Introduction
Aquaculture increasingly relies on refined feed ingredients that may compete with resources suitable for human food production. Greater use of low-opportunity-cost biomass (LOCB), including processing by-products and other underutilised resources, could improve feed circularity (Verreth et al., 2023), provided fish performance and edible-product quality are maintained without unacceptable trade-offs in nutrient utilisation or waste management. Nile tilapia (Oreochromis niloticus) is well suited to testing this concept. We evaluated whether increasing LOCB use could reduce feed-food competition while maintaining production performance and edible-product nutritional quality.
Materials and Methods
Two diets were formulated: a conventional control (CON) based predominantly on refined ingredients and a treatment (TREAT) with increased locally available LOCB. Both targeted comparable predicted digestible protein and energy; TREAT contained higher crude nutrient concentrations to compensate for expected lower digestibility of selected LOCB ingredients. Both diets met essential amino acid requirements and were produced as floating extruded pellets.
A 90-day feeding trial used mixed-sex Nile tilapia (~144 g initial body weight) in three tanks per feeding system. CON and TREAT were maintained in separate ~4,500-L recirculating aquaculture systems and fed three times daily at 2% biomass. Growth, feed utilisation, survival and fillet nutritional quality were evaluated. Because diet was confounded with RAS, comparisons were exploratory and based on tank means. A separate replicated trial (four tanks per diet) determined apparent digestibility and faecal particle integrity. Resource-use indicators quantified LOCB and food-competing feed/protein inputs and conversion into edible fish nutrients.
Results
LOCB inclusion increased from 21.5% in CON to 52.4% in TREAT, while food-competing ingredients decreased from 76.5% to 44.6%. FCR was comparable (1.74 ± 0.03 vs. 1.76 ± 0.04; p = 0.410), while exploratory comparisons indicated higher weight gain (202.7 ± 5.0 vs. 216.0 ± 3.8 g fish-1; p = 0.024) and SGR (0.976 ± 0.015 vs. 1.015 ± 0.008% day-1; p = 0.030) in TREAT; survival exceeded 97% in both systems. Food-competing feed input decreased by 41% (1.331 to 0.785 kg kg-1 biomass gain) and food-competing protein input by 54%. With similar net edible protein gain, TREAT converted food-competing dietary protein into edible fish protein approximately 2.2-fold more efficiently. Final fillet yield (36.31 ± 2.28 vs. 36.47 ± 1.39%), proximate and amino-acid composition, and EPA+DHA concentration (0.468 ± 0.018 vs. 0.446 ± 0.036 mg g-1 wet weight; p = 0.423) were comparable. TREAT showed lower apparent protein digestibility (82.1 vs. 90.1%) and more disintegrable faeces (54.0 ± 11.1 vs. 25.7 ± 4.4%; p < 0.01), with fewer easily removable solids (46.0 ± 11.1 vs. 74.3 ± 4.4%; p < 0.01).
Discussion
Increasing LOCB use substantially reduced dependence on food-competing biomass and protein without an evident penalty in feed conversion, survival, fillet yield or nutritional quality. This advantage became clearer when feed efficiency was evaluated according to resource opportunity cost rather than total feed or protein input. However, lower protein digestibility and poorer faecal particle integrity reveal a trade-off between food-system circularity and nutrient and solids management in intensive RAS. LOCB-based formulation is therefore promising, while digestibility and physical waste quality remain important optimisation targets.
Acknowledgement
The study was funded by the HORIZON-CL6-2025-02 Project: 101286791 — DIVERSIFY4Future.
References
Verreth, J.A.J., Roy, K., Turchini, G.M., 2023. Circular bio-economy in aquaculture. Reviews in Aquaculture, 15(3), 944–946.