*6.1.3.1 Zootechnical parameters*

As previously indicated, the fry obtained after 60 days of larval rearing was transferred to the two study areas, where the rearing and grow-out phases were initiated. The zootechnical parameters obtained in the pre-grown fry in the cages

**Figure 5.** *Average evolution of the water physicochemical parameters.*

*Nile Tilapia "Oreochromis niloticus" Farming in Fresh and Geothermal… DOI: http://dx.doi.org/10.5772/intechopen.106646*


#### **Table 4.**

*Zootechnical performances recorded in pre-grown tilapia in fresh and geothermal waters.*

placed in the large rearing pond in Bechima Station (geothermal water) and the enclosures set in Smati Reservoir (freshwater) are reported in **Table 4**.

Growth performance expressed by the daily growth rate, the survival rate, and the conversion rate are in favor of the fish reared in the freshwater reservoir. Indeed, the growth rates recorded after 50 days of rearing are around 10.51 and 12.7 respectively for fish in geothermal water and in fresh water. Similarly, the conversion rates are of the order of 1.36 for fry reared in geothermal water and 1.05 for those reared in fresh water. For the fry length-weight relationship, we recorded a variation in *O. niloticus* growth. **Table 5** summarizes the different results obtained as well as the type of allometry.

In the freshwater pens we found that the relationship between total weight of *O. niloticus* fry and total length can be expressed as follows Pt = 0.0096Lt3.1041


#### **Table 5.**

*Tests of slopes and allometry positions of the weight-total length relationship per month in fresh and geothermal waters. (FW: fresh water; GW: geothermal water).*

(n = 41, R2 = 0.6962). On the other hand, it is expressed as Pt = 0.0075Lt3.256 for cages placed in geothermal water. Moreover, the allometry coefficient is greater than three for the fry raised in both areas. Thus, this species exhibits a positive allometry between total weight and size. This asserts that this fish gains in weight more than in length. This result is congruent with the findings achieved at Lake Kainji in Nigeria [69] but it differs from the results found by Coulibaly [68] at Lake Volta in Burkina Faso (Minority allometry) and Derouiche et al. [17] in Lebna Reservoir (isometric allometry).

The monthly analysis of growth performance shows a slight difference in growth between the two zones over the study period. The value of the t-test is inferior to 1.96 at the 5% threshold. The month of July is characterized by a positive allometry in the reservoir and in Bechima station where the individual weight grows faster than the length.

Comparing the results of zootechnical performance between geothermal and fresh waters, it can be noted that the growth of pre-grown fry in pens at Smati Reservoir is superior to the fry raised in cages at Bechima fish farm. This may be due to the water quality at the rearing pond where the turnover rate is low. This is caused by the low flow of used water to maintain the temperature around 28–30°C. On the other hand, the water in Smati Reservoir seems to have physico-chemical parameters that are favorable to rearing.

Comparing our results with other studies, we note that at the Blonbey fish farm, fry of 1660 mg stocked in ponds at a density of 500 ind/m<sup>3</sup> reach an average weight of 5980 mg. The daily growth rate is 210 mg/day, while the specific growth rate is about 5.98% per day. Our results are consistent with those found by Azaza *et al.* [58]. These authors indicate that they were able to achieve a daily weight gain of 0.4666 g/day during 15 days of hatching from fry of 2 g to an average weight of 9 g. The rearing adopted by these authors was conducted on a feed with the same composition as used in our experiment.

On the other hand, FAO states that a good daily growth rate can be obtained during 30 days and under intensive conditions (0.5 g/d daily gain) [73]. In Côte d'Ivoire, conducted hatching experiments have shown that growth rates are much better in 1m3 cages with a density of 1500 fry/m<sup>3</sup> . These experiments produced a fry weight of 25 g during 1 month with an estimated daily growth rate of 0.22 g.

The results from the pen rearing in Smati Reservoir are similar to those achieved by Azaza et al. [58] who obtained a daily growth rate of 0.43 g/day during 45 days. However, our results differ from those found by Lazard and Legendre [59] who got an average individual weight of 5 g after 2 months of pond rearing. This experiment was based on a dry feed composed of 20% of fishmeal, for fry with an average weight of 0.9 g, which corresponds to a daily weight gain equal to 68.33 mg/day. On the other hand, the good zootechnical performance observed during our experiment in fry pregrown in fresh and geothermal waters is comparable to that of *H. longifilis* pre-grown made during 4 weeks in 4 m<sup>2</sup> tanks and whose average weight evolved from 4.5 g to 50 g [74].

#### *6.1.3.2 Evaluation of physicochemical parameters during pre-growth phase*

Tilapia farming is relatively difficult because it depends on the environmental factors of the water used. Indeed, the good management of a better water quality is the key to produce fry and fish in good conditions. The average values of the physicochemical parameters recorded at the two sites during the pre-growth phase are shown in **Table 6**.

