Introduction
High inclusions of fish meals (FM) and oils (FO) in aquaculture feeds are not considered indispensable ingredients anymore . To meet future the demand for aquaculture products and to reduce completely the dependence on FM and FO, it is necessary to find new profitable and more sustainable sources of protein and lipids. Insect meals and single-cell proteins from bacterial or microalgal sources have been recently investigated and pointed out as promising future feedstuffs for the aquaculture industry due to their high nutritional potential for fish, as well as their low ecological impact. Added to novel nutritional strategies, the implementation of genetic selection programs can also be a complementary tool to improve the robustness of farmed fish and their plasticity to deal with nutritional innovations and challenging feeds with low FM/FO. Some studies reported that fish that are selected for fast growing on FM/FO diets display also higher growth on plant-based diets (Palti et al., 2006), while some others, reported a significant diet x genotype interaction, which means that fish that are selected for their fast growth on FM diets may not be the ones that grow faster when facing a challenging diet (Geay et al., 2011). Therefore, the aim of the present study , as part of the AquaIMPACT project (Genomic and nutritional innovations for genetically superior farmed fish to improve efficiency in European aquaculture; EU Horizon 2020, 818367), is to determine the effectiveness of genetic selection for growth in gilthead sea bream, in response to a challenging low FM/FO diet that aimed to partially replace FM by two emergent ingredients: insect meal from black soldier fly or single-cell protein from M. capsulatus. These diets also aimed to totally replace FO by a blend of poultry oil with a novel microalgae oil. The response of selected sea bream to the novel dietary interventions was assessed on fish productive parameters, fish proximate and fatty acid composition, the apparent digestibility coefficients of the dietary nutrients, as well as on fillet quality properties at commercial size.
Materials and Methods
Three diets were formulated to be isoproteic and isoenergetic to meet the nutritional requirements of gilthead sea bream juveniles. Control diet (C) contained 15% of FM and 5.9 % of fish oil to mimic the composition of a current commercial diet, as well as completed with some vegetable meals and rapeseed oil as protein and lipid sources, respectively. Insect meal diet (INS) was included at 5% of the diet to replace 33.3% of the dietary FM. Single-cell protein was included at 10% of the diet and replaced 66.7% of the dietary FM. Fish oil was also totally replaced in these diets by a blend of poultry oil and Veramaris algal oil. Yttrium premix was added at 0.1% to both diets to further determine the apparent digestibility coefficients (ADC) of nutrients. The nutritional trial was carried out at the experimental facilities of the ULPGC. Gilthead sea bream from each experimental group (HG genotype vs REF genotype) and with an initial body weight of 49.91 g (average body weight), were randomly distributed in 12 experimental tanks, at a density of 45 fish/tank (3 tanks/treatment). Fish were initially allocated in cylinder-conical tanks of 500 L. Fish were manually fed until apparent satiation with one of the two experimental diets for 12 weeks (4 times a day, 6 days a week). Fish growth performance was monitored every 4 weeks until the end of the feeding trial and fish whole-body, fillets and liver samples were collected at the end of the trial for biochemical composition . Furthermore , fish faeces were collected after a digestibility trial to calculate the apparent digestibility coefficients analysis of the dietary protein and amino acids. T exture properties at 1 and 4 days post-slaughter, a s well as the sensorial attributes of fish fillets, scored by a human panel , was also assessed.
Results
Diet showed an effect on fish growth performance sooner (after 4 weeks of feeding) than genotype . INS and SCP diets led to a poorer growth performance in terms of body weight, length and/or SGR and TGC than C diet from 4 weeks of feeding and this difference was maintained until the end of the feeding trial (12 weeks) . INS and SCP diets also significantly decreased fish FI compared with C diet, irrespective of the genotype, but showed no effect on FCR. In contrast, selected genotype increased fish growth in terms of body weight, total length and productive parameters like SGR and TGC, as well as optimized FCR, irrespective of the diet. No significant interactions between g x d were observed in any productive parameter evaluated after the 12-week feeding trial. HG genotype significantly increased the ADC values of protein and individual and total amino acids, except for tyrosine , whereas diet showed no effect on ADC values.
Genotype did not affect fatty acid profile of fish tissues, but SCP and INS diets increased the n-3 LC-PUFA content of fish fillets.
As expected, time post-slaughter significantly affected many texture properties of fish fillets, irrespective of the diet or genotype, with the values for fracturability , hardness elasticity, gumminess, and resilience of fish fillets decreasing with time. However, no significant effect of genotype or diet were noted in the texture properties of fish fillets, neither at 1 nor 4 days post-slaughter. Concerning the sensory properties of sea bream fillets, scored by the evaluation panel, no significant effects of diet, genotype or an interaction between g x d were observed for any of the sensorial attributes of fish fillets .
Conclusions
Overall, the results reaffirm the positive effects of genetic selection in improving sea bream productive key indicators, as well as support the use of insect meal and microalgal oil as replacers of FM and FO, respectively, in diets for selected sea bream, especially microalgae oil to increase n-3 LC-PUFA of fish fillets and improving the nutritional value of fish final products for consumers. However, the use of feed attractants might be useful to achieve similar growth as when a Control diet with FM is used, since the inclusion of insect meal and single-cell protein at 5% and 10% of the diet, respectively, might compromise feed intake.
References
Geay , F., Ferraresso , S., Zambonino -Infante, J. L., Bargelloni , L., Quentel , C., Vandeputte , M., ... & Mazurais, D. (2011). Effects of the total replacement of fish-based diet with plant-based diet on the hepatic transcriptome of two European sea bass (Dicentrarchus labrax) half-sibfamilies showing different growth rates with the plant-based diet. BMC genomics, 12(1), 1-18.
Palti , Y., Silverstein, J. T., Wieman, H., Phillips, J. G., Barrows, F. T., & Parsons, J. E. (2006). Evaluation of family growth response to fishmeal and gluten-based diets in rainbow trout (Oncorhynchus mykiss). Aquaculture, 255(1-4), 548-556.