ASSESSING A REDUCED-FISHMEAL ORGANIC DIET IN GILTHEAD SEABREAM Sparus aurata: COMPARATIVE RESPONSES ACCROSS TWO GENETIC LINES

Introduction

Organic aquaculture requires feeds that meet strict standards for fish quality, animal welfare, and regulatory compliance. In 2020, it produced 74,000 tonnes (6.4% of EU aquaculture), with gilthead seabream and European sea bass contributing only 1.5% (Estévez & Vasilaki, 2023). Its expansion is limited by strict organic rules, including scarce availability of certified feed ingredients, especially fishmeal, and the ban on synthetic amino acids, which increase costs and reduce formulation flexibility. Alternative proteins such as land-based animal meals and single cell proteins are therefore being explored to overcome these constraints (Sarmiento et al., 2025; Oliva-Teles & Gonçalves, 2001; Marchi et al., 2023). This study evaluated growth performance and quality in two gilthead seabream (Sparus aurata) genetic lines (fast-growing and standard) fed a novel organic diet featuring partial fishmeal replacement with hydrolysed yeast extract and inclusion of organic poultry meal.

Materials and Methods

Three isoproteic (43%) and isolipidic (17%) diets (conventional (CC), commercial organic (CO), novel organic (NO)) were tested in a 3 × 2 factorial design across eighteen 800-L tanks in a controlled RAS. The NO diet reduced fishmeal, partially replaced by hydrolysed yeast extract, and included organic poultry meal as an alternative protein source. Fish (133.5 ± 0.25 g) were PIT-tagged, reared under optimal conditions (23–24 °C; salinity 28–33 ppt; dissolved oxygen ~100%; pH 7.8–8.2; 12L:12D), and fed ad libitum until tripling in weight over 94 days. Growth performance, biometric indices, and feed efficiency were evaluated at tank level. Fillet composition and quality, gene expression (muscle, liver, and bone), and plasma biochemistry were analysed in subsampled fish to assess overall responses.

Results and Discussion

Preliminary results showed that SGR, FCR, and final body weight were significantly affected by both diet and genetic line, with a post-hoc A,B,A pattern (CC, CO, NO). Feed intake was uneffected, whereas the fast-growing line exhibited improved FCR (Fig.1). Somatic indices (HSI and MFI) were uneffected across treatments, while condition factor was higher in the fast-growing line. Fillet quality was mainly diet-driven, with firmness highest in CO and lowest in NO, while no consistent differences were observed for pH or colour. These results align with previous studies showing that well-balanced organic diets can match conventional performance despite reduced fishmeal and absence of syntetic amino acids, while genetic background remains a key determinant of growth efficiency (Estévez & Vasilaki, 2023; Sarmiento et al., 2025) with fillet quality largely unaffected, except for minor diet-related differences in texture (Sarmiento et al., 2025) .

Conclusion

Reducing fishmeal through partial replacement with hydrolysed yeast extract, alongside organic poultry meal as an alternative protein source, sustains growth performance and product quality in gilthead seabream, reinforcing the scalability of EU organic aquaculture.

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Fig. 1: Growth performance parameters: final body weight (FBW), specific growth rate (SGR), and feed conversion ratio (FCR) as affected by diet (CC, CO, NO) and genetic line (standard, in blue vs fast-growing, in light blue). Two-way ANOVA results with P-values reported in the figure.

Acknowledgment

This work was part of EUAqua.Org project founded by the European Union's Horizon Europe Programme under Grant Agreement No 101181589. https://www.euaqua.org/

References

Estévez, A., Vasilaki, P. (2023). Organic production of gilthead sea bream (Sparus aurata) using organic certified green pea protein and seaweed. Effects on growth, feed conversion and final product quality. Aquaculture, Volume 571. https://doi.org/10.1016/j.aquaculture.2023.739490.

Marchi, A., Bonaldo, A., Scicchitano, D., Candela, M., De Marco, A., Falciglia, S., Mazzoni, M., Lattanzio, G., Clavenzani, P., Dondi, F., Gatta, P.P., Parma, L. (2023). Feeding gilthead sea bream with increasing dietary bacterial single cell protein level: Implication on growth, plasma biochemistry, gut histology, and gut microbiota. Aquaculture, Volume 565. https://doi.org/10.1016/j.aquaculture.2022.739132.

Oliva-Teles, A., Gonçalves, P. (2001). Partial replacement of fishmeal by brewers yeast (Saccaromyces cerevisae) in diets for sea bass (Dicentrarchus labrax) juveniles. Aquaculture, Volume 202, Issues 3–4. https://doi.org/10.1016/S0044-8486(01)00777-3.

Sarmiento, P.; Castro, P.L.; Ginés, R. (2025) Impact of Alternative Feed Ingredients and Feeding Strategies on Growth, Muscle Morphology, and Fillet Quality of Genetically Selected Gilthead Seabream (Sparus aurata) in a Long-Term Feeding Trial. Animals, 15, 1913. https://doi.org/10.3390/ani15131913