**4.2 Ecological requirements**

Thanks to its high adaptability to biotic and abiotic ecological factors, Tilapia rearing can be carried out in fresh and warm waters, as well as in well-controlled conditions after acclimation [17]. *O. niloticus* is a eurylecious species that adapts to large variations of ecological factors as it is able to adjust to extremely different environments [29, 30].

**Table 1**, below, summarizes the various average values of the physicochemical parameters that this fish tolerates for its survival.

#### *4.2.1 Temperature*

Temperature represents a main factor that conditions either way the properties of the water required for rearing and the different growth phases of tilapia. In natural environment, tilapia is a eurythermic fish that can withstand important temperature disparities [32]. Thus, it is possible to find this fish at temperatures ranging between 14 and 33°C. Within breeding conditions, the lower and upper lethal temperatures recorded are 7.4 and 40.73°C respectively [33, 34]. Above 16–17°C, the fish stops feeding and becomes increasingly susceptible to a series of diseases [35–38]. For reproduction, the appropriate temperature ranges between 22 and 30°C [39–41]. In Tunisia, the optimal average temperature for tilapia breeding is between 9 and 28°C in the reservoirs. Moreover, Derouiche et al. [17] reported that this species can adopt a hibernation strategy to survive and grow when the winter temperature is around 9°C (Lebna Reservoir). On the other hand, conducted experiments of the species farming in southern geothermal groundwater show a tolerance of 36 and 40°C [42, 43].

#### *4.2.2 Salinity*

Although most tilapia are freshwater species, their ability to adapt to different salinities is clearly remarkable [44, 45]. For example, *O. niloticus* can adjust to waters with salinity between 0.015 and 30 PSU [46]. Similarly, in Tunisian geothermal waters, tilapias show their ability to withstand high salinities up to 28 g/l [43]. However, with regard to reproduction, this fish would be unable to reproduce in a salinity that exceeds 15–18‰ [47, 48].


**Table 1.**

*Water quality requirements for Nile Tilapia culture [31].*

#### *4.2.3 Dissolved oxygen*

Tilapia are capable of surviving in conditions where the dissolved oxygen concentration is very low [49]. Indeed, they can even withstand levels below 0.5 mg/l, which is considered to be below the threshold level tolerated by most aquaculture species [50, 51]. However, a minimum level of 2 to 3 mg/l is recommended in rearing, otherwise, a depression of the metabolic rate and growth can affect the production.

#### *4.2.4 The hydrogen potential (pH)*

Nile Tilapia presents a capacity of survival in environments of extreme pH. However, the optimal pH advised for its survival and its breeding oscillates between 7 and 8 [39, 45, 49].

#### *4.2.5 Nitrogenous compounds*

In fish farming, ammonia poisoning is closely related to pH. When this substance increases, it leads to the transformation of a significant amount of total ammonia into its toxic form (NH3) [16, 52]. The concentration of nitrogenous metabolic waste excreted through the gills, urine depends mainly on the temperature, the size of the individuals, and the quantity and amount of food distributed. This concentration must be kept below the critical threshold of *O. niloticus*, not exceeding 5 mg/l for nitrate, 500 mg/l for nitrite, and 15 mg/l for total ammonia [53].

### *4.2.6 Photoperiod*

The action of light, although closely related to temperature, acts on the species' growth via the endocrine system. Mélard [54] explain that an optimal photoperiod stimulates the secretion of growth hormone (GH) in *O. niloticus.* Moreover, larvae are more sensitive to photoperiod than fry and juveniles [48, 55]. Experimentally, larvae that are exposed to a long period of light (18-24 h) have a better growth and a significantly higher food efficiency than those exposed to a short or intermediate period between 6 and 12 h [55].

#### **4.3 Reproductive biology**

#### *4.3.1 Reproductive behavior*

In the wild, when abiotic conditions are appropriate, adults migrate to a shallow area with a loose substrate (gravel, sand, clay). After choosing the site for their own nest, each male aggressively defends its territory and digs a plate-shaped nest with its mouth. The females living in a school near the breeding grounds move between the males and each one tries to acquire a partner [15, 56].

#### *4.3.2 Sexual maturity*

The size of the first sexual maturity of *O. niloticus* varies between 14 and 20 cm. However, under stressful conditions, this species can reproduce as early as 3 months of age, at a weight of less than 50 g [57]. Moreover, the reproduction period of this species is exponentially continuous during the whole year when the water

temperature is higher than 22°C [52]. Thus, in Tunisia, the study conducted by Azaza et al. [58] on the reproduction of Nile tilapia in captivity in the geothermal waters in southern Tunisia, showed that this fish reaches its first sexual maturity during the first year of rearing, with an Lm50 equal to 11.3 cm for females and 12.3 cm for males.

## *4.3.3 Fertility*

Absolute fecundity is defined as the number of eggs freshly recovered from the oral cavity of a female. In tilapia, as in other fish, this fecundity increases with the size of the females. As reported by Mélard [54], the minimum absolute observed fecundity for a 26 g female is 340 ovules and the maximum fecundity for a 550 g female is 3500 ovules. In addition, Dhraief et al. [8] proved that this parameter increases with the length of the females. On the other hand, Mélard [54] demonstrated that relative fecundity (expressed as the number of fertilized eggs or fry produced/kg of female) varies inversely with the average weight of tilapia females.

#### **4.4 Growth**

It is commonly accepted that Fish have a predetermined typical growing course dependent on genetic factors which interact with other environmental aspects. Thus, the growth rate is extremely alterable depending on the controlling factors, such as temperature and limiting factors including food, oxygen, and ammonia which affect the amount of energy available for growth. Similarly, other secondary factors such as stocking density and photoperiod, can certainly affect the growth of the species [59]. Moreover, in *O. niloticus* there is a phenomenon of sexual dimorphism of growth which appears very quickly in rearing: the males have better growth performances than females, due to the particularity of the reproductive process in females (oral incubation) and social behavior (territoriality, etc.) [24].

#### **4.5 Production of single-sex male population**

In order to optimize *O. niloticus* production systems, rearing of single-sex male populations is sought more and more in tilapia farming for many reasons. First, males grow twice as fast as females [35, 60]. Another reason is to avoid reproduction which would result in an overpopulation of small individuals in the rearing environment [61], and eventually ensure a homogeneous population at the time of harvest, with an interesting individual size and good commercial value.

#### **4.6 Pathological risks**

Like all aquatic species, Nile tilapia can be prone to a range of diseases resulting from the proliferation of certain pathogenic organisms. Generally, bacteriological diseases remain the most prevalent, namely Mobile *Aeromonas septicemia* and *Vibriosis*, resulting primarily from stress and poor water quality. Affected fish show signs of burns on the skin and fins and a loss of balance associated with abnormal behavior [9, 52]. Results obtained from a study done in tilapia farms in Ghana, revealed three types of ectoparasites: *Trichodina sp*, *Monogenes* and *Tetrahymena sp*, of which the first two were common in most farms, but did not pose real problems.

### **4.7 Diet**

In the wild, tilapia is an omnivorous fish. In aquaculture, however, it shows an ability to consume various products, co-products and waste products that are considered valuable, such as palm nuts, soybean or cotton cakes, rice flour, rapeseed, alfalfa and animal excrements [51, 62]. In Tunisia, a study conducted on the development of dry feeds for *O. niloticus* by Derouiche *et al.* [17] showed that the best growth and feed conversion rates were obtained by feeds containing 20% and 30% of fish meal, with conversion rates of 1.71 and 1.49.
