**6. Results and discussion**

#### **6.1 Evaluation of the rearing parameters of** *O. niloticus*

#### *6.1.1 Evaluation of broodstock fecundity*

In this study, broodstocks were maintained under optimal abiotic conditions, thus 13 females ranging in size from 99 to 183 g were collected in the post-spawning phase. We established relationships linking the number of larvae obtained to the size and females body weight in order to study the absolute fecundity. In addition, the data were fitted with power curves, whose equation is y = aX<sup>b</sup> .

The absolute fecundity observed during the first cycle varied between 451 and 1598 larvae/female (L/F), for respective weights of 113.4 and 183 g. However, the calculated average fecundity (806 L/F) was highly significant compared to research made by NISTS (600 L/F) and TCA (510 L/F) [30, 67]. The study of different parameters that can have a direct effect on fertility allowed us to establish the following relationships: height/weight, height/fertility and weight/fertility.

The analysis between the studied parameters shows a strong correlation between the size or total weight and the number of larvae produced by a female (R2 = 0.6 and 0.645) (**Figure 2**).

The relationship between total tilapia broodstock weight and total length can be expressed as follows: Pt = 0.0324 Lt2.7504 (R<sup>2</sup> = 0.88) (**Figure 3**).

With regard to the allometry coefficient, for all the broodstock exploited during the first rearing phase, it was below 3 (b = 2.75 < 3) indicating that allometry is negative for this species. This result affirm that *O. noloticus* gains in length more than it does in weight. Our results are concordant with those found by Coulibaly [68] at Lake Volta in Burkina Faso, but different from those obtained by Derouiche *et al.* [17] at Lebna Tunisian Reservoir (isometric allometry), and Du Feu [69] at Lake Kainji in Nigeria (positive allometry).

Mélard [54] observed variability in fecundity for the same size. The fecundity increases significantly as a consequence of the length of the females. These findings are similar to other results achieved by authors who studied Nile tilapia in Tunisia and demonstrated that b oscillates around 1.96 [8]. Despite the small size range in our sample (between 18.8 and 23.5), it was shown that absolute fecundity evolves proportionally with size, which is congruent with results obtained by other authors [15, 54, 70]. Generally, this is largely attributed to a genetic difference, and a possible complex interaction between fecundity, egg size obtained and the staggered periodicity of egg laying.

In contrast, for the same size range, fecundity was much lower in the *Oreochromis niloticus* buccal incubator compared to substrate-laying tilapia. These species fecundity

**Figure 2.** *Relative growth and fecundity curves for* Oreochromis niloticus *broodstock.*

**Figure 3.** *Growth curve of female Tilapia.*

ranged from 2314 to 5178 eggs in *Tilapia zillii* [71]. On the other hand, the results obtained on absolute and relative productivity are in close agreement with the research results achieved by Dhraief et al. [8]. In addition, the estimated relative productivity, which evolves significantly as a function of total female weight (2.43 to 10.08 larvae/g female), remains significant compared to that found in 2015 by TCA [30] (between 0.5 and 4.02) and Dhraief et al. [8] and which ranges between 1.4 and 6.8 larvae/g female.
