*6.1.2 Growth during broodstock phase*

### *6.1.2.1 Evaluation of zootechnical parameters*

Monitoring the evolution of the average weight of larvae, from the first phase, for 60 days allowed an estimation of pre-growth rates. A total of 6138 *O. niloticus* larvae, with an average weight of 0.01 g, were collected from 13 broodstocks on April 5th, 2017. *Nile Tilapia "Oreochromis niloticus" Farming in Fresh and Geothermal… DOI: http://dx.doi.org/10.5772/intechopen.106646*

After 36 days, all the 0.2–0.4 g larvae were distributed to two tanks with a volume of 1m3 . The average weight of the larvae in the first batch increased from 0.4 g (initial weight: day of collection) to 2.55 g at the end of the experiment, which corresponds to a growth rate of 0.059 g/day. For the second batch, the larvae monitoring showed a growth rate about 0.023 g/day. This rate is considerably lower than that of batch 1 whose average weight evolved from 0.01 g to 0.85 g. The evolution of the larvae weight from the first phase of rearing which lasted 60 days allowed to estimate the growth rates. These are shown in **Figure 4**.

During this period, larvae were fed manually at a rate of 2 to 3 meals per day with ground powdered food consisting of fish meal, soybean, corn, vegetable oil and a mineral-vitamin complex. The product is fed slightly in excess with a feeding rate of 20% of the total larval biomass. The initial daily intake is 7 g of food/day. Regarding the mortality, we noted that at the end of the larval rearing cycle, the survival rate was about 70%. This rate is comparable with that obtained by the NISTS (in geothermal waters) and the TCA (in fresh waters). **Table 3** summarizes the different parameters, estimated during the larval rearing phase, obtained in previous studies.

Towards the end of larval rearing, the average recorded weight of larvae ranged from 0.85 to 2.55 g between the tail and head in the batches, respectively. Our results are slightly above the individual weights estimated by NISTS and TCA [30, 67].


#### **Table 3.**

*Comparative zootechnical parameters during larval rearing in* Oreochromis niloticus*.*

The conversion index obtained in this study is 2.25 and 2.51 for batches 1 and 2 respectively. These values are higher than those attained by Philippart et al. [72] (IC = 1.1), TCA in 2015 (CI = 1.32) [30] and NISTS in 2009 (IC = 1.97) [67].

The comparison of our results with those achieved by other authors shows poor growth performance at the larval stage especially with the weight larvae heterogeneity (0.02-5 g) (size: 0.8-7 cm). This poor growth is firstly due to the: Quality and physiological state of the spawners; Egg and larval quality; Alterations during the manual counting of the larvae and during the cleaning operations of the larval rearing tanks (siphoning); Disturbance of water quality especially during the night and at dawn when oxygen is lacking; Quality of the dry food produced in the station; Low frequency of food distribution during the day (2–3 times/day), which seems to be lower than recommended (up to 8 times/day); Cannibalism phenomenon (this prompted us to make regular sorting, based on size, by separating the larvae into two batches); and High density of larvae placed in the larval rearing tanks.
