Introduction
The gilthead seabream Sparus aurata is one of the most produced species in Mediterranean Sea and has a great potential of expansion for other regions were the species occurs. The species have been cultivated in the Red Sea in commercial farms, and studies on nutrition in the early grow-out phase and at harvestable size are always on demand. The Kingdom of Saudi Arabia has a large coastline suitable for aquaculture
with favorable conditions and environment. Aquaculture is a growing sector in KSA and nutrition is one of the key factors , since feeds are responsible for up to 60% of the total production costs.
So, i n order to have the optimal feed conversion ratio and high feed efficiency, in the present work we evaluate the best diets commercially available in KSA and investigated the growth performance, survival and fish incorporation of the feeds in order to understand seabream performance in different sizes under the Red Sea conditions.
Materials and methods
In the first feeding experiment at the Coastal Marine Resources Labs (CMOR) of the King Abdullah University of Science and Technology (KAUST) , 20 juvenile of gilthead seabream (Run 1) with an average body weight of 119.6 ± 8.9g were randomly stocked in 600 L tanks. In the second experiment, 30 pre- adult seabream (Run 2) with an average body weight 310.3 ± 41.9g were randomly stocked in 1m3 tanks. For Run 1, the fish were fed with four commercial diets (diet 1, 2, 3, and 4) and for Run 2, three commercial diets were tested (diet 5, 6 , and 7). Fish were hand-fed until apparent satiation two times per day, sampling was conducted every 2 weeks to determine growth performance and survival, and p roximate composition of the diets was performed. The parameters that fulfilled the prerequisites of analysis of variance (ANOVA) were performed with a one-way ANOVA followed by Fishers Least Significant Difference (LSD) and the parameters that did not fulfill the prerequisites were compared using Kruskal-Wallis-H test followed by Student Newman Keuls
using Sigmaplot version 11.0. T he level of significance adopted were P<0.05.
Results
The survival of the gilthead seabream in both runs was around 98.3-100% and showed no significant difference among the different diets tested. In this study, the 119 g juvenile gilthead seabream showed a maximum daily weight gain of 2.36 g day-1 and an SGR of 1.27% day-1 . Comparing the different diets, the FCR value was significantly lower (more efficient) in diets 2 and 4 as compared to diets 1 and 3, so, the higher energy diets,
showed better FCR. For the pre-adult gilthead seabream (run 2), the FCR value was significantly lower (p<0.05) in diet 6-fed fish (1.95) as compared to fish fed the diet 5 (2.16) and 7 (2.02).
Diet 3 exhibited lower body mass at the end of the experiment (p=0.001) (figure 1a), while diet 4 showed the highest PER. In the pre-adult fish, diet 7 showed the highest SGR and growth gain, while, diet 6 showed the lowest FCR, but no differences was observed in the body mass (figure 1b) .
In the juvenile fish, the feed intake was highest in diet 1 that has the lowest energy content while the lowest feed intake in diet 4, which has the highest energy.
Discussion
These growth results are comparable or even better to the results obtained by previous researches wherein juvenile gilthead seabream of about 105-254 g size cultured at salinity conditions lower than the Red Sea (<41 ppt) attained a daily weight gain of around 1.15-2.87 g day-1 and an SGR of around 0.56-0.96% ( Ribeiro et al., 2015) . The FCR values obtained from this study (1.57-1.7= juvenile; 1.95-2.16= preadult), were also comparable from previous reports (1.5-2.1) (Araújo‐Luna et al., 2018; Ribeiro et al., 2015).
It has been observed even with other fish species that the energy density of the feed directly influences the amount of feed consumed, such that a high energy diet can result in a reduction in the amount of food eaten by the fish. In many fish species, that protein (nitrogen) retention can be improved by partly replacing dietary proteins with lipids resulting in a protein-sparing effect (Santinha et al., 1999). PER, which is a good measure of the protein-sparing effect by non-protein energy sources , showed a significantly higher PER in diet 4, suggesting that the lipid in the diet has probably spared protein. Our result for diet 4 is comparable to previous results wherein PER was highest in a lower protein (40%) as compared to a higher protein diet (55%) but with a higher energy diet (Santinha et al., 1999). However, optimum lipid content should be determined in the diet considering that a high-energy diet generally leads to fat deposition and thus can affect the fish commercial value.
Conclusions
The growth results from this study showed comparable with past researches despite the fish were grown in slightly higher temperatures (>24°C), probably due to the higher protein and/or energy in some of the diets tested. Our results suggest that gilthead seabream can attain high survival in the Red Sea with good growth performance and FCR levels showing potential economic feasibility for a commercial operation.
References
Araújo‐Luna, R., Ribeiro, L., Bergheim, A., Pousão‐Ferreira, P. 2018. The impact of different rearing condition on gilthead seabream welfare: dissolved oxygen levels and stocking densities. Aquaculture Research. 49, 3845-3855.
Ribeiro, A.R., Goncalves, A., Colen, R., Nunes, M.L., Dinis, M.T., Dias, J. 2015. Dietary macroalgae is a natural and effective tool to fortify gilthead seabream fillets with iodine: Effects on growth, sensory quality and nutritional value. Aquaculture, 437, 51-59.
Santinha, P.J.M., Medale, F., Corraze, G., Gomes, E.F.S. 1999. Effects of the dietary protein: lipid ratio on growth and nutrient utilization in gilthead seabream (Sparus aurata L.). Aquac. Nutr. 1999 v.5 no.3, pp. 147-156.