in data analysis.

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