Fish production is very important not only as a source of protein but it ensures food security, employment and income for many people in developing countries (Sheikh and Sheikh, 2004). Aquaculture is carried out  not only for increasing the availability of fish for food but also to conserve the natural stock and thereby protect the biodiversity. Pond culture is the most widespread type of aquaculture in Africa today. In Ethiopia with a large number of lakes and rivers has more diversified fish fauna (Abebe Getahun, 2002). However, Oreochromis niloticus is the commercially important and cultivable  species in the country (Gashw and Zenebe Tedesse, 2008; Zenebe Tedesse, 2010). Fish growth parameters in terms of weight gain, feeding rate and feeding efficiency show an increasing trend with the increase in water temperature. Below an optimum temperature, fish stop feeding and faces stress condition, fungal infection and high mortality (Britz et al., 1997). The growth of tilapia depends upon the stocking density, feed quality, energy content of the diet, physiological status, reproductive state, and environmental factors such as water temperature and pH (Lovell, 1989; Ali, 1993). 

 

Animal manures are used in fish ponds as a source of soluble phosphorus, nitrogen and carbon to maximize the algal growth and natural food production (Ansa and Jiya, 2002). The type of manure input and fish yield are directly related with each other (Garg and Bhatnagar, 1999; Yadava and Garg, 1992). Ponds manured with cattle dung show higher production by encouraging plankton than supplementary feed since small tilapia filter significantly more phytoplankton than larger individuals. Manuring in earthen ponds is found to be the efficient tool for maintaining water quality due to higher assimilation capacity and plankton population, which enhances fish growth and benefit the farmer economically to reduce 50% cost of inorganic fertilizer and supplementary feed (Desilva and Anderson, 1995).

 

The study was conducted in the ponds located in Toke Kombolcha, Lencha and Imala Dawe Ajo, Toke Kutaye Woreda of Oromia Regional state, Ethiopia (Figure 1).

 

 

Length and weight of tilapia fingerlings is presented in table 1 and 2. The initial body length (IBL) and initial body weight (IBW) were found to be 5±0.03 cm and 5.7±0.02 gm respectively. The final body length (10.1 cm) was observed in P3 after 120 days of culture and it was 8.1 cm in P1 situated at highest altitude (Table 1). The maximum final body weight (IBW) noticed was 63.7±0.50 g in P3while it was  52.4±0.57g in P2 and 18.2±0.02g in P1. 

 

Statistical analysis showed significant difference (P<0.05) for average individual body weight among all the treatments (Table 2).

 

Average body weight gained by fish in ponds located in different altitudes was 0.2±0.03, 1.2±0.05 and 3±0.09 g/fish in15 days. The values increased during the study period and reached 2.6±0.03, 10.1±0.78 and 12.3±0.53 g/fish at the end of the experiment (120 days) in P1, P2 and P3, respectively (Table 3). The values of average body weight gain among all ponds  showed significant differences (P<0.05). 

 

Changes in SGR value of O. niloticus at the end of the experiment were 1.0±0.23, 1.2±0.11 and 1.4±0.06% at the altitude of 2,500, 2,100 and 1,710 msl respectively (Table 4). The highest mean growth rate was recorded in P3 and the minimum in P1. The observed SGR were significantly different (P<0.05) between the ponds. 

 

During the study period all the important water quality parameters in the three ponds were analyzed. Of which only variations in temperature, nitrate and total phosphorus showed distinct variations in the ponds and are for discussion in this paper.

 

3.2.1 Temperature

During the experimental period the overall range of water temperature were 15.5±0.46 to 16.7±0.55, 20.5±0.56 to 22.2±0.80 and 25.4±0.65 to 27.1±0.27°C in P1, P2 and P3, respectively (Table 5). The minimum average values were recorded during October and maximum in all the ponds. Statistical analysis showed significant differences (P<0.05) for temperature among the ponds during the experimental period.

 

Nitrate concentration in all the ponds showed gradual increase in all the three ponds. The pond at the lowest altitude (1,710 msl) showed maximum value (6.9 mg/l) followed by 5.3 mg/l (2,100 msl) and 2.8 mg/l (1,710 msl). The phosphate level was found to be minimu in P1 and maximum in P3 (Table 6). A wide range of variation in phosphate found in P3 compared to P1 at the end of the experiment. There was a statistically significant difference (P <0.05) for total nitrate and total phosphate among all the treatments (Table 6).