*Nile Tilapia "Oreochromis niloticus" Farming in Fresh and Geothermal… DOI: http://dx.doi.org/10.5772/intechopen.106646*


#### **Table 6.**

*Evolution of physicochemical parameters in fresh and geothermal waters.*

#### *6.1.3.3 Temperature*

During the pre-growth phase, the monitoring of physico-chemical parameters in the freshwater and the geothermal water areas show that the maximum temperature values were recorded in July. The average temperature recorded in Smati Reservoir is estimated at 26.3°C against 30°C at the level of Bechima station. Eer *et al*. [75] have shown that a temperature between 20 and 30 °C is optimal for fish farming.

#### *6.1.3.4 PH*

Monitoring this parameter is momentous in farm management since inadequate pH values can influence the physiological fish state and their growth. Furthermore, it can lead to fish mortality, especially during the early developmental stages [53]. Throughout our study, pH values in the larval tanks, cages and pens showed small variations through time, but were still within the optimal tolerance range of the species (6.5–8.5) [53]. The average pH values recorded in the cages and pens are close and show a basic character of the rearing water (7.3–8.23). This species is tolerance to pH variations and it is found in waters with pH values ranging from 5 to 11; the ideal being located between 6.5 and 8.5 [53].

#### *6.1.3.5 Dissolved oxygen*

The dissolved oxygen levels measured in the two areas are within the scale of values recommended for fish farming [50]. The average oxygen level recorded varies between 6.94 and 8.28 mg/l in fresh water and geothermal water.

### *6.1.3.6 Salinity*

Salinity monitoring showed that this parameter was almost constant during the 3 months of the experiment in both study sites. The values recorded are 2.03 g/l and 0.2 g/l respectively in the geothermal water and freshwater.

The analysis of the evolution of the physico-chemical parameters in the two areas showed that the values are well within the ranges recommended for Nile Tilapia farming. Indeed, this species is found in natural environment between 13.5–33°C [47, 76] and does not feed below 15°C [53].

In addition, *O. niloticus* tolerates both high oxygen deficits and saturations. Thus, up to 3 mg/l of dissolved oxygen, this species does not present any particular metabolic

difficulty. However, below this value, respiratory stress occurs, although mortality only occurs after 6 hours of exposure to 3 mg/l. Nevertheless, this species can withstand low concentrations of dissolved oxygen for short periods. The optimum required is 5 mg/l [53].

## *6.1.3.7 Effect of water on tilapia growth*

To test the prior effect of abiotic factors on the zootechnical performance of fish during the pre-growth phase, we established statistical tests based on general linear models (GLM), by means of quantitative variables. These tests allow the complex parametric relationships between response variable and explanatory variables to be modeled; as well as to look for the most parsimonious relationship including only the relevant variables (using the AIC selection criterion). In addition, they make it possible to test the effects of explanatory variables and their interactions.

We followed the effect of physicochemical water parameters on the evolution of the total biomass of tilapia. We found that only temperature and dissolved oxygen govern the growth of individuals reared in freshwater and geothermal waters alike. Besides, the predictive study allowed to retain the variables mentioned above, showing the lowest value of AIC.

### *6.1.4 Growth parameters of tilapia during the grow-out phase*

#### *6.1.4.1 Physicochemical parameters*

The table below (**Table 7**) illustrates the average values recorded of the water physicochemical parameters during the fattening phase. The study made during the grow-out stage allowed us to point out a clear difference in temperature between the two study areas. For the other parameters (dissolved oxygen and pH) remained very close and did not show significant variations.

### *6.1.4.2 Nitrogenous elements during breeding phase*

Nitrogen is an essential compound in living structures, depending on the degree of oxidation; it exists in three forms in water: nitrites (NO2-), ammonium (NH4+) and


**Table 7.**

*Variation of physicochemical parameters between Bechima Station and Smati reservoir.*


**Table 8.**

*Evolution of nitrogenous elements for pre-growth and grow-out in the two environments.*

nitrates (NO3-). The latter must be well controlled throughout the rearing period due to their toxic nature. The results of these analyses on the quality of the rearing water are reported in the following table (**Table 8**).

During the pre-growth phase, the analyses obtained in the two study areas are within the recommended ranges for rearing *O. niloticus*. In fact, the concentration of nitrogenous waste excreted by the gills and urine depends on various factors, namely temperature, fish size, ammonia concentration in the environment and the quality of the feed which must be kept below the critical threshold for survival of *O*. *niloticus*. Concentrations should not exceed 15 mg/l nitrate, 2 mg/l nitrite, 0.95 mg/l ammonium ions and 0.3 mg/l orthophosphate in any case [53].

However, at the grow-out phase, we recorded an increase in nitrite, nitrate and orthophosphate levels (at the reservoir). A high concentration of ammonia may have altered the taste and odor of the water. This may be explained by the increase in temperature during the summer season, and the phytoplankton bloom at the reservoir impoundment caused by the discharge of agricultural wastes and nitrogenous products. However, the majority of the waters of Tunisian reservoirs are classified as eutrophic to hypertrophic. These values remain below those recorded in the reservoir of Bir M'chergua whose nitrate concentration is estimated at 16.9 mg/l [77].