 

Phytoplankton belonging to Chlorophyceae, Bacillariophyceae, Cyanophyceae and Euglenophyceae  were  identified  in  all  the  ponds (Figure 1). Cyanophyceae was the dominant group in pond 1 while this was the least abundant group in the other ponds (Figure 3, Figure 4). The chlorophyceae members are the most abundant in pond P2and P3. Bacillariophyceae and Euglenophyceae were relatively less abundant in pond1than the P2 and P3. 

 

 

The zooplankton was represented by Rotifers, Copepods and Cladocerans. Rotifers were more dominant in P2 and P3 followed by Copepods (Figure 5, Figure 6, Figure 7). The highest number of Rotifers and Copepods were recorded in February in P3. But the higher number of Cladocera was recorded in P1. (Figure 5). The species of Rotifers identified during the experimental period were, Brachionus, Horaella, Testudinella and Keratella. Keratella was the most dominant in P2 and P3 and Brachionus in P3. Horaella was absent in P1 and rare in P2. Cyclopoid and Calanoid were the most abundant groups of copepods. However, copepods were less abundant in the higher altitude pond. Calanoids were the most dominant in P3 (Figure 7). Daphnia was the only genus among Cladocera found in P1 than the other ponds. 

 

Each aquatic organism has specific survival range of environmental temperature for their efficient existence and beyond the limit, the conditions become lethal. Water temperature is known to prominently affects the fish life by directly or indirectly through influencing the physical and chemical properties of the water in which it lives. Growth performance of fish is dependent on the temperature (Britz et al., 1997; Azevedo et al., 1998; Lei and Li, 2000) which in turn is dependent on the altitude. O. niloticus fingerlings were maintained in fish ponds located at altitudes of 2,500 masl, 2,100 masl and 1,710 masl fertilized with organic fertilizer and supplementary feed. There was a significant difference (P<0.05) in water temperature among the treatments which caused variation in growth performance of O.niloticus. In the present experiment when the temperature increased growth rate was observed in pond 3 located at low altitude  where the temperature was higher than pond1 indicating the influence of temperature on the growth of O. niloticus. The highest weight gain was also observed for the fish maintained in P3. Fish being cold blooded, is easily influenced by the surrounding water temperature that shows a prominent effect on body temperature, growth rate, feed consumption, FCR and other metabolic function [Lovell, 1989; lei and Li, 2000). The present results revealed that when the altitude is decreased the temperature increases and as a result growth performance increases (El-Sherif and El-Feky, 2009). The range of temperature in the study sites varied between 21.6 and 27°C that prevailed at the altitudes of 2,100 masl and 1,710 masl. It is inferred that the water temperature in the low altitude was more favorable for the growth of O. niloticus as reported earlier that the optimum water temperature is between 21 and 28°C (Popma and masser, 1999; Bhijikajee and Gobin, 1997).The average body weight gain of the fish decreased on day 60 and 105 in P1, on day 90 in P2 due to low water temperature. Similar findings on the average daily body weight gain decrease at temperatures below 21.1oC was reported earlier (Khouraiba, 1989) which may be attributed to decrease in feed consumption and metabolic rate. An increase in temperature enhances the activity of digestive enzymes, which may accelerate the digestion of the nutrients, thus resulting in better growth (El-Sherif and El-Feky, 2009). In the present study, SGR in the three treatments were positively correlated with temperature. There was a decrease in SGR at the average temperature of 16.1°C (P1) and it was better in P3 than P2. This observation is in agreement with the specific growth rates observed by many workers who stated that  between 16 and 20°C SGR is significantly (P < 0.05) high however it was highest at 25°C (El-sherif and El-Feky, 2009). The best FCR was observed in P3, followed by P2 and P1 (Table 4). The variation in water temperature also influences the solubility of gases, pH, conductivity and plankton distribution (Boyd, 1998). At the altitude of 2500 masl higher transparency and low concentration of nitrate and total phosphorus was noticed associated with low rate of decomposition of the organic manure applied and the resultant  low rate of plankton production. The plant nutrients nitrate and phosphorus recorded in the ponds are however optimum for the fish culture and phytoplankton growth (Boyd, 1998). High abundance of plankton in P2 and P3 might be due to the higher nutrient (nitrogen and phosphorus) concentrations and temperature recorded in thosetreatments other than in P1(Yadava and Garg, 1992). In this study, the high numbers of plankton were recorded at P3 followed by P2; this condition indicates that there is good growth performance and survival rate for fish in these ponds.